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Summary The paper presents the views of the 2‐D interpretation of of magnetotelluric (MT) data that are characteristic of the Russian magnetotelluric school. Discussing the strategy of 2‐D interpretation of MT data, we have to answer three questions. In decreasing order of importance, these questions are as foloows. (1) Which field mode is more sensitive to the near‐surface and deep structures that are the targets of MT surveys? (2) Which field mode is more robust to the 3‐D effects caused by real geological bodies? (3) Which field mode is more susceptible to the static shift induced by near‐surface inhomogeneities? We examine the transverse magnetic (TM) and transverse electric (TE) modes of the 2‐D magnetotelluric field and show that they satisfy the principle of information complementarity: (1) while the TM mode is more sensitive to the near‐surface structures, the TE mode may be more sensitive to the deep structures; (2) while the TM mode is more robust to the 3‐D effects caused by resistive structures; and (3) while the TM mode is more susceptible to the static shift, the TE mode may be almost iundistorted. Thus, the gaps left by one mode can be filled by another mode. If so, the most comprehensive and reliable information on the conductivity of the Earth's interior can be obtained using both modes, i.e. the transverse and lingitudinal MT curves. The general scheme of this bimodal MT inversion is rather simple. The transverse curves provide details of near‐surface structures (e.g. the sediments) and allow one to evaluate the lithosphere resistance and outline the deep conductive faults, while the longitudinal curves help one to detect the conductive zones in deep layers of the lithosphere and in the asthenosphere. An efficient two‐level algorithm for the bimodal MT inversion realizing this scheme is suggested. As an illustration, the paper presents the geoelectrical model of the Kirghiz Tien Shan constructed by means of the bimodal MT inversion.

The quasi‐linear approximation for electromagnetic forward modeling is based on the assumption that the anomalous electrical field within an inhomogeneous domain is linearly proportional to the background (normal) field through an electrical reflectivity tensor λ⁁. In the original formulation of the quasi‐linear approximation, λ⁁ was determined by solving a minimization problem based on an integral equation for the scattering currents. This approach is much less time‐consuming than the full integral equation method; however, it still requires solution of the corresponding system of linear equations. In this paper, we present a new approach to the approximate solution of the integral equation using λ⁁ through construction of quasi‐analytical expressions for the anomalous electromagnetic field for 3-D and 2-D models. Quasi‐analytical solutions reduce dramatically the computational effort related to forward electromagnetic modeling of inhomogeneous geoelectrical structures. In the last sections of this paper, we extend the quasi‐analytical method using iterations and develop higher order approximations resulting in quasi‐analytical series which provide improved accuracy. Computation of these series is based on repetitive application of the given integral contraction operator, which insures rapid convergence to the correct result. Numerical studies demonstrate that quasi‐analytical series can be treated as a new powerful method of fast but rigorous forward modeling solution.

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