P. Yu. Pushkarev

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.

Possibilities of interpretation of the magnetotelluric data, obtained on a single profile over 3D resistivity structures

Magnetotelluric soundings are frequently carried out on a single profile or on profiles remote from each other. Interpretation of the obtained data is difficult in the presence of spatially heterogeneous geoelectric structures. We evaluate its capabilities on the basis of the synthetic data, that correspond to a geoelectric model, which consists of a three-layered section in the background and three rectangular prisms, differently arranged relative to the profile. Using the simple methods of analysis of magnetotelluric data, we succeeded in allocating all three heterogeneities over the area that surrounds the profile of observations. As a result of the fast smoothed-structure 1D and 2D inversion of different components of data, taking into account their specific features, the depths of the occurrence of anomalies and the order of the values of their electrical resistivity were evaluated, and the background section was also reconstructed. On this basis, and, also, with the use of a priori geological-geophysical information, the construction of a 3D model in a more or less broad band around the profile and its correction with the aid of 3D data inversion are possible.

Chapter 13 Regional Magnetotelluric Explorations in Russia

Publisher Summary This chapter discusses the regional magnetotelluric (MT) explorations in Russia. Electromagnetic (EM) geophysical methods (telluric current method, magnetotelluric sounding, frequency sounding, and transient sounding) have been used in the erstwhile Russia to study a deep structure of sedimentary basins and of the consolidated crust since the 1950s. Tectonic schemes of the principal sedimentary basins of Russia were constructed and several large hydrocarbon deposits, for example—the Urengoy gas field, were discovered using the telluric currents method and MT soundings, in combination with other geophysical methods. The basic regional EM method is the MT method. MT provides the largest exploration depth and is inexpensive and mobile, as it does not require an artificial field source MT investigations essentially expand the existing ideas about the structure and geodynamics of the Earths interior, based on the results of drilling and of seismic, gravity, and magnetic studies. MT investigations provide unique information about the structure and reservoir properties of sedimentary complexes, the state of active geodynamic regions, the graphitization and fluid regime of the consolidated crust, and the permeable and fluid-saturated crustal zones.

The first ever application of electromagnetic sounding for mapping the submarine permafrost table on the Laptev Sea shelf

The inconsistency between the position of the submarine permafrost table in the East Arctic seas revealed by simulation and anomalies in the distribution of dissolved methane associated with ascending torchlike ejections of bubbling methane made it necessary to develop a representative geophysical express method, which allows the position of the submarine permafrost table to be determined. The method is based on sounding of the generated primary electromagnetic field in the near zone of the medium and measurement of the induced secondary EM field. The reliability of the method is confirmed by core drilling in the Laptev Sea.

The creation of a geophysical observatory at the Aleksandrovka research base of the Geological Faculty of Moscow State University in the Kaluga Region

Since May 2011, long-period magnetotelluric (MT) data have been collected in the non-magnetic pavilion at the geophysical base of Moscow University in the Kaluga Region. The non-magnetic pavilion was constructed in compliance with the following conditions: selection of non-magnetic materials, control of local magnetic field anomalies, and setting the basement for magnetometers separately from the main building. Recording equipment with fluxgate and optomechanical magnetometers with three sets of electric lines with different electrodes was installed. The parallel testing of channels was performed. Currently, the seismo-logical equipment and meteorological station are being prepared for installation.

Three-dimensional inversion of the single-profile magnetotelluric data

It is preferable to use the three-dimensional (3D) magnetotelluric inversion, which provides volumetric geoelectric models, to handle the array input data. However, the soundings are frequently conducted on the single profiles or on the profiles that are considerably spaced apart from each other. We explore the possibilities of the 3D inversion of such data by the example of a three-layer model containing three local inhomogeneities. We previously showed that the simple processing of the data and their 1D or 2D inversion enable reconstructing the background cross section and locating all the three inhomogeneities. In the present paper, we use this information for constructing several versions of the starting model and carrying out the smoothing 3D inversion of the data. The experiments show that if the background cross section is incorporated into the starting model, the final model provided by the inversion closely reproduces the real distribution of all geoelectric parameters. At the same time, if the starting model that hosts the inhomogeneities has the form of a homogeneous half-space, the inversion is not able to reconstruct an adequate final model.

Magnetotellurics and radio-wave interference sounding

The plane harmonic electromagnetic fields are considered in the theory of magnetotelluric methods in the range of frequencies from 0.0001 Hz to 20 kHz. These fields are natural by their origin and contain information on the depths from tens of meters up to 100 km and more. The magnetotelluric soundings, which use the fields of radio stations, expand the frequency band almost up to 1 MHz and make it possible to study the depths from the first few meters. The method of radio-wave interference sounding supplements geoelectric prospecting on plane waves into the range of even higher frequencies (up to 100 MHz). In this case, the conduction and displacement currents become comparable, which makes it possible to distinguish objects both by their electrical conductivity and by their dielectric permittivity. For the two-layered model of a medium, there exist simple kinematic methods of data interpretation of a radio-interferometry sounding. Within multilayer, and especially horizontally heterogeneous, media, methods for solving equations of electrodynamics and inverse problems of geophysics are required. In the present paper, the foundations of the theory of radio-interferometry sounding, the methodology, its role in geoelectric prospecting, and the opportunities for the solution of geological problems are discussed.

Electromagnetic soundings of the sedimentary cover and consolidated crust in the transition zone from the Moscow syneclise to the Voronezh anteclise: Problems and prospects

The methods and results of electromagnetic soundings (EMS) performed in the transition zone from the Moscow syneclise to the Voronezh anteclise in the vicinity of the MSU geophysical base are considered. This base is located in the village of Aleksandrovka in the Yukhnov district of Kaluga area. The composite EMS curves characterizing rock complexes composing the sedimentary cover are constructed, and changes in these complexes within the specified transition zone are traced. The standard curves of magnetotelluric (MT) and magnetovariational (MV) soundings are constructed from the results of long-term measurements at the ALX observation point located at the Moscow State University’s (MSU) geophysical base. The maps of thickness and total longitudinal conductance of the sedimentary cover are constructed from the results of interpretation of MT data obtained in the region. A conductor in the consolidated Earth’s crust is identified within the Voronezh anteclise. Prospects for further investigations of the region are associated with the tracing of the crustal conductor within the Voronezh anteclise, as well as with the organization of an observatory at the MSU’s geophysical base in order to perform long-term measurements of the electromagnetic (EM) and other geophysical fields.

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.