Mark N. Berdichevsky

Models and Methods of Magnetotellurics

Magnetotelluric methods are finding increasing applications for imaging electrically conductive structures below the Earths surface in both industrial and academic research. In Models and Methods of Magnetotellurics, the authors provide a systematic approach to understanding the modern theory of ill-posed problems which is essential to making confident meaningful interpretation of magnetotelluric and magnetovariational soundings. The interpretation is conducted out in an interactive way, including the hypotheses tests and successive partial inversions with priority of the tippers, magnetic tensors and impedance-phases, which keep out the destructive static effects of near-surface inhomogeneities. The efficiency of the interpretation is exemplified by new geoelectric models of the Baikal rift /one and the Cascadian subduction /one.

The fundamental model of magnetotelluric sounding

It is widely believed that the Tikhonov-Cagniard model can be applied in the magnetotelluric method of exploration geophysics if the horizontal field changes are sufficiently slow. Actually, this is overly restrictive and may give rise to unjustified doubts about the validity of the Tikhonov-Cagniard model. This paper shows that the condition determining the applicability of the model is the linearity of the horizontal field changes, rather than the slowness of these changes. This considerably extends the sphere of practical applications of the Tikhonov-Cagniard model.

Magnetotellurics in the context of the theory of ill-posed problems

This volume serves as an introduction to modern magnetotellurics originating with the pioneering work of Tikhonov and Cagniard. It presents a comprehensive summary of theoretical and methodological aspects of magnetotellurics. It provides a bridge between textbooks on electrical prospecting and numerous papers on magnetotelluric methods scattered among various geophysical journals and collections. The book has been written in the terms of the theory of ill-posed problems and contains a special chapter encouraging readers to master the elements of this theory that defines the philosophy of the physical experiment. The book thus offers the connected and consistent account of the principles of magnetotellurics from that single viewpoint. The book also brings together developments from many sources and involves some little-known results developed in Russia in Tikhonovs magnetotellurics school. Of particular interest are concluding chapters of the book that demonstrate the potential of magnetotellurics in oil and gas surveys, including discovery of the Urengoy gas field in Western Siberia, one of the largest gas fields in the world. This potential also is revealed in studies of the earths crust and upper mantle.

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.

Analysis of magnetovariational response functions

Advances in magnetovariational sounding are stimulating further development of this area of deep geoelectrics. The paper contains a review of magnetovariational response functions revealing their main properties and proposing new methods of their analysis. The theory of magnetic perturbation ellipses having a higher resolution is set forth and methods of the decomposition of magnetovariational functions are discussed that allow one to reduce 3D magnetovariational interpretation to two or three independent 2D inversions and eliminate the effect of 2D regional structures of the geoelectric background.

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.

Magnetotelluric Studies in Russia: Regional-Scale Surveys and Hydrocarbon Exploration

We briefly cover the current state of regional-scale and oil and gas magnetotelluric (MT) exploration surveys in Russia and present the findings of some studies, completed recently. These include regional-scale surveys aimed at mapping basement surface, recovering most prominent features of the sedimentary cover, and detecting crustal conductors. The study carried out in the Moscow syneclise reveal an inverse structure traced across the sedimentary formations. MT surveys in East Siberia discover interesting conductivity patterns beneath known oil fields and ore mineral deposits. Another group of studies has primary focus on hydrocarbon prospecting. The survey in Taimyr region allowed imaging the main tectonic elements and delineating the areas prospective in connection with hydrocarbons. Also, we cover the results of three surveys conducted in the salt-dome tectonic settings (Pre-Caspian depression). They provided important information on both salt and subsalt formations. Finally, we offer our view with respect to the further development of the method.

Possibilities and problems of modern magnetotellurics

Approaches to the solution of three-dimensional inverse problems are considered in the paper. The main methods for regularization of inverse problems of electromagnetic sounding are reviewed. Basic scenarios of three-dimensional and multicriterion interpretation are considered.

The Recent Developments

Growth in the East Asia and Pacific (EAP) region is estimated to have slowed to 6.3 percent in 2018 and momentum has weakened, reflecting diminishing support from exports partly offset by robust domestic demand (Figure 2.1.1; Table 2.1.1). External conditions have deteriorated, reflecting moderating global demand, heightened trade tensions, and a substantial tightening of financing conditions. This less favorable international environment has been accompanied by financial stress in some large emerging market and developing economies (EMDEs).