Array EM Studies Chairpersons: M. Engels and A.Viljanen 7.1 PROCESSING AND INTERPRETATION OF ELECTROMAGNETIC INDUCTION ARRAY DATA: A REVIEW (INVITED REVIEW PAPER) Gary D. Egbert College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503, USA egbert@oce.orst.edu 7.2 DC TRAINS AND PC3S: SOURCE EFFECTS IN MID-LATITUDE GEOMAGNETIC TRANSFER FUNCTIONS G.D. Egbert(1), M.E. Eisel(2), O.S. Boyd(3), and H.F. Morrison(3) (1) College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503, USA (2) GeoForschungsZentrum Potsdam, Germany (3) Engineering Geoscience, University of California, Berkeley, CA, USA egbert@oce.orst.edu Magnetotelluric (MT) data from two sites 150 and 300 km southeast ofSan Francisco, California (geomagnetic dipole latitude: 43 degrees, L approximately 1.9) show that the usual MT assumption of spatially uniform external magnetic fields is violated to a significant degree in the period range 10-30 s. Inter-station transfer functions exhibit large systematic temporal variations which are consistent with a combination of two distinct sources: electromagnetic noise due to the San Francisco Bay Area Rapid Transit (BART) DC electric railway, and Pc3 geomagnetic pulsations. There is a suggestion in the data that some of the Pc activity may actually be excited by BART. 7.3 3-D INTERPRETATION OF THE ARRAY MAGNETOTELLURIC DATA Michael Zhdanov(1), Nikolay Golubev(1), Sheng Fang(2), Koichi Matsuo(3), and Tateyuki Negi(4) (1) University of Utah, USA (2) Baker Atlas, USA (3) Nittetsu Mining Co, Japan (4) Japan National Oil Corporation, Japan mzhdanov@mines.utah.edu 3-D electromagnetic inversion continues to be a challenging problem in electrical exploration. We have developed recently a new approach to the solution of this problem based on quasi-linear, QL, approximation of forward modeling operator. It generates a linear equation with respect to the modified conductivity tensor, which is proportional to the reflectivity tensor and the complex anomalous conductivity. We solve this linear equation by using regularized conjugate gradient method. After determining a modified conductivity tensor we use the electrical reflectivity tensor to evaluate the anomalous conductivity. Thus, the developed inversion scheme reduces the original nonlinear inverse problem to a set of linear inverse problems. The developed algorithm has been applied to the array magnetotelluric data. The case histories include interpretation of the array MT survey, conducted by the New Energy and Industrial Technology Development Organization in the Minamikayabe area located in the southern part of Hokkaido, Japan, and 3-D inversion of the array MT data, collected by Japan National Oil Corporation for hydrocarbon exploration. 7.4 INTEGRATED MODELING OF EM RESPONSE FUNCTIONS FROM PENINSULAR INDIA AND BAY OF BENGAL Baldev R. Arora and P.B.V. Subba Rao Indian Institute of Geomagnetism, Colaba, Mumbai 400 005, India bra@iig.iigm.res.in Existing sets of magnetovariational data from large number of sites spread widely across the Peninsular India and those collected through Ocean Bottom Magnetometer (OBM) surveys in the adjoining Bay of Bengal are revisited to obtain inter-site vertical and horizontal field transfer functions using the multiple reference site technique. The vertical field response functions at sites, bordering the coastline, have been shown earlier to be controlled by current concentrations in the multiple conductors related to offshore geology. In contrast, the vertical field variations with periods up to several tens of minutes are highly attenuated at all OBM sites. The regional difference in the vertical field response across the coastline is modeled in terms of varied conducting layered configuration beneath the continent and sea. Also, observed induction response functions, corrected for the deep electrical lithospheric inequalities across coastline, are modeled as a thin- sheet of laterally variable conductance. The inferred conductivity distribution, besides providing better insight on the off-shore conductive structures, also brings out evidence on the number of land anomalies that were overlooked on the maps of observed response functions due the dominating influence of off-shore structures. The tectonic significance of the land conductive anomalies is emphasized in terms of the surface geology and metamorphic history of the Peninsular India. 7.5 BALTIC ELECTROMAGNETIC ARRAY RESEARCH: AN OVERVIEW Toivo Korja and BEAR Working Group Geological Survey of Finland, Espoo, Finland toivo.korja@gsf.fi The Baltic Electromagnetic Array Research -BEAR- project uses data from a shield wide magnetotelluric and magnetometer array. The project aims at resolving deep conductivity taking into account distortions caused by polar magnetotelluric sources and influence of crustal conductors, and to integrate conductivity models with other Earth model, e.g. thermal, seismic tomography, xenolith, available from the EUROPROBE SVEKALAPKO project to determine the thickness of lithosphere, properties of asthenosphere, disposition of major lithospheric structures, and evolution of the Baltic Shield. On June-July 1998 45 days long magnetic and electric time series were simultaneously acquired from 46 magnetotelluric and 20 magnetic sites. Processing has resulted in a full set of stable electromagnetic transfer functions covering periods from 10 s to several hours. Magnetotelluric impedances from robust processing apparently show very small polar source distortions up to periods of 3-6 hours or even 12 hours whereas tipper estimates show contamination already at periods of c. 2-3 hours. Special tools are being developed to investigate source field distortions at very long periods. Transfer functions and preliminary 1D inversions indicate the existence of crustal and upper mantle conducting features. Decomposition analysis, however, indicate strong 3D effects and a full 3D inversion is therefore required to extract reliable conductivity structure. This paper aims to be a general overview of the current status of the BEAR project while the results from various of lines of investigations, e.g. time series processing, distortions due to non- uniform source field and crustal conductors, are given in separate contributions. 7.6 DATA PROCESSING TECHNIQUES FOR THE ARRAY ELECTROMAGNETIC SOUNDINGS Ivan Varentsov, Elena Sokolova, and BEAR Working Group Geoelectromagnetic Research Institute RAS, 142090, P.O.B. 30, Troitsk, Russia igemi1@pop.transit.ru The discussion of various array specific aspects of MT and GDS data processing algorithms, codes and their application graphs is presented, basing on our recent experience in the BEAR project. We concentrate on the proper balance of the traditional local approaches and the multi-site simultaneous data processing schemes, and on the strategy of the accuracy control for the estimated transfer functions. The multi-window robust processing technique (Varentsov et al., 1997) in the single-site (SS) and remote reference (RR) modifications is formulated, and a special "multi-remote" scheme is suggested, basing on the robust stacking of RR transfer function (TF) estimates for a number of different remote sites. The sequence of tests is constructed to verify the stability of TF responses in respect to different time and frequency domain disturbing effects. This sequence includes the application of different preprocessing schemes (conventional and first difference), the comparison of single-record and multi-record stacking estimates, the "multi-remote" test in the case of RR processing, and the "multi-base" test for the estimation of simultaneous TF responses (geomagnetic and telluric). The excessive sequence of quasi-independent TF estimates gives the valuable resources to create smooth and reliable final responses and to give reasonable error bars resulted from the "jack-knife" analysis. The effectiveness of the developed processing techniques is illustrated on the material from the BEAR project. More results and specific details are given in the related posters (Sokolova, Varentsov; Sokolova et al.; Martanus et al.), presented at this session. 7.7 INTERPRETATION OF SOURCE EFFECT-DISTORTED BEAR-DATA Kerstin Roden and BEAR Working Group Institute of Geophysics and Meteorology, Technical University Braunschweig, Mendelssohnstr. 2-3, D- 38106 Braunschweig, Germany K.Roden@tu-bs.de Source-effect distorted electromagnetic data can be treated, using an integral method, developed by Prof. U. Schmucker. The electric field is connected with the magnetic field via a convolution integral in space-domain. The integral kernel depends on the conductivity structure. The method is particularly suitable for the treatment of non-uniform source fields. Application of the integral method will first be shown on theoretical data, from which the limits of the method and the conditions on real data can be derived. Several conductivity structures with different source fields will be shown and the resolvability of the conductivity parameters will be studied. The inversion will be done by global optimization via Simulated Annealing. A perfect data set for this methods represents the BEAR data set, where 1998 an array of about 50 MT-station measured six weeks simultaneously the electromagnetic field on the Baltic Shield. Special inhomogeneous events will be selected and interpreted. 7.8 MAGNETIC FIELD SEPARATION: IMPORTANCE OF THE INTERNAL PART IN IONOSPHERIC STUDIES Ari Viljanen, Eija I. Kallio, Tuija I. Pulkkinen, Antti Pulkkinen, Lasse Hakkinen, Risto Pirjola, Olaf Amm, and BEAR Working Group Finnish Meteorological Institute, Geophysical Research Division, P.O.B. 503, FIN-00101 Helsinki, Finland ari.viljanen@fmi.fi The ground magnetic field is used as a standard measure of ionospheric and magnetospheric currents. In the normal practice the total field is assumed to be purely of external origin. We will consider this assumption in connection to magnetospheric substorms, and specifically concerning the local AL index, defined as the minimum of Bx in a given region. The AL index is used to estimate the Joule heating power in the ionosphere. We show by using the BEAR array and the long chain of IMAGE magnetometers in Fennoscandia and Svalbard that the internal contribution to AL can be remarkable especially at the onset time of a substorm. Consequently, the quantitative use of AL requires some care at certain situations. 7.9p MODELLING OF STRONG 3D EFFECTS IN MT DATA FROM NAMIBIA Ute Weckmann, Oliver Ritter, and Volker Haak GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany uweck@gfz-potsdam.de Within the framework of two field campaigns in Namibia we recorded MT and GDS data in a broad frequency range between 0.001 s and 1000 s. In this paper we concentrate on data from 61 sites which were recorded in 1999 covering an area of approximately 20 x 20 km. Most of the sites are aligned along two parallel, 18 km long profiles with a dense site spacing of 500 m and 2 km, respectively. The two profiles are extended to the east and west with an additional set of 20 sites. They cross the Waterberg fault- Omaruru lineament, a major tectonic boundary that correlates with a zone of high conductivity discovered by the regional MT study of 1998. The MT data are generally of excellent quality since there are no sources of cultural noise in this region. However, some of the sites are strongly affected by 3D effects as we observe high skews, phase values leaving the quadrant and a strong coherency between Ex and Bx for periods longer than 10 s. Only sites within a restricted area in the vicinity of the shear zone show this behaviour. The dense site spacing is essential for a profound investigation of these 3D effects. We present MT and GDS results, test several tensor decomposition methods and show first 3D models. At least some of the observed effects can be explained by shallow, conductive bodies embedded in a resistive environment. 7.10p ANOMALOUS GEOMAGNETIC VARIATION IN THE SOUTHERN CENTRAL ANDES Wolfgang Soyer Freie Universitaet Berlin, Fachrichtung Geophysik, Malteserstrasse 74-100, D-12249 Berlin, Germany lobo@geophysik.fu-berlin.de Whithin the framework of the special research programme 'Deformation processes in the Andes' founded by the german research community, four great field campaigns have been carried out yielding long period electromagnetic data from about 80 field sites in the southern Central Andes, mostly aligned on two profiles, one of them following the great ANCORP seismic reflection line at 21°S, which reaches from the Chilenean Pacific coast via the Bolivian Altiplano to the Eastern Cordillera. In the present work, intersite geomagnetic transfer functions have been calculated, in a way that all magnetic channels from almost all sites from the four campaigns can be referred to two channels of any desired reference site. For the processing of the data Gary Egberts program 'multmtrn' has been used to compute spectral density matrices within small arrays of four to six simultanously running stations. From the eigenvectors corresponding to the two dominant eigenvalues of these SDMs one can easily obtain transfer functions between arbitrary channels within these arrays. To join non- simultanous arrays e.g. from different field campaigns in a straight- forward way, spatial overlaps of arrays are mandatory. The execution of the campaigns - several previous telluricly bad stations had been rebuilt to record better data - outlines a very specific joining-scheme for the sub-arrays to construct one final transfer function array. 7.11p MAGNETIC FIELD SEPARATION: USE OF A TWO-DIMENSIONAL MAGNETOMETER ARRAY WITH SOME IMPROVEMENTS OF THE FIELD INTERPOLATION Ari Viljanen, Olaf Amm, Antti Pulkkinen, and BEAR Working Group Finnish Meteorological Institute, Geophysical Research Division, P.O.B. 503, FIN-00101 Helsinki, Finland ari.viljanen@fmi.fi We investigate the internal part of geomagnetic variations in Fennoscandia to provide information about large-scale conductivity anomalies in the earth. We use magnetometer recordings of the BEAR array whose two-dimensionality allows to perform the separation without any assumptions about the spatial behaviour of the field. We also present some improvements of the field interpolation by applying a recently developed concept of elementary current systems. 7.12p POLAR CUP CURRENTS AS A POSSIBLE SOURCE FOR MT SOUNDINGS IN BEAR PROJECT L.L. Vanyan, N.A. Palshin, and BEAR Working Group Shirshov Institute of Oceanology, Moscow, Russia vanyan@geo.sio.rssi.ru palshin@geo.sio.rssi.ru EM data obtained in the framework BEAR give us a unique possibility to study the source effect in more detail then it was achievable before. Transfer functions were estimated accurately for the periods up to 12-24 hours at several tens of sites at latitudes from 60N to 67N. The preliminary results of BEAR experiment give an impression that our traditional approaches and estimations could not adequately describe source effect manifestations. MT responses for the periods greater than 10000 sec are not influenced by the source effect at first sight, while the pronounced changes in induction vectors take place at the same period. One can expect that we face another type of the source, which is not of auroral electrojet nature. We proposed that polar cap current systems could be responsible for a plane-wave response at long- period band at high latitudes. Polar cap currents flow through the geomagnetic pole in sunward direction and produce two broad streams of return currents in geomagnetic latitudes lower than 75. Intensity and geometry of current systems strongly depend on interplanetary magnetic field. Two-step numerical procedure was applied to estimate transfer functions. Firstly, EM field of arbitrary polar cap currents is simulated over 1D model. At the second step impedance tensor is calculated using inhomogeneous EM field obtained at the first step as a "normal field" at the surface. This work is supported by INTAS Project 97-1162. 7.13p INVESTIGATION OF THE SOURCE INFLUENCE ON THE RESULTS OF MAGNETOTELLURIC DATA ROBUST PROCESSING FOR THE PERIOD RANGE OF GEOMAGNETIC PULSATIONS FROM THE BEAR DATA Maxim Smirnov(1), Valeriy Ismagilov(2), and BEAR Working Group (1) Institute of Physics, St.Petersburg University, Ulyanovskaya 1, Petrodvorets, 198904, St.Petersburg, Russia (2) St.Petersburg filial of IZMIRAN, Muchnoy per. 2, 191023, p.b.188, St.Petersburg, Russia msmirnov@pcland3.phys.spbu.ru There was investigated the influence of the source field on the results of magnetotelluric measurements in the period range of geomagnetic pulsations using 2D magnetotelluric BEAR array. One of the problem that might aroused for MT soundings at the high latitudes is source influence on the results of magnetotelluric transfer functions estimations. This may cause not only increasing the instability of the impedance tensor determination but the uncontrolled estimation bias as well. The most part of BEAR array sites placed near to auroral zone and, hence, the source effects might produce significant MT transfer functions distortions. There were processed separately day and night events, disturbed and quite days. The larger instability for all such events processing was observed in opposite to the whole data set processing, that due first of all to the less statistics. So the possible source effects were under the confidence limits of the estimations. Besides there are another reasons that also could produce the differences between the events processing such as industrial noises, could be day, night depended, possible spectral estimations distortions due to the large difference in signal properties for disturbed and quite days. In this study we have been trying to coincide the MT transfer functions disturbances, bias from the results of the whole data processing results, depended on the sources positions. For the selected days there were constructed sources positions above BEAR 2D-array. There was determined the sources distribution averaged for some time. Here we used the averaging from 10 min up to 2 days for the period range of 30-1000 sec. For the same moment of time the MT transfer functions were calculated and their bias for each site. Then the sources distribution above the area of measurements and the distribution of the bias, as averaged deviation among whole period range as well as for some periods, of MT curves, for both amplitudes and phases of main and additional impedances, for the same area as a surface plots were compared. The results of this comparison are presented. 7.14p ANALYSIS AND DECOMPOSITION OF THE BEAR ARRAY IMPEDANCE TENSORS Ilkka Lahti(1), Toivo Korja(2), Maxim Smirnov(3), and BEAR Working Group (1) Department of Geophysics, University of Oulu, P.O. Box 3000, FIN-90014 University of Oulu, Finland (2) Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland (3) Physics of the Earth Department, Institute of Physics, St.Petersburg State University, Ulyanovskaya 1 , St.Petersburg 198904, Russia ilkka.lahti@oulu.fi The Baltic Electromagnetic Array Research (BEAR) project uses a long period (T over 10 s) magnetotelluric and magnetometer array data to determine the electrical conductivity of the upper mantle beneath the Fennoscandian Shield. Crustal conductors can produce e.g. galvanic distortions to regional electric field and consequently to measured impedance tensors. Thus both the amplitude and the phase of the MT tensor data may be distorted. In order to obtain information on distortions and apparent dimensions of the geoelectric structures, the BEAR MT data are analysed using different dimensionality and strike indicators (Swift, 1967, Bahr, 1991). Results indicate, in general, regional 3D or 2D behaviour of the MT-transfer functions but at some sites local distortions are identified. Groom and Bailey (1989) decomposition and its McNeice and Jones (1996) extension are applied to data to remove local distortions and to obtain regional tensors. As a main result, we present recovered regional MT parameters of the BEAR data as well as the apparent dimensions of the deep geoelectric structure. 7.15p COMPLETE SET OF TRANSFER FUNCTIONS FOR THE BALTIC ARRAY ELECTROMAGNETIC RESEARCH (BEAR) Elena Sokolova, Ivan Varentsov, Elena Martanus, Kira Nalivaiko, and BEAR Working Group Geoelectromagnetic Research Institute RAS, 142090, P.O.B. 30, Troitsk, Russia igemi1@pop.transit.ru We present the complete system of transfer functions (TF) for the BEAR array (impedance, tipper and two-site magnetic operators) in the range from 8 s to Sq periods at 46 MTS and 20 GDS simultaneous observation sites. Our processing codes (Varentsov et al., 1997, 2000) were preliminary verified using the COMDAT synthetic time series. The robust single-site (SS), two-site and remote reference (RR) analysis in all sites was done with the extended accuracy control, including the comparison of results for different preprocessing schemes, various spectral windows, single- and multi-record stacking selections, and providing sufficient resources to construct accurate and smooth final estimates with the reasonable error bars. The polar source distortions were favorably reduced (Sokolova, Varentsov, 2000). The RR impedance and tipper estimates at each site were obtained for a number of reference sites located at different distances and azimuths, giving the amazingly close fit and the valuable quality improvement in some period intervals comparing with the SS estimates. The perfect fit of our impedance and tipper operators was observed in the most of sites with the same responses from BEAR teams in St. Petersburg and Oulu Universities. The system of simultaneous magnetic field operators (both horizontal and vertical) including 66 observation sites is unique in the BEAR data processing. This estimation was done independently for two common reference sites (B22 and B27), providing an extra error control from the comparison of direct and transitive estimates for each pair of sites. The evident signatures of conductivity anomalies at the crustal and upper mantle depths are traced in the estimated transfer functions. However, these effects have a strong 3D structure in the most of sites, and a full 3D inversion of the whole set of array TF responses should follow. 7.16p INVESTIGATION AND ELIMINATION OF THE POLAR SOURCE DISTORTIONS IN THE BEAR PROJECT TRANSFER FUNCTIONS Elena Sokolova, Ivan Varentsov, and BEAR Working Group Geoelectromagnetic Research Institute RAS, 142090, P.O.B. 30, Troitsk, Russia igemi3@pop.transit.ru The deep array EM soundings of the BEAR project took place in the high latitude area approaching the polar circle for MT sites and going far north for GDS observations. It was natural to expect there the significant disturbing source effects, but our robust processing codes brought the surprising temporal stability of different transfer functions (TF) and no significant evidence for the polar source distortions at the whole array. To understand this situation we present the detailed study of TF temporal changes in relation with variations in the geomagnetic activity and the source field inhomogeneity. The sequence of the partial single-extent TF estimates obtained for 3 different time windows, namely 4.5, 11 and 73 hours, was inspected in the site B11, located just at the polar circle. This monitoring of single site and two-station TF estimates (impedance, tipper and horizontal magnetic tensor) had shown significant distortions in the whole period range, being the most common at night hours and appearing during the time intervals of increased source field activity and inhomogeneity. Hopefully, such distorted time intervals are reliably characterized by the decreased quality of the estimated linear relations and are obviously seen in the coherencies of partial TF estimates. This fact explains how our robust processing schemes applying mutual and input coherency criteria to reject or downweight the unfavorable data extents may be so effective in the elimination of the polar source distortions. The undistorted character of all TFs at the site B11 may be traced homogeneously up to 3-hour period (even up to 12 hours for the impedance tensor). We observe a very good fit between the TF estimates stacked for the whole amount of data and the corresponding single-record results. The favorable elimination of the source distortions is also well seen in the remote reference estimates compared for a number of different remote sites. 7.17p MORPHOLOGY OF GEOMAGNETIC TRANSFER OPERATORS IN THE POLAR EUROPE (BEAR EXPERIMENT) Elena Martanus, Elena Sokolova, Ivan Varentsov, Kira Nalivaiko, and BEAR Working Group Geoelectromagnetic Research Institute RAS, 142090, P.O.B. 30, Troitsk, Russia igemi3@pop.transit.ru The spatial and frequency domain morphology of magnetic field transfer functions (TF) is discussed in specific details for the northern part of the BEAR array from Polar Scandinavia to Spitzbergen islands. The tipper data Wz and simultaneous magnetic (both horizontal M and vertical Sz) responses are considered. We concentrate on the problems of long-distance geomagnetic field linear relations, polar source distortions and coast effects in GDS responses. The simultaneous TF responses were created in the following scheme: the direct estimation for two reference (base) sites (B22 and B27) at the continent, and the use of an extra reference A13 for sites on the polar islands with further transitive recalculations using A13/B22 and A13/B27 responses. The better quality was generally achieved for the B22 reference. The maps of M-tensor components and invariants, as well as Wz and Sz arrow diagrams are presented for the area of interest. The obvious anomalies in these GDS data are discussed. The difference in Wz and Sz anomalies is specially studied. We further project a number of sites in the central column of the BEAR array on the submeridional profile, and study the variation in different GDS data from south to north across the interval of the most probable electrojet locations. For the completeness we extended this profile to the south till the Estonian BEAR sites. Finally, we apply the approach to trace temporal variations in partial (single- extent) GDS responses (Sokolova, Varentsov, 2000) at the Spitzbergen site A13, and estimate in this way the level of source distortions and the quality of their reduction in the data processing. Another control of source distortions comes from few dual (direct and transitive via A13) simultaneous responses at the islands. The conclusion is made, that the studied magnetic transfer functions in this polar area at the periods below 3 hours are estimated with the proper accuracy and dominantly reflects the geoelectric structure, but not the source. 7.18p MULTISHEET MODELING OF THE FENNOSCANDIAN SHIELD: SOURCE EFFECT STUDIES Martin Engels(1), Olaf Amm(2), Antti Pulkkinen(2), Ari Viljanen(2), and BEAR Working Group (1) Uppsala University, Department of Geophysics, Villavaegen 16, SE-752 36 Uppsala, Sweden (2) Finnish Meteorological Institute, Finland me@geofys.uu.se During the BEAR-experiment (Baltic Electromagnetic Array Research) in summer 1998, an array of about 50 magnetotelluric and 20 magnetic stations was operated on the Fennoscandian Shield. The unique database consists of 6 weeks simultaneous recordings with a sampling rate of 2-sec (typical site spacing of 150 km). Long period array processing and modeling is aimed to resolve the deep conductivity structure of the lithosphere- asthenosphere system of this ancient shield. Two major challenges have to be tackled: Distortion effects i) due to prominent crustal conductivity anomalies and ii) due to inhomogeneous source fields of the near polar current systems. This poster is to study source effects of realistic source field current systems in presence of crustal conductors by forward modeling using the powerful multisheet AKP-code (by Avdeev, Kuvshinov, Pankratov). The Earth model is based on a priori conductivity information from former field campaigns (compiled by Korja and BEAR working group): It consists of a 1-D reference structure and three embedded sheets with lateral inhomogeneous conductances (model cell size of 15x15km). A surface sheet is representing the upper crustal conductances including crystalline basement, anomalies, seawater and sediments. Additional deeper sheets are introduced to represent the middle and lower crustal conductors, even an asthenospheric inhomogeneous layer may be added optionally. The source model consists of realistic equivalent current systems derived from the BEAR database by the co-authors from the FMI. Source effects of these inhomogeneous sources (as well as prototypes of typical source field structures) are studied in contrast to quasi-homogeneous sources. Electromagnetic responses by multisheet modeling are compared with results from data processing. These studies for selected data events are to estimate and understand the superimposed distortion effects of crustal conductors and realistic inhomogeneous sources on BEAR data.