ON 2-DIMENSIONAL INTERPRETATION OF MAGNETOTELLURIC SOUNDINGS

The paper presents the views of the 2-D interpretation of magnetotellu ric (MT) data that are characteristic of the Russian magnetotelluric s chool. Discussing the strategy of 2-D interpretation of MT data, we ha ve to answer three questions. In decreasing order of importance, these questions are as follows. (1) Which field mode is more sensitive to t he near-surface and deep structures that are the targets of MT surveys ? (2) Which held mode is more robust to the 3-D effects caused by real geological bodies? (3) Which field mode is more susceptible to the st atic shift induced by nearsurface inhomogeneities? We examine the tran sverse magnetic (TM) and transverse electric (TE) modes of the 2-D mag netotelluric held and show that they satisfy the principle of informat ion complementarity: (I)while the TM mode is more sensitive to the nea r-surface structures, the TE mode may be more sensitive to the deep st ructures; (2) while the TM mode is more robust to the 3-D effects caus ed by conductive structures, 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 undist orted. Thus, the gaps left by one mode can be filled by another mode. If so, the most comprehensive and reliable information on the conducti vity of the Earth's interior can be obtained using both modes, i.e. th e transverse and longitudinal MT curves. The general scheme of this bi modal MT inversion is rather simple. The transverse curves provide det ails 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 conducti ve zones in deep layers of the lithosphere and in the asthenosphere. A n 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 t he bimodal MT inversion.