GEOCIENCIAS-VOL 14-1 2010.vp EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 14, No. 1 (June 2010): 76-87 CSEM IMAGING OF THE NEAR SURFACE DYNAMICS AND ITS IMPACT FOR FOUNDATION STABILITY AT QUARTER 27, 15 th OF MAY CITY, HELWAN, EGYPT Magdy A. Atya1, Olga A. Khachay2, Mamdouh M. Soliman1, Oleg Yu Khachay3, Ahmed B. Khalil1, Mahmoud Gaballah1, Fathy F. Shaaban1,4 , Ibrahim A. El Hemali1 1 National Research Institute of Astronomy and Geophysics (NRIAG), 11722 Helwan, Egypt. 2 Institute of Geophysics, Russian Academy of Science (RAS), Ural Division, Russia. 3 Ural State University, Russia. 4 King Khaled University, Faculty of Science, Physics Department, Abha, Saudia Arabia. magdy@igse-eg.com ABSTRACT In the present work, we involve the Control Source Electromagnetic (CSEM) Technique to image the dynamic migration center of the near surface fractures, fissures, and cracks in a new dwelling area at 15th of May city close to Cairo. This area forms the center of the zonal weakness of the subsoil, which in turn, interact with the weight center of the construction leading to cata- strophic collapses. The control source electromagnetic technique has been developed recently to monitor the migration of the weakness center represented as the accumulative fissures and cracks in the near surface. Three composite profiles of wide and planshet mesh data have been collected in 2008. This survey has been followed by performing two profiles in 2010; one of these profiles has been repeated to observe the situation changes, and the second profile has been measured between the other two profiles of 2008. The last profile had been performed to study the mutual relation between the measurements of the two cycles. The objec- tives of the whole process are to validate the capability of the technique to pick the minor changes of the weakness center, con- sequently, study its relation to the weight center of the adjacent construction to produce a recommended procedure to minimize the destruction resources at the site of investigation. The measured data has been interpreted and represented in graphs showing the distribution of the heterogeneity of the geoelectric parameters in the subsoil, furthermore, in a series of geoelectric cross section representing the applied frequencies used during the survey. The study concludes that (1) the center of the cracked zone is moving upward closer to the surface of the ground and heaver, (2) the water content is moving downward producing soil dryness at the shallow depths, and (3) the site became more stable in 2010 than 2008, however, the destruction resources remain warning with collapse events. Furthermore, the situation reflects the relation between the water content and the changes in the weakness center. Key words: electromagnetic technique, en helwan, weakness center, control source electromagnetic (CSEM) 76 Manuscript received: 28/03/2010 Accepted for publication: 20/05/2010 RESUMEN En el presente trabajo, aplicamos la Técnica de Fuente Electromagnética Controlada (CSEM) para obtenerla imagen del centro dinámico de la migración de las fracturas, fisuras y grietas de la superficie somera en una nueva área de vivienda en la ciudad de 15 de mayo cerca a El Cairo. Esta zona constituye el centro de la debilidad del subsuelo, que a su vez, interactúacon el centro de peso de la construcción que ha llevado a colapsos catastróficos. La técnica de Control de Fuente electromagnética se ha desarrollado recientemente para monitorearla migración del centro de la debilidad, representados por la acumulación de fisuras y grietas de la superficie somera. Tres perfiles compuestos se han recogido en 2008. Este estudio se ha seguido realizando con dos perfiles en 2010. Uno de estos perfiles se ha repetido para observar los cambios, y el segundo perfil se ha medido entre los otros dos perfiles de 2008. El último perfil se ha realizado para estudiar la relación mutua entre las mediciones de los dos ciclos. Los objetivos de todo el proceso es validar la eficacia de la técnica para recoger los mínimos cambios del centro de la debilidad, y así, estudiar su relación con el centro de peso de construcciones cercanas, para recomendar un procedimiento que reduzca al mínimo la destrucción de los recursosen el sitio de la investigación. Los datos medidos se haninterpretado y representado en gráficos que muestran la distribución de la heterogeneidad de los parámetros geo-eléctricos en el subsuelo; además, en una serie de seccionescruzadas geoeléctrica se representa las frecuenciasutilizadasdurante el estudio. El estudio concluye que (1) el centro de la zona agrietada se mueve hacia arriba cerca de la superficie de la tierra, (2) el contenido de agua está bajando, produciendola sequedad del suelo apoca profundidad, y (3) el sitio se volviómás estable en 2010 que en 2008, sin embargo, los producto de la destrucción siguen susceptiblesa colapsos.Además, la situación refleja la relación entre el contenido de agua y los cambios en el centro de debilidad. Palabras clave: técnica electromagnética, en helwan, centro de debilidad, control de código fuente electromagnética (CSEM) 1. Introduction The idea behind the present paper reflects the capability of the applied and environmental geophysics to be in- volved in the community problems. Accordingly, it is strongly recommended to establish a monitoring system for the detection of possible occurrence of disasters. Cur- rently, various kinds of geophysical systems are being op- erated for environmental monitoring purpose and detecting the subsurface temporal changes. The problem under investigation is related to the unstable subsurface. The subsurface heterogeneity caused by the intensity of the fractures, the fissures, the cracks, and the water con- tent, is a factor of the instability level of the base ground beneath the constructions. Such instability of the rock sta- tus resulted in several destructions at many places such as the stone block fallen at Al Deweaka (Fig. 1) and at the Second Level of Al Mokkatam Plateau, furthermore, the constructions deformation like that happened in the 15th of May City, Quarter 27 (Fig 2), and land subsidence like that occurred at several parts of Wadi Hof province and along the Autostrade form Al Maadi to the margins of 15th May towards Al Tebbeen. The research layout is devoted to observe and monitor the near surface structures and to follow the growth of the in- tensity of the fractures, the fissures and the cracks. The water content and its salinity is also an important factor to be ob- served and followed. It is planned that, the observation and monitoring phase will run arbitrary at the start of the work to calculate the optimum measurement circulation comparable to the cyclic stress forces or loads affecting the near surface structures. Then the situation will be subject of vector analy- sis to provide a management procedure to reduce the de- struction resources and overcome or at least minimize the collapse. Geophysics can easily detect the presence of different objects in wide range of applications. However, if those geophysical measurements are repeatedly obtained and compared, additional temporal dimension analysis makes interpretation and characterization of target much easier (Hachay, 2004, Hachay et al., 2007). The CSEM approach involves the possibility to survey the subsurface in volume block as wide profiling and planshet pickets along profiles. Therefore, the technique is capable to produce an image of the subsurface representing 77 CSEM IMAGING OF THE NEAR SURFACE DYNAMICS AND ITS IMPACT FOR FOUNDATION STABILITY AT QUARTER 27, 15 TH OF MAY CITY, HELWAN, EGYPT the near surface structural setting of the fissures, cracks, fractures, and also the hydrologic situation. Getting enough information about the structural image of the surface including the heterogeneity intensity and the concentration depth of the fractures, cracks, and the devel- opment of their growth, and also the water content will help to provide an objective procedure assures the best manage- ment of the site, therefore, the catastrophic and dramatic ac- cidental results will be minimized. The technique is proposed to be applied on a site with destruction resources that is warning with collapse, the site has been measured in 2008 and repeated in 2010. The com- parative study of the subsoil images obtained in the two cy- cles represents enough information about the subsurface structural changes occurred during the time interval between the two cycles. The study concludes that, the center of weakness zone is moving upwards and became denser as the water saturation became less in 2010 than 2008. That means that the fracture contacts at the subsurface became closer to the surface and became heavier, while the water is moving downwards. The technical impact of this result is the site became more stable but still dangerous and is subject to collapse. 2. Objectives The expected objectives and outcome of the present paper could be declared in the following items: 1) Full understanding of the site of investigation, pro- posed here which is the quarter 27 at 15th of May city, and a more instable site in Al Mokkatam pla- teau. 2) A documentary archive of the situation including the hydrologic setting, the subsurface structure, the geoelectric heterogeneity move of the center, and upgradable conclusions. 3) Vector analysis with locations of the forces applied to the area in reference to the center of heterogene- ity. 4) A monitoring protocol and a management proce- dure for the site of investigation. 5) And finally the main object of the research which is the overcome or at least reduce the catastrophic im- pact via pre read of the destruction resources and probably prediction of the accident. 78 MAGDY A. ATYA, OLGA A. KHACHAY, MAMDOUH M. SOLIMAN, OLEG YU KHACHAY, AHMED B. KHALIL, MAHMOUD GABALLAH, FATHY F. SHAABAN, IBRAHIM A. EL HEMALI Figure 1. Accidental fall down of stone block ad Al Deweaka, 2008. 3. Geological Background and Site Description The city of 15th of May is constructed over a limestone pla- teau on the eastern bank of the River Nile (Fig. 3). This pla- teau is covered mainly with Wadi Garawi and El-Qurn Formations of middle Eocene age. El-Qurn formation is of about120 m thick and is composed of chalky and marly lime- stone intercalated with shale, sandy marls and shale banks (Strougo, 1985; Said, 1990). In addition, these formations are highly jointed, highly fossiliferous, vuggy and sandy dolomitic limestone. Struc- turally, the area is affected by three major sets of faulting systems striking mainly in NW-SE, E-W, and NE-SW direc- tions. All of them are normal faults, with dip ranges from 7u to 40u, where the beds are locally dragged. In addition to faults, there are at least many sets of joints dissect the area (Moustafa et al., 1985). These joints trend in E-W and NW-SE directions (Moustafa et al., 1985; Farag, and Ismail, 1959). The major fractures in the area around Cairo and its vi- cinity were interpreted based on satellite image of large scale (El-Shazly et al., 1980). These fractures were analyzed sta- tistically and each trend of fractures sets was treated sepa- rately. It was found that there are three main trends which are EW, WNW and NW. 4. Theory and Mathematics of CSEM 4.1 Principal The CSEM provides an enhanced geophysical technique and device, in addition to, the precise analytical interpreta- tion of the geophysical complexes for subjects such as the topic of this research, especially when the primary informa- tion about the site is not enough. The device has been estab- lished to use the planshet method of electromagnetic induction (Hachay, 1997a, Hachay and Bodin, 1997b, Hachay et al., 1999, and 2000, Hachay, 2002) in the fre- quency domain. The method was adapted to map and moni- tor the high complicated geological mediums, to determine the structural factors and criteria of stability of the rock mas- sif in the mine subsurface. The field observation and the way of interpretation make the new technology differ from other known earlier methods of field raying or tomography (Atya et al., 2010, Hachay and Novgorodova, 1997c, Hachay et al., 1999, and 2000, Lau and Cheng, 1977). The concept to construct the 3D geoelectric medium is based on interpreting the alternating electromagnetic field in the frame of a block-layered isotropic medium with inclu- sions over three stages (Hachay 1997a, and 2002); in the first stage, the geoelectric parameters of the horizontal block-layered medium, which includes the block heterogeneities, are defined, in the second stage, a geometri- cal model of the different local heterogeneities or groups in- side the block-layered medium is constructed based on the data of local geoelectrical heterogeneities, while in the third stage, the surfaces of the searched heterogeneities could be calculated in account of the physical parameters of the anomalous objects. 4.2 Data collections The CSEM S*data collection is a measure of the geoelectric parameters of a medium, through which the electromagnetic waves propagates from the transmitter to the receiver. At each location of the transmitter and re- 79 CSEM IMAGING OF THE NEAR SURFACE DYNAMICS AND ITS IMPACT FOR FOUNDATION STABILITY AT QUARTER 27, 15 TH OF MAY CITY, HELWAN, EGYPT Divergence joints Figure 2. Constructions showing the joints of divergence at Quarter 27, Fifteenth of May City. ceiver, the three components of the electromagnetic field are measured. Figure (4) explains the layout of the acquisi- tion process. (a) Shows an arbitrary position of the trans- mitter and receiver during a wide profile survey, (b) shows the measurement of the vertical component Z, while the re- ceiver coil is placed vertical, (c) shows the measuring of the first horizontal component H1, while the receiver coil is placed horizontal at right angle with the line between the transmitter and the receiver, and (d) shows the measuring of the second horizontal component, while the receiver coil is placed horizontal inline coaxial with the line between the transmitter and the receiver. The transmitter can inject more than a controlled frequency to scan different depths at the survey area. The measuring procedure could be classified into tow configurations, in correspondence to the objects of the sur- vey. Planshet data collections; is a procedure to collect the electromagnetic response over a 2D plan underneath the line between the transmitter and the receiver. Figure (5) shows the process of gathering the planshet data. In this procedure, the transmitter and receiver are placed on the same line. The transmitter is placed at its first station, as the receiver moves stepwise at pre defined pickets along the profile. Then the transmitter is moved to the next station and the receiver re- peat the stepwise movement along the profile. This process is repeated until the last transmitter station. Wide profile data collection; is a procedure employed to measure the electromagnetic response over a volume, therefore, it may be called volume CSEM. In this procedure, the transmitter is placed on a line, while the receivers are placed on a parallel line. Both of the transmitter and receiver stations on the lines are marked as pickets. The survey vol- ume is margined with the two lines. The data collected on two phases, the first is measures as the transmitter is placed on a line and the receiver is placed on a parallel line (Fig. 6a), while in the second phase, the lines are exchanged and the transmitter is placed on the receiver line and vise versa (Fig. 6b), the combination of the two phases produces the final image of the volume block. 4.3 Interpretation For each array and fixed frequency ù two interpretation pa- rameters are defined: � � � � �eff r r Hz Hr r H Hr( ) ( / ) ( ) ( / ) %� � 2 100 Where ñeff (r) is the apparent resistivity, r is the distance between the source and the point of observation, ù is the fre- quency f multiplied by 2ð, �Hz� is the module of the vertical magnetic component, �Hr� is the horizontal component of magnetic field oriented to the source, ä(r) is the parameter of geoelectrical heterogeneity, and �Hö� is the second horizon- tal component of magnetic field perpendicular to the direc- tion of the source. 80 MAGDY A. ATYA, OLGA A. KHACHAY, MAMDOUH M. SOLIMAN, OLEG YU KHACHAY, AHMED B. KHALIL, MAHMOUD GABALLAH, FATHY F. SHAABAN, IBRAHIM A. EL HEMALI Quarter 30 299.5N 299.5N 299N 299N 0 200m 111.0 112.0 111.9 110.0 110.0 105.0100.7108.0 108.0 120.0 59.2 112.0 103.0 100.0 100.0 Buildings Roads Mediterranean Sea Cairo 15th of May City Nile Delta Quarter 27 3 4 3 E Survey Site 3 4 3 .5 E Su rv ey sit e 3 4 3 .5 E Egypt location map S uez G ulf 3 4 3 E Figure 3: Location map of Quarter 27 at 15 th of May City. That data are the information base for the further inter- pretation. On the first stage we define the geoelectrical pa- rameters of the 1-D section for each array and each frequency after the preliminary filtration of the data: ñeff (r): ä(r)