Science and technological developments: article

Geological Institute FRC “Kola Science Center of the Russian Academy of Sciences”
Journal: Science and technological developments
Tome: 98
Number: 4
Year: 2019
Pages: 19-33
UDK: 550.837.61+550.837.63+550.837.211
DOI: 10.21455/std2019.4-2
Аnnotation file
Bibliographic list
Keywords: CSAMT, electrical conductivity, frequency sounding, audio-magnetotellurics, CSAMT, FFT, autocorrelation function, cross-correlation function, power spectrum, impedance, apparent resistivity, controlled transformation, static shift
Аnnotation: 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.