A computational method for automatic 3D MT modelling is described. Making use of a recent publicly available forward algorithm, our method allows unattended search for a 3D conductivity model. The geometry of the conductivity features is described by a set of mathematical functions of the horizontal coordinates x and y and of a fixed number of parameters. Starting from a presumed conductivity distribution, our scheme automatically varies the parameters in a steepest descent control loop, until the misfit between the model response and the measured data reaches an allowable value. To illustrate the method, we apply it to MT and induction data gathered in the Swiss Alps and determine the depth and lateral extension of a highly conductive, graphite-bearing layer.

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Using the staggered grid full domain 3D modeling schemes of various accuracies have been developed. This study focuses on the second order finite difference method with the 13-point rule for meshes extending into the air. Tests with Krylov space iterative solvers indicate that the restarted Bi-CG Stablised method offers the best convergence for our problems. Because the air and the conductive earth have distinctive physical properties which greatly broaden the spectra of the whole matrix system, the whole mesh with both domains in one system either converges very slowly or fails to converge completely. However, the matrix systems for each domain have much smaller condition numbers. To overcome instability caused by the inclusion of the air in the mesh a domain decomposition method are experimented. Tests show that the adaptive iteration amongst the subdomains converges exponentially, which implies that large models can be solved by using the domain decomposition method.

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For models having moderate departures from the basic distortion model (from the so called “principal superimposition” model, that is where local three-dimensional and regional two-dimensional structures are superimposed) a special tensor decomposition method (the so called phase deviation method) was suggested by Bahr (1991). As far as we know, this technique has never got a wide field application in the field. In a careful examination of the suggested solution, an error was observed in the original derivation of the formulas. In this paper Bahr's (1991) solution is corrected. Using the new equations, more understandable and interpretable results are obtained, as it is illustrated on synthetic examples.

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We carried out sixty-four MT and GDS soundings in the eastern Swiss Alps to obtain information on the electrical conductivity distribution. One of the main findings is an anomalous directional behaviour of the real induction arrows over the entire period range (T = 1-300 s) on the Bündnerschiefer. The Bündnerschiefer occurs between the crystalline Aar and Gotthard massives to the North and the crystalline Penninic nappes (Adula, Tambo, Suretta) to the South within the investigation area. The sediments form an elongated eastwards plunging ramp with a possible conductive link to the Northern Swiss Permo-carboniferous trough (Molasse basin). We consider electric currents induced in various local and regional conductive structures and leaking into the Bündnerschiefer as possible causes for the observed effect upon the electromagnetic field. Applying Hypothetical Event Analysis (HEA) we find a spatial decoupling of the induction processes with depth and a conductivity anomaly presumably due to the northward indentation by the Adriatic plate.

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Wide-band (0.002-20, 000 Hz) magnetotelluric measurements (MT) observations have been conducted along three traverses in the central Tohoku district of the northeastern Japan arc at 86 observation sites since 1990 in order to image the electrical resistivity structure. We used the impedance tensors fully corrected for the three-dimensional galvanic distortion effects including static shift effects in order to refine previous 2-D models (discussed in Fujinawa et al., 1997). The subsurface 3-D effects are found to be generally small with the result of a slight difference between the present model and the previously reported one. The modeling results indicate that the crust is homogeneous without an enhanced conductivity zone in the lower crust, in general agreement with results in the northern part of the Tohoku district. The refined resistivity profiles delineate more clearly two near-surface conductive anomalies located in the fracture zone between the Dewa Hill and the Central Basin Range, and in the zone between the Kitakami and Abukuma River regions. Conductors in the crust west of the Sekiryo Mountain Range generally correlate well with mapped faults or pre-Tertiary tectonic lines. Several buried faults are also suggested from the conductivity data. The electrical resistivity distribution and known active faults are integrated to better understand the seismo-tectonics and geologic regime associated with the subduction processes in the Japan arc region.

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A reconnaissance MT survey was conducted in the southern portion of the Sao Francisco Sedimentary Basin, Brazil, for mapping the subsurface structure. The objective was to provide a regional model and in helping to select areas to be further surveyed with seismics. The data collected at seven EMAP-style spreads of 500 m, with an average spacing of 30 km. The field procedure allowed for the correction of statics and the production of 7 inverted models, smooth representations of the regional structure as seen below each site. A 2-D forward model tested and refined the regional structure revealed by the inversions and yielded an earth composed of 4 main geologic units. A 100 ohm.m limestone layer on the top of 20 ohm.m shales, followed by a 1000-200 ohm.m structured basement. Crustal resistivities do not change until reaching depths of more than 30 km. The model ends at a conductive half-space of 30 ohm.m. A suture zone between an orogenic belt and the Sao Francisco craton was modeled below the sedimentary section bringing its know limits 60 km to the east.

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The use of Mohr circles into magnetotelluric (MT) intepretation was introduced by Lilley (Lilley, 1976). By plotting Z_xx-rotated versus Z_xy-rotated (real and imaginary parts) important information on the conductivity structure is obtained. In this paper Mohr circles are employed to reveal the directions of polarisation of the electric field. Furthermore, the following procedure for Mohr circle analysis is suggested: plotting Z_xx-rotated versus Z_yx-rotated (instead of Z_xy-rotated) the principal axes system i.e., the strike angle, of the regional 2D structure can be resolved in some cases. The latter analysis is implemented to MT data from Ioannina area, in NW Greece. Results from other intepretation techniques applied to the same dataset, such as decomposition methods, tipper and induction arrows analysis, provide support for the validity of the conclusions reached.

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The method proposed is based on the Arrenius' equation, that links electrical conductivity of solids with an enthalpy of activation, is studied and well-known distributions of temperature, pressure and electrical conductivity in the mantle. Formulas for the quantitative estimation of parameters, that characterize energy state of a lattice of the mantle, are received. Dependence of electrical conductivity on activation energy of free electron or ion, mobility of which has a power dependence on temperature, and of polaron with an exponential dependence on temperature was studied. The analysis of results has reveled that an electrical conductivity in the middle and in the lower mantle is realized by a change of activation energy of a polaron.

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Initially, two 2D models are presented in order to emphasize the possible placement of the Trans-European Suture in the rooted zone of the Eastern Carpathians' Flysch Nappes System. Then, a conclusive review of the distribution of the main geoelectrical properties on this area is accomplished, pointing out the accuracy of the image reflecting the two types of contrasting basement, without neglecting the complex aspect related to the Carpathian Arc Bend. By correlating the two maps, at the lower crust and basement levels, elaborated by electromagnetic data, new information concerning the particularities of this distinctive tectonic element are brought to light. In addition to it, the two extreme top limits on the map of the lower crust (brittle/ductile transition zone) are revealed-the highest one (30 km) eastwards of the Pannonian Basin, and the deepest one (about 50 km) in the Vrancea area.

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There exists a number of models that account for the deep structure of the Sayan-Baikal province on the basis of different geophysical data. The models based on magnetotelluric soundings differ from those obtained from seismology with no regard to magnetotelluric soundings (MTS) results. To decide between the active and passive mechanisms of Baikal rifting, we propose to correlate the modeling results to the available seismological and petrological data, as well as to the model of asthenospheric diapirism put forward by Zorin et al. (1989, 1990, 1995). Inasmuch as the reality of MTS-based models depends on removal of static-shift effects, the reported threedimensional modeling of the Baikal rift deep structure was preceded by normalization of the original MT curves by the global magnetovariation curve. The enhanced electrical conductivities of the crust and upper mantle of the region is interpreted in terms of geodynamics where the Cenozoic activity is suggested to be driven by convective mantle flow and triggered by the associated plate collisions. The origin and evolution of the Baikal rift can be explained in terms of reactivation of a pre-existing lithospheric inhomogeneity between the Siberian craton and the Sayan-Baikal fold area.

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The interpretation of actual geophysical field data still has a problem for obtaining a unique solution. In order to investigate the groundwater potentials in Saqqara archaeological area, vertical electrical soundings with Schlumberger array have been carried out. In the interpretation of VES data, 1D resistivity inversion has been performed based on a horizontally layered earth model by El-Qady (1995). However, some results of 1D inversion are not fully satisfied for actual 3D structures such as archaeological tombs. Therefore, we have carried out 2D inversion based on ABIC least squares method for Schlumberger VES data obtained in Saqqara area. Although the results of 2D cross sections were correlated with the previous interpretation, the 2D inversion still shows a rough spatial resistivity distribution, which is the abrupt change in resistivity between two neighboring blocks of the computed region. It is concluded that 3D interpretation is recommended for visualizing ground water distribution with depth in the Saqqara area.

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Diagrams from electromagnetic data that were recorded at Muntele Rosu Observatory during the periods from December 1996 to January 1997, and December 1997 to September 1998 were plotted. The times when Vrancea earthquakes of magnitudes M ≥ 3.9 occurred within these periods are marked on the diagrams. The magnetic data prove that Vrancea earthquakes are preceded by magnetic perturbations that may be regarded as their short-term precursors. Some of these perturbations may also be regarded either as geomagnetic storms or as instrument effects. Perturbations, which could likewise be seen as short-term precursors of Vrancea earthquakes, are also noticed in the electric records. Still, a number of electric data do cast a doubt on their forerunning nature. Some suggestions are made in the end of the paper on how electromagnetic research should go ahead to be of use for Vrancea earthquake prediction.

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Extensive EM studies have been made in the Ukraine during the last 40 years and a set of reliable results have been received. Data from 3000 Magnetotelluric soundings are presented as maps of effective apparent resistivity at periods of 150 s and 500 s and impedance phase at T = 150 s. The principal results are: S1 map at the scale 1:2500000, tracing and study of Carpathian, Kirovograd and Donbas regional conductivity anomalies, a map of the crustal conductivity for Ukrainian Shield, a study of faults.

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In the last two Magnetotelluric Data Interpretation Workshops (MT-DIW) the participants were asked to model the Kayabe magnetotelluric dataset, a dense (100 m) grid of thirteen lines, with thirteen stations in each line. Bahr's phase-sensitive skew and the Groom and Bailey decomposition were used to select those lines for which the data could be considered two-dimensional. For these lines we used a 2Dinversion algorithm to obtain a series of resistivity models for the earth. Finally, we constructed a 3D model using the 2D models and critically examined the validity and practicality of this approach based on 3D model study. We found that in the Kayabe dataset case the common practice of using 2D models to depict 3D models, can only be used to create a starting model for 3D interpretation. The sequential 2D models as a representation of a 3D body is unacceptable in terms of fit to the observed data. We question the validity of some of the conductivity structures in the 2D models, as they can be mere artifacts created by the algorithm to match 3D effects.

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A two-dimensional inversion method was applied to the COPROD-2S2 dataset, which consists of apparent resistivity, phase, and magnetic transfer functions at 33 sites over 3.5 decade period band. The modeling procedure adjusts tradeoff between the data misfit, model roughness and static shift L2 norm. Two different models were made using strong and weak constrains on the shallower structure than the typical shortest skin depth (1.5 km). Stronger constraint on the shallow horizontal roughness lead to larger static shifts in TM mode and horizontally smoother structure. This shows tradeoff between the shallow resistivity roughness and static shift in TM mode. However, both models had consistent features in the deep resistivity structure and the static shift in TE mode. Both models showed consistent results on the background structures, which were estimated by the resistivity blocks outside the data area.

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