ИСТИНА

The technique and results of the joint interpretation of EM data along two lines across paleovalley arepresented. The paleovalley is located near Aleksandrovka Geophysical Field Camp in Kaluga region (Russia) and EM data was acquired during summer field courses for students of Moscow State University. EM methods using in this survey were: 1) vertical electrical sounding with induced polarization (VES-IP), 2) time-domain electromagnetic sounding (TDEM) and 3) audio-magnetotelluric sounding (AMT). Interpretation of apparent resistivity curves obtained for layered geological section, allows defining some parameters, which have a narrow range of equivalence. They are different for direct current methods (VES) and low-frequency induction methods (TDEM, AMT). If the geological section consists of alternation of thin layers with different resistivity, in order to satisfy the observed data of both groups of methods, it is necessary to set, in addition to the resistivity of the layer, its anisotropy or to increase the number of layers. The possibilities and advantages of joint interpretation of apparent resistivity curves for VES and TDEM using special version of IPI 2 Win software are shown. The results of joint interpretation of EM data showed that the TDEM data determines the position of the roofs of the low-resistivity layers and the VES data determines the thickness and the resistivity of high-resistivity layers. The upper part of the resistivity image obtained from joint interpretation VES and TDEM data was used as the starting model for 2D inversion of AMT data. Thus, the joint interpretation of the TDEM and VES curves makes the range of possible equivalent resistivity models considerably narrower and allows obtaining new and more reliable information about the resistivity structures than using each of the data of the methods separately. Involving AMT data to the interpretation allows increasing depth of investigation.