Vladimir Yu. Semenov

Deep electromagnetic soundings conducted in Trans‐European Suture Zone

A consortium of nine geophysical institutions recently carried out a large-scale geomagnetic experiment focused on revealing the deep electrical structure beneath central and eastern Europe around the Trans-European Suture Zone (TESZ), the regions first-order geological lineament. The TESZ is considered a broad zone of deformation that crosses all of Europe, from the British Isles in the northwest to the Black Sea area in the southeast, and it most likely continues in North America [Keller and Hatcher, 1999]. The geomagnetic experiment was called Central Europe Mantle Geoelectrical Structure (CEMES). Initiated by Polish scientists, geophysicists from eight other countries joined the project during a NATO Advanced Research Workshop held in the spring of 2001 in Belsk, Poland. The experimental phase of the project was held from 2001 through 2002; and altogether, 12 geomagnetic observatories, the international codes of which are shown among others in Figure 1, took part in acquiring the data. They will serve the objective of inferring information on the mantle conductivity structures beneath the region of TESZ, as well as beneath surrounding units, specifically the western part of the East European Craton (EEC), Variscides, including the Bohemian Massif, the Carpathians, and the Pannonian Basin.

Updating the map of Earth's Surface conductance

Studying the Earths deep conductivity structures, important for developing our understanding of the dynamics of the Earth, is complicated due to effects of the shallow conductive structures on the electromagnetic (EM) responses for periods larger than hours. The results of the deep EM soundings can be heavily distorted by the surface shell conductance, which varies from fractions of Siemens (S) inland to up to tens thousand of Siemens in the oceans. Thus, separating the effects caused by those variations and by deep conductivity structures is an important step during interpretation of the data. This article reports on efforts to overcome these difficulties by providing high-resolution, global maps of the Earths surface shell conductivity structure, from which deep conductivity can be interpolated. Using finescale regional surface schemes of conductance for the shallow structures (S-maps) overlain and compiled into broader spatial maps, scientists will b e able to use data products from these efforts to accomplish research goals of the currently running USArray (http://www.emscope.org) and for the planned Euro-Array (http://www.euroarrayorg), projects that aim in part to regionally map the conductivity structures at upper and middle mantle depths by using magnetotelluric (MT) and magnetovariation (MV) methods.

Conductivity structure of the upper mantle in an active subduction zone

Abstract The results of deep electromagnetic soundings for the active transition ocean-continent zone at Sakhalin Island are presented. After an averaging procedure of the magnetotelluric response functions, the period range was extended up to 500 days by using the geomagnetic soundings of the Yuzhno-Sakhalinsk, Kakioka and Memambetsu observatory data. The existence of the asthenosphere and a high conductivity layer located at the base of the upper mantle was established by one-dimensional inverse methods. High resistivity revealed at depths of 250–450 km appears to be connected with the penetration of the cold slab into the mantle. The possible nature of a mid-mantle conductive layer and the relation of its conductances with the tectonic history are discussed.

Results of Crust and Mantle Soundings in Central and Northern Europe in the 21st Century (Review)

The first decade of 21st century is characterized by the appearance of new approaches to deep induction soundings. The theory of magnetovariation and magnetotelluric soundings was generalised or corrected. Spatial derivatives of response functions (induction arrows) were obtained for the ultra-long periods. New phenomena have been detected by this method: secular variations of the Earth’s apparent resistivity and the rapid changes of induction arrows over the last 50 years. The first one can be correlated with the number of earthquakes, and the second one–with geomagnetic jerks in Central Europe. The extensive studies of geoelectrical structure of the crust and mantle were realized in the frame of a series of international projects. New information about geoelectrical structures of the crust in Northern Europe and Ukraine was obtained by deep electromagnetic soundings involving controlled powerful sources. An influence of the crust magnetic permeability on the deep sounding results was confirmed.

Modeling of deep magnetovariation soundings on the rotating earth

Induced magnetic fields in the Earth arise due to two phenomena: induction generated by the time-variable exciting field and the motional induction caused by movement of the conductive planet in the outer magnetic fields. The comparison of both approaches on the spherical Earth has been analyzed in the present work for two sources in the ionosphere and magnetosphere. For this aim, both sources with their natural sizes and positions have been modeled analytically to obtain the fields on the layered sphere at the middle latitudes. The conditions when the steady ring current field is not influenced by the Earth’s rotation have been established theoretically. The synthetic diurnal magnetograms were used for the deep sounding by the magnetovariation spatial gradient method and the result was compared with the one obtained on the nonrotating sphere. Sounding results using both approaches were found different above the 2D inhomogeneous mantle. The precessions of the magnetospheric belt current pole for daily sampling frequency were presented using several geomagnetic observatory data in the northern hemisphere.

Modeling of Deep Soundings

The impedances for the deep electromagnetic soundings of the Earth are obtained from the relations for the Fourier amplitudes of the observed field components. These relations are essentially different for the magnetotelluric and magnetovariation sounding methods. In order to increase the reliability of investigations, studies of electrical properties of the Earth’s mantle are often carried out by the joint inversion of impedances obtained by both methods of sounding. The forward modeling is a unique tool to verify the accuracy of merging the differently obtained long-period impedances because those simplified relations were derived theoretically for the radio-wave periods only. The spherical modeling of the responses above 2D mantle inhomogeneities presented in this paper has shown that the induction methods can give mutually inconsistent results and the combinations of their responses can be problematic in practice. For this reason, much attention is given to the generalized magnetovariation sounding method which results in both, regularized impedance functions in space and frequency domains and closely resemble the magnetotelluric ones devoid of the subsurface galvanic distortions. In this study, some peculiar properties of the induction arrows above a spherical inhomogeneity excited by an inhomogeneous external field are estimated for long periods. The final comprehensive model, assuming a shell of the realistic Earth’s surface conductance, is an evidence that the generalized magnetovariation method is promising for the study of mantle inhomogeneities and can be used with the magnetotelluric method in a specific way.

Several Impedances from One Equation

The chapter is dedicated to estimate impedances and their space derivatives using the theory of the random processes. This approach is effective to estimate several unknown values from one equation. Results are prepared in the frequency domain and characterized by confidence limits. Selections of directions for interpretation are discussed. The theoretical principles considered below deal with fields induced by sources in the ionosphere or magnetosphere of the Earth only.

Impedances, Sources and Environments

The Earth does not have its own impedance and that is why the impedance definitions are numerous and, in some sense, arbitrary. Impedances adapt to the properties of the investigated media, introducing a priori information, for example, about their dimensionality. In this way, considering 1D, 2D or 3D environments means to mimic homogeneous and inhomogeneous media. Additionally, the geometry of external sources must be taken into consideration. As far as we know, the notion of “response function” appeared for the first time in the middle of the XIX century in the writings of J. Lamont [see in: Haak in The experiments with telluric currents and magnetic fields of Johann von Lamont in 1861, the sedimentary layer beneath Munich and the color theory of Johann Wolfgang von Goethe. Abstracts of 22 Electromagnetic Induction Workshops, (2014)] in Germany. At the end of the same century, Schuster and Lamb (Philosophical Transactions of the Royal Society of London. A 180:467–518, 1889) in the U.K. attempted to estimate the deep structure of the Earth using diurnal variations in Earths magnetic field for this purpose. Further development of the use of impedance occurs in the 1930s–1940s, probably due to the development of aircraft design, radar and active mineral exploration. The use of deep magnetovariation soundings is usually referred to the second half of the twentieth century, although sounding theory was developed as early as 1940 by S.M. Rytov and independently by A.N. Schukin in Russia.

Induction Soundings of the Earth's Mantle

The Earth does not have its own impedance and that is why the impedance definitions are numerous and, in some sense, arbitrary. Impedances adapt to the properties of the investigated media, introducing a priori information, for example, about their dimensionality. In this way, considering 1D, 2D or 3D environments means to mimic homogeneous and inhomogeneous media. Additionally, the geometry of external sources must be taken into consideration. As far as we know, the notion of “response function” appeared for the first time in the middle of the XIX century in the writings of J. Lamont [see in: Haak in The experiments with telluric currents and magnetic fields of Johann von Lamont in 1861, the sedimentary layer beneath Munich and the color theory of Johann Wolfgang von Goethe. Abstracts of 22 Electromagnetic Induction Workshops, (2014)] in Germany. At the end of the same century, Schuster and Lamb (Philosophical Transactions of the Royal Society of London. A 180:467–518, 1889) in the U.K. attempted to estimate the deep structure of the Earth using diurnal variations in Earths magnetic field for this purpose. Further development of the use of impedance occurs in the 1930s–1940s, probably due to the development of aircraft design, radar and active mineral exploration. The use of deep magnetovariation soundings is usually referred to the second half of the twentieth century, although sounding theory was developed as early as 1940 by S.M. Rytov and independently by A.N. Schukin in Russia.

Results of Deep Soundings in Europe

In the first half of the XX century, there appeared new approaches to deep induction soundings. The theory of magnetovariation as well as magnetotelluric soundings was formulated just before the World War Two. Spatial derivatives of response functions (induction arrows) were obtained for the long periods. New phenomena have been detected by this method: secular variations of the Earth’s apparent resistivity and the rapid changes of induction arrows over the last 50 years. The first ones can be correlated with the number of earthquakes and the second ones—with geomagnetic jerks in Central Europe. Extensive studies of geoelectrical structure of the crust and mantle were realized in the frame of a series of international projects. New information about geoelectrical structures of the crust in Northern Europe and Ukraine was obtained by deep electromagnetic soundings involving powerful, controlled sources. An influence of the crust magnetic permeability on the deep sounding results was confirmed.