A Method for Quantifying Static Shift Distortions Using a Magnetic Field of Controlled Source (CSAMT)
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thod for Quantifying Static Shift Distortions Using a Magnetic Field of Controlled Source (CSAMT) A. A. Zhamaletdinova, b, * a
St. Petersburg Branch of Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation Russian Academy of Sciences, St. Petersburg, 199034 Russia b Geological Institute “Kola Science Center of the Russian Academy of Sciences,” Apatity, 184209 Russia *e-mail: [email protected] Abstract—Static distortions are known abroad by the term “static shift distortions.” They have a strong influence on the results of deep electromagnetic soundings made with both natural (AMT-MTS) and controlled (CSAMT) sources. The main cause of static shift distortions is the local inhomogeneities of the upper halfspace in comparison with the electromagnetic wavelength in the ground. Static shift distortions are frequency-independent and manifest themselves in a parallel shift of the apparent resistivity curves relative to the resistance scale. The shape of the apparent resistivity curves containing information on the section of the model is preserved, but the ideas about the depths of the layer locations and their conductance values change. Currently, many methods have been developed for quantifying static shift distortions, but they all have a quality, phenomenological character. This work proposes a quantitative method for the correction of static shift distortions. It is based on the use of the apparent resistivity curve of a controlled source, normalized to the value of the total horizontal magnetic field. The use of magnetic measurements with induction coils that do not have a galvanic connection with the ground allows quantifying static shift distortions by comparing the magnetic field with apparent resistivity curves normalized to the total electric field or the total input impedance value. The proposed method is applicable only for frequency soundings with controlled sources CSAMT. Nevertheless, it allows one to correct the MTS and AMTS curves measured at the same installation and using the same sensors in the low-frequency region, outside the wave zone, where the CSAMT method loses its sounding functions. The results of the experimental works that justify the applicability of the new method by the example of controlled-source soundings are presented. Keywords: soundings, controlled sources, static shift distortions, Baltic shield DOI: 10.3103/S0747923920050138
INTRODUCTION Static shift distortions of the results of electromagnetic soundings appear due to the influence of nearsurface inhomogeneities. They produce a shunting effect on the electric components of the field and lead to frequency-independent shifts of the apparent resistivity curves, sometimes by a few orders of magnitude. There are many ways of quantifying the estimation of static shift distortions. The most popular ones are the following: (1) For quantifying estimation of static shift distortions, the work (Berdichevskii, 1968) concerned with the first experiments in developing the methods of magnetotelluric sounding (MTS) and
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