On two-dimensional interpretation of magnetotelluric soundings

Abstract

The paper presents the views of the 2-D interpretation 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 follows. 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 conductive structrures, the TE mode may be 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 undistorted. 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 longitudinal MT curves. The general scheme of this bimodal MT inversion is rather simple. The transverse curves provide details of near-surface structures (e.g. in 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 realizinig 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.