A. Rybin

Geoelectric section of the Central Tien Shan: Analysis of magnetotelluric and magnetovariational responses along the Naryn geotraverse

During the past two decades, at the Research station (Bishkek) more than a hundred magnetotelluric and magnetovariational soundings were carried out on the Naryn geotraverse that intersects the Tien Shan region from Lake Balkhash to the Tarim Basin along the 76° E meridian. Integration and complex interpretation of the data of these soundings with improved resolution and reliability of the geoelectric model of the Central Tien Shan section became an urgent challenge. Our paper presents a complex of methods for processing and invariant analysis of the electromagnetic data developed for the solution of this problem. Its application allowed us to validate the choice of the 2D interpretation model for the Naryn Line and to form the adequate ensemble of the data to be inverted. The developed approaches will also be useful in similar studies in the other mountain regions.

Estimation of the correlation between magnetotelluric and geothermal data in the Bishkek geodynamic research area

The paper analyzes the correlation between the electrical conductivity and temperature in the upper crust of the Bishkek geodynamic research area (the Northern Tien Shan). Electrical conductivity profiles constructed from magnetotelluric data and thermograms from the boreholes near magnetotelluric sounding (MTS) points are used for estimations. The correlation analysis of conductivity and temperature profiles to depths of 3–4 km showed that, first, the correlation coefficients do not depend on the distance between the borehole and the nearest MTS point; second, the good correlation between the conductivity and temperature observed for the majority of borehole-MTS point pairs is accounted for by the fact that the study parameters vary with depth in a manner normal for laminated sedimentary rocks; and, third, a low correlation is due to specific features of the geological structure between the borehole and MTS point under consideration.

Geoelectric section of the Central Tien Shan: Sequential inversion of the magnetovariational and magnetotelluric data along the Naryn Line

The paper presents the results of 2D inversion of deep magnetotelluric (MT) and magnetovariational (MV) soundings along the Naryn Line. The method of partial (sequential) inversions is used. According to this method, at the first stage, magnetovariation responses are used for the localization of deep anomalies of electrical conductivity, and then the magnetotelluric sounding data are invoked to refine the structure of the host medium and the structural details in the upper part of the section. It is shown that this approach enables one to estimate the informativeness of separate components of the electromagnetic field, to reduce the distorting influence of the near-surface geoelectric inhomogeneities, and to increase the stability of the final solution of the inverse problem.

New geophysical data on the depth structure of the Tien Shan intermontane depressions

New results of a detailed study of the Issyk Kul intermontane depression system by electromagnetic and geomagnetic methods are presented. A profile-based magnetometric survey across the water area of Lake Issyk Kul revealed a positive 30-km-wide magnetic anomaly at approximately +100 nT in intensity. The nature of this anomaly can be explained by the exposure of Late Paleozoic granitoid intrusions. The problems of the construction of a regional geoelectric model along the Barskaun magnetotelluric profile, which crosses the central Issyk Kul Depression in the meridional direction, are discussed. On the basis of generalization of previously obtained data from complex studies along the MANAS geophysical transect, a structural-geodynamic lithosphere section was constructed along this transect. In this section, the interrelationships between the surface and deep seismic and geoelectric structures are clearly reflected in the lithosphere of the Naryn-Atbashi intermontane depression system.