N. Golubtsova

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.

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.

Magnetovariational Method in Deep Geoelectrics

Deep resistivity structure of the Earth’s crust and upper mantle can be studied by two natural-source methods: magnetotelluric (MT) sounding, which uses electric and magnetic field variations, and magnetovariational (MV) sounding, which uses magnetic field variations only. Integrated MT and MV data interpretation is a multicriterion problem. MT and MV data have different sensitivity to resistivity structures, as well as different robustness to their 2-D approximation. We consider two approaches to MT and MV data integrated interpretation: parallel inversion and successive inversions of data components. In the first case, the selection of weights of data components is critical. We suggest successive inversions approach and test it on model data, calculated for the schematic 2-D resistivity model of the Kyrgyz Tien Shan, and experimental data, collected along the profile in the Cascadia subduction zone.

Chapter 2 Magnetovariational Method in Deep Geoelectrics

Publisher Summary This chapter discusses the magnetovariational method in deep geoelectrics. Deep geoelectrics studies of the Earths crust and upper mantle include two methods: (1) The magnetotelluric (MT) method using the electric and magnetic fields and (2) the magnetovariational (MV) method using only the magnetic field. The MT–MV geoelectric complex is widely and rather successfully used throughout the world. It provides an unique information on the Earths interior (porosity, permeability, graphitization, sulfidizing, dehydration, melting, fluid regime, ground-water mineralization, rheological characteristics, thermodynamic, and geodynamic processes). The inverse problem of MV and MT soundings is unstable. An arbitrarily small error in the measurement data can give rise to an arbitrarily large error in the conductivity distribution. The weak point of deep geoelectrics with MT priority is that inhomogeneities in the uppermost layers may severely distort the electric field and consequently the impedance tensor along with the apparent resistivity. It is commonly supposed that ‘‘MV studies determine only horizontal conductivity gradients, while the vertical conductivity distribution is not resolved.’’