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About the journal

NAUKA I TEKHNOLOGICHESKIE RAZRABOTKI (SCIENCE AND TECHNOLOGICAL

DEVELOPMENTS), ISSN: 2079-5165, eISSN: 2410-7948, DOI: 10.21455/std; https://elibrary.ru/title_

about.asp?id=32295; http://std.ifz.ru/. The journal was founded in 1992.

PROCESSING AND INTERPRETATION OF DEPTH CONTROLLED SOURCE AUDIO-MAGNETOTELLURICS (CSAMT) SOUNDING
IN COMPLEX WITH AUDIO-MAGNETOTELLURICS (AMT) DATA (THE MURMAN-2018 EXPERIMENT)

A.N. Shevtsov

Geological Institute FRC “Kola Science Center of the Russian Academy of Sciences”,
Murmansk region, Apatity, Russia

e-mail: anshev2019-01@mail.ru

Highlights

– A technique has been developed for estimating the static shift of the apparent resistivity curves of controlled source audio-magnetotellurics (CSAMT) and audio-magnetotelluric sounding (AMTS) based on the horizontal components of the vectors of the electromagnetic field of the horizontal electric dipole

– Field measurements of a horizontal electric dipole were carried out with a domestic measuring station VMTU-10 and an Energy-4 generator with a power of 29 kW at distances of 12–105 km from the source in combination with AMT sounding.

– Developed technique for taking into account static distortions made it possible to coordinate the results of measurements of AMTS and CSAMT of the Murman-2018 experiment for two polarizations of the primary field of the source.

Abstract. The purpose of the work is to study the deep electrical conductivity of the Murmansk block of the Earth’s crust on the Fennoscandinavian shield using controlled source audio-magnetotellurics (CSAMT) and audio-magnetotellurics (AMT) methods. One of the most important tasks of in-depth studies using the complex of AMT and CSAMT methods is to take into account the influence of surface conductors on the measurement results (static shift). To solve this problem, we develop a methodology for processing and interpreting the measurement data of the CSAMT and AMT system using the results of the deep-sounding experiment in the frequency range from 1 to 500 Hz performed in 2018 (Murman-2018) as an example. Measurements with a controlled source in the form of two mutually orthogonal horizontal electric dipoles were performed at distances of 12–105 km. The generator-measuring complex consisted of the domestic measuring station VMTU-10 (VEGA LLC) and the Energy-4 generator (KSC RAS). Using synchronous time series of the field components at the observation points and the current strength in the supply dipole, the power spectra of the autocorrelation and cross-correlation functions of the recorded values were calculated based on the fast Fourier transform (FFT). The obtained spectral characteristics were used both to determine the amplitudes of the source field components and the phase shifts between them, and to evaluate the components of the AMT field impedance tensor. Using the amplitudes of the horizontal components of the electromagnetic field and the impedance ratios, the values of apparent resistivity were calculated. The obtained measurement results were corrected for the static shift. For this, correction coefficients were calculated with respect to the ratio of the apparent resistivity values for the horizontal component of the magnetic field to the apparent resistivity values by impedance and electric field. The data for the axial and equatorial installations along with the data of AMT and DC sounding made it possible to establish a high degree of homogeneity of the deep electric section in the horizontal directions and to expand the frequency range of the obtained curves of apparent resistivity and impedance phase. A one-dimensional interpretation is made and estimates of the distribution of resistivity in depth are obtained.

Keywords: electrical conductivity, frequency sounding, audio-magnetotellurics, CSAMT, FFT, autocorrelation function, cross-correlation function, power spectrum, impedance, apparent resistivity, controlled transformation, static shift

Cite this article as: Shevtsov A.N. Processing and interpretation of depth controlled source audio-magnetotellurics (CSAMT) sounding in complex with audio-magnetotellurics (AMT) data (the Murman-2018 experiment), Nauka i Tekhnologicheskie razrabotki (Science and Technological Developments), 2019, vol. 98, no. 4, pp. 19–33. [Special issue “Methodological developments for electromagnetic controlled source soundings”]. [in Russian]. https://doi.org/10.21455/std2019.4-2

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About the author

SHEVTSOV Alexander Nikolaevich – Candidate of Physical and Mathematical Sciences, Senior researcher, Geological Institute FRC “Kola Science Center of the Russian Academy of Sciences”. 184209, Apatity, Murmansk region, Fersman st., 14. Tel.: +7(815) 557-92-08. E-mail: anshev2019-01@mail.ru