DOBRICA_final:Layout 6 On the crustal bias of repeat stations in Romania Venera Dobrica1,*, Crisan Demetrescu1, Razvan Greculeasa1, Anca Isac2 1 Institute of Geodynamics, Romanian Academy, Bucharest, Romania 2 Geological Survey, Bucharest, Romania ANNALS OF GEOPHYSICS, 55, 6, 2012; doi: 10.4401/ag-5442 ABSTRACT A magnetic induction model has been applied to recordings obtained in 2010 during the field campaigns for geomagnetic measurements at the 26 repeat stations of the Romanian secular variation network. The model is based on the observation that a variable external magnetic field induces a response of the Earth's interior not only by electromagnetic induction, but also by magnetic induction in the magnetic rocks above the Curie temper- ature. The model computes coefficients of a linear relationship between recorded values of a certain geomagnetic element (X, Y, Z, or F) at the re- peat station and recorded X, Y, Z values at a reference station (in this case, SUA observatory). Coefficients depend on magnetic permeabilities of rocks beneath the station and stand as a proxy for the anomaly bias character- izing the site. Maps of the lateral variation of this type of information were obtained and discussed. 1. Introduction At present the geomagnetic field is monitored both at ground, by means of magnetic observatories and repeat sta- tions, and from space, by means of satellites, leading to com- plex studies and modeling of the geomagnetic field [e.g. Olsen et al. 2007, Finlay et al. 2010, Matzka et al. 2010, Thébault et al. 2010, Mandea and Korte 2011]. At global scale the secular variation is found by processing continuous records of the geomagnetic field at magnetic ob- servatories. Generally, a country of Romania's size has only one observatory. The Romanian one, namely Surlari geomag- netic observatory (IAGA code, SUA), was established in 1942. The information from geomagnetic observatories is supplemented at regional level by repeated measurements in a network called secular variation network. In the last years, as a result of a better understanding of the importance of re- peat station data in learning about the spatial-temporal evo- lution of the geomagnetic field, a project is going on at European level, MagNetE (Magnetic Network of Europe), initiated in 2003 [Korte and Mandea 2003], on systematic measurements in national secular variation networks. As a result of this initiative the map of declination in Europe at the epoch 2006 was published [Duma et al. 2012]. The repeat station data were discussed in several studies [e. g. Korte and Thébault 2007, Thébault 2008] as regards the crustal bias and/or modelling at regional scale. In Romania, since 1964 systematic geomagnetic meas- urements have been done in a network of 26 repeat stations. Reports on measurement results and their interpretation in terms of secular variation and normal field distributions were published by Atanasiu et al. [1965, 1967, 1970, 1974, 1976], Anghel et al. [1980], Demetrescu et al. [1985]. Recently, Demetrescu et al. [2011] have reported measurements of the horizontal component at the repeat stations, in the time in- terval 1980-2004, interpreted according to insights from the analysis of long time series provided by geomagnetic obser- vatories [Demetrescu and Dobrica 2005, 2012]. Beginning with the 2009 campaign, recording fluxgate and proton mag- netometers have been deployed, in order to better control the data correction for diurnal and disturbed variations. In this study we focus on the recordings taken in 2010 in the stations of the repeat network, by means of a LEMI-018 magnetic variometer and of a G-856 Geometrics proton magnetometer, in order to infer information on the lateral variation on the Romanian territory of the crustal bias that characterizes each repeat station. 2. Method Usually, the interpretation of the geomagnetic data from magnetometric arrays is based on the assumption that the external variable magnetic field induces in the conduc- tive structures of subsurface electrical currents that, in turn, produce secondary, detectable magnetic fields. Our method is based on the observation that the variable external mag- netic field induces variable internal magnetic fields not only by electromagnetic induction, but also by magnetic induc- tion in rocks below the Curie temperature. In case of pure magnetic induction, the temporal variation of the field com- ponents at a given observing site is a linear combination of the components of the magnetic inducing force [Demetrescu et al. 1985, 1988], Article history Received October 14, 2011; accepted January 30, 2012. Subject classification: Geomagnetic field, Secular variation, Repeat station network, Romania. 1145 MagNetE workshop 2011 (1) where D represents variations about temporal averages, E(S) is the geomagnetic field component at the station S (E can be X, Y, Z, or F), Fk, k = 1... 3, are the components of the in- ducing magnetic force and are coefficients that depend on the effective magnetic permeability characterizing the site. The calculated values (DE(S)calc) of the model would repre- sent the pure magnetic induction component of the ob- served signal and the residuals (DE(S)res = DE(S) − DE(S)calc) would contain information on electromagnetic induction in the Earth at the observing site. The coefficients , can be de- termined by a standard least squares procedure and be mapped, resulting in images of the lateral distribution of the magnetic properties characterizing the volume of rocks above the Curie temperature (generally the crust). In order to ensure the comparability of the coefficients obtained for the stations, the recordings at all stations should be simultaneous, to meet the requirement that variations recorded at each station sam- ple the same time interval as the external inducing force. Con- sequently, temporal averages mentioned above refer to the common recording time interval. So far, the method has been successfully applied in the case of the Romanian network of repeat stations [Deme- trescu et al. 1985] and the network of European geomagnetic observatories [Demetrescu et al. 1988, Demetrescu and An- dreescu 1992, Demetrescu and Andreescu 1994, Demetrescu and Dobrica 2003] for the case of the solar-sunspot-cycle re- lated variations, as well as in the case of the Hokkaido mag- netometric array [Dobrica et al. 2008/2009] for the diurnal variation. In the present paper we apply this method to vari- ations recorded at the Romanian network of repeat stations in a time span of several hours. Slight changes in the method, discussed in the next two sections, have been introduced due to the particular setting of our measurements. 3. Repeat station and observatory data The input data in the magnetic induction model are the three orthogonal components and total intensity of the ge- omagnetic field observed in the Romanian network of re- peat stations and the components of the inducing magnetic force. In the absence of independent data on the external ge- omagnetic field related to the variations observed (i.e. the field produced by the ionospheric and magnetospheric cur- rent systems responsible for the regular diurnal variation (Sq) and, respectively, for the geomagnetic activity), we took as estimates for the components of the inducing magnetic force, the components of the variation at a nearby geomag- netic observatory (a reference station, as is usually done in the interpretation of data from magnetometer arrays [Gam- ble et al. 1979, Gough and Ingham 1983, Harada et al. 2004]). In this study, the Surlari geomagnetic observatory has been used as the reference station, so the input magnetic inducing force in the model is in fact a resultant of the external field and the induced response of the Earth beneath the observa- tory. As the latitudinal spread of the study territory is rather small in comparison to the lateral scale of the Sq current sys- tem, we did not account for the small possible variation of the external Sq field from station to station. The actual equa- tions expanded from Equation (1) read: DX(S) (t) = axDX SUA (t) + ayDY SUA (t) + azDZ SUA (t) DY(S) (t) = bxDX SUA (t) + byDY SUA (t) + bzDZ SUA (t) (2) DZ(S) (t) = cxDX SUA (t) + cyDY SUA (t) + czDZ SUA (t) DF(S) (t) = gxDX SUA (t) + gyDY SUA (t) + gzDZ SUA (t) The distribution of the 26 repeat stations and SUA ob- servatory over the national territory is presented in Figure 1 and the geographical coordinates are given in Table 1. The measurements were taken in 2010 during four field campaigns: June 17 to July 7, July 30 to August 18, September 14 to 22, and October 1st to 14. At each station the field com- ponents X, Y, and Z were recorded for 7-8 hours, generally between 8-17 standard local time (SLT) (5-14 UT), by means of a LEMI-018 magnetic variometer. We also recorded, by means of a G-856 Geometrics proton magnetometer, the total field intensity, F. One minute data have been used in the subsequent processing. As an example of data appearance we show, in Figure 2, the records in case of three stations, as compared to observatory recordings. Generally low geo- magnetic activity characterized the recording time span, but disturbed days were also present in a few instances. A strong disturbance seen on data in the recordings of Lipova station, on the 4th of August is shown as an example as well. The ge- omagnetic conditions during the measurement campaigns can be seen in Figure 3, where the field evolution recorded by the geomagnetic observatory during the four campaigns is shown together with the evolution of the Dst index. 4. Results and discussion Results for each repeat station consist of the coefficients of Equation (2), the calculated values of the model, and the residuals. In Figure 4 we give, as an example, the calculated field and the residuals of the induction model for X, Y, Z, and F, in case of Cluj repeat station. Figure 5 synthetically illus- trates, by means of the standard deviation about the mean (SD), the variability of initial data, of calculated ones, and of residuals for the 26 stations, in case of each of the four geo- magnetic elements investigated. Generally the calculated val- ues, that is the magnetically induced response of rocks above Curie temperature, explain the most part of the recorded sig- nal in all components. The X record at station no. 24 and the Y record at station no. 18 could not be used due to a malfunction DOBRICA ET AL. 1146 ( ) ( ),E t C F t( )S k E k k 1 3 D D= = / Ck E Ck E 1147 ON THE CRUSTAL BIAS OF REPEAT STATIONS IN ROMANIA No. Station Latitude (degrees) Longitude (degrees) 1 Saveni 47.96593 26.88277 2 Livada 47.84867 23.13362 3 Radauti 47.82142 25.94800 4 Somcuta 47.49997 23.43900 5 Vaida 47.25157 21.98520 6 Bistrita 47.19485 24.48570 7 Varatec 47.15150 26.29175 8 Cluj-Faget 46.69750 23.54715 9 Husi 46.67880 28.00265 10 Chisineu-Cris 46.54443 21.53978 11 Bretcu 46.05767 26.35652 12 Lipova 46.05202 21.72387 13 Deva 45.85712 22.91722 14 Dumbravita 45.83292 21.29002 15 Selimbar 45.75188 24.18742 16 Stamora 45.28383 21.24357 17 Gropeni 45.08573 27.86850 18 Mizil 44.99238 26.37497 19 Herculane 44.92215 22.44942 20 Babadag 44.87207 28.76385 21 Costesti 44.65898 24.89332 22 Strehaia 44.61763 23.16890 23 Tonea 44.20200 27.41502 24 Alexandria 43.96683 25.36662 25 Sadova 43.89513 23.93970 26 Negru-Voda 43.82022 28.24305 27 *Surlari Geomagnetic Observatory (SUA) 44.68000 26.25330 Figure 1. The Romanian repeat stations network and Surlari geomagnetic obser- vatory (SUA). Table 1. Geographical coordinates of the repeat stations of the Romanian network. DOBRICA ET AL. 1148 F ig u re 2 .E xa m pl e of X , Z , a nd F r ec or ds ( fu ll lin e) a t C lu j ( le ft ), V ar at ec ( m id dl e) , a nd L ip ov a (r ig ht ), a s co m pa re d w it h re co rd s at S U A ( br ok en li ne ). 1149 ON THE CRUSTAL BIAS OF REPEAT STATIONS IN ROMANIA Figure 3. The horizontal northward component evolution recorded at SUA during the field campaigns. The Dst index plotted along illustrates the evolution of the geomagnetic activity. DOBRICA ET AL. 1150 Figure 4. Example of modeling (Cluj station): a) the north component (X); b) the east component (Y); c) the vertical component (Z); d) the total field (F). In each plot the recorded data (upper curve), the calculated values of the model (middle curve) and the residuals (lower curve) are shown. Figure 5. Standard deviation about the mean of raw (measured) data, calculated values of the magnetic induction model, and model residuals for the 26 stations of the repeat network (identification numbers as in Table 1). a) X; b) Y; c) Z; d) F. a) a) c) c) d) d) b) b) 1151 of the recorder. The corresponding SD analysis and the coef- ficient values are not displayed in Figure 5 and respectively in Figure 6. The ratio of residuals SD to calculated values SD is, however, larger in case of the vertical component, meaning that the latter is more responsive to electromagnetic induction than the horizontal components, which are more responsive to the magnetic induction. In case of Z, the Lipova station shows the largest residuals SD, of about ± 4 nT, and individ- ual residual values that reach 30 nT, which is related to the ge- omagnetic storm that occurred during measurements. The Z residuals for other stations are of the order of ± 1 nT (18 sta- tions) and of ± 2 nT (7 stations). The accuracy of coefficients varies between 3% and 10% of the coefficient values. In Figure 6 we present maps of the lateral variation of the coefficients we determined in case of the four geomag- netic elements measured. In order to compare coefficients (and consequently magnetic properties of crust) at different stations, the ideal situation would be to have simultaneous measurements at all network stations, to ensure that the same inducing field is applied to the laterally varying mag- netic properties of the study area. Having in view that meas- urements at the repeat stations were not simultaneous and the daily LT variations have different amplitudes in different days, we selected for each station only data in the time in- terval common to all stations, namely 8:24-13:05 UT, to en- sure at least that we sampled the same part of the regular ON THE CRUSTAL BIAS OF REPEAT STATIONS IN ROMANIA Figure 6. The lateral variation of the repeat stations coefficients. From top to bottom: ax, ay, az; bx, by, bz; cx, cy, cz; gx, gy, gz. diurnal variation. The dominant period we sampled this way was the 6-hours harmonic. The higher-frequency UT varia- tions characteristic to the geomagnetic activity were differ- ent, of course, in successive days of the survey, but we believe that due to their much lower amplitude (compare 0.5-3 nT to 40-50 nT; Figure 2) their contribution intervene to a much lesser extent in the actual values of coefficients. Having in view (1) that coefficients represent the lateral variation of magnetic properties by means of relative values and (2) that we use the proxy for the external variation that induces the magnetic response (records at SUA) we checked if the map patterns are stable when the proxy was changed: we compared our results with coefficients that resulted using a second inducing source, namely the recordings of the Niemegk observatory (NGK). In Figure 7 we plotted the val- ues of coefficients at successively numbered repeat stations (Table 1) in case of X and Z, resulted from the magnetic in- duction model that used SUA (full line) and NGK (broken line) as reference stations. The agreement between the two sets of coefficients is good, meaning that maps based on the two sets will show similar patterns. Of course, one has to keep in mind the fact that the coefficient values do not rep- resent absolute permeabilities of the underground, but rather quantities relative to each other. Also, the external source and the internal magnetic and electric structures are different for the two observatories, so at least amplitudes of the recorded signal differ in the two cases. Anyway, this issue needs more investigation than that undertaken so far. For the moment, a second type of experiment was not possible, namely checking if the magnetic induction model gives sim- ilar results when applied to other time intervals. This would be a matter for future studies. DOBRICA ET AL. 1152 Figure 7. Comparison of the induction model coefficients ax and cz using as proxy for the inducing force SUA (full line) and NGK (broken line). 1153 Each repeat station was carefully selected when the net- work was established [Atanasiu et al. 1970], from about 500 stations of the regional geomagnetic survey with 'absolute' measurements done in the 1960s (a station per 400 km2), in order not to reflect regional and local anomalies. The maps reveal, however, some regional pattern, with smaller values in NW, S, and E, and higher values in the central and south- western part. The lower values might be associated with structure peculiarities of the Pannonian Depression, and Moesian and East-European Platforms, and the higher ones to peculiarities of the eastern and southern Carpathians. As the mapped coefficients reflect magnetic properties of rock volumes down to the Curie temperature surface, the shape and thickness of the lower crust might also contribute to the observed pattern. A quantitative interpretation would be a matter for future research. Of course, a much denser net- work would be necessary to detect smaller scale structures. 5. Conclusion X, Y, Z, and F data recorded in 2010 at the Romanian re- peat stations network were used to infer information on the lateral variation of the crustal bias that characterize each sta- tion. A magnetic induction model based on the observation that a variable external magnetic field induces a response of the Earth's interior not only by electromagnetic induction, but also by magnetic induction in the magnetic rocks above the Curie temperature, was applied. The calculated values of the model represent the pure magnetic induction com- ponent of the observed signal and the residual would con- tain information on electromagnetic induction in the Earth at the observing site. The coefficients of the linear model relating the meas- ured values to component of the inducing external field were mapped. The resulting patterns show regional distribution of values that correlate with large scale crustal compart- ments in the study area, but no quantitative interpretation has been undertaken as yet. The methodology and the information acquired in this stage of research would contribute to a better correction and interpretation of geomagnetic measurements at repeat stations. Acknowledgements. The study has been supported by the Institute of Geodynamics (Project 2/2011) and by the Ministry of Education and Re- search (PN-II UEFISCDI IDEI 0262/2011). We thank two anonymous re- viewers for constructive comments that helped improving the manuscript. 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