It is known that magnetic poles of the Earth is accelerated and is now being ≈ 50 km/year (Olsen & Mandea, 2007) while the geomagnetic pole (the dipole part), which is computed (fictitious) value, has much less velocity. It is believed that the magnetospheric outer ring currents are held by the dipole part of the Earth’s magnetic field. The low frequency magnetic variations of that source allow determine the current position of the source axis and its corresponding pole which as shown experimentally precesses around the geomagnetic pole.
Electromagnetic (EM) methods of geophysics allow to image the subsurface conductivity distribution by analyzing data measured on the Earth surface. The transient electromagnetic (TEM) sounding method is a time-domain controlled source one which utilize a non-stationary transient process of the EM field decaying in the conductive medium due to the step current excitation which is described by the Heaviside function. The mathematical and algorithmic tools for express analysis of such experimental data acquired for the TEM method are presented in this article, the practical application of which is sometimes important for experimental in field measurements, since it allows as to make immediate decisions on optimization of experimental field work as well as to qualitatively estimate the state of the object under study. The proposed express analysis algorithm is developed on the basis of a widely used transformation of the experimental TEM curve, also known as the S-inversion, which in turn is based on the approximation of a conductive half-space by a thin sheet which is immersing into a non-conductive half-space with the decaying of transient process in the EM field. To reduce the influence of noise into the measured data, we carrying out the approximation of the experimental sounding curves accounting several aspects: the solutions of equations which describe the penetration of EM field into a conducting medium; the function of the cubic spline; and elements of mathematical analysis of continuous functions. According to profile measurements, one dimensional models of resistivity distribution were obtained using the created express analysis tools and a pseudo two dimensional model of the geoelectric cross-section of the waste reservoir dam which is next to the Stebnyk town was constructed on their basis. The analysis of the obtained models allowed us to detect two areas where it is desirable to carry out additional studies to clarify the geological situation. Of course, algorithms for inversion of experimental data, both as from a mathematical point of view as well as from the point of view of algorithm implementation, are much more complicated. However, if one implements a fast and efficient one-dimensional inversion algorithm for the TEM data instead of the transformation mentioned above, it will be possible to significantly improve the reliability of the result.
The transient electromagnetic (TEM) method is a time-domain, controlled source, electromagnetic (EM) geophysical technique which is often applied to image the subsurface conductivity distributions of shallow layers due to its effectiveness and adaptability to complex site working conditions. The means for an express analysis of such experimental data in several practical cases have advantages and are suitable for use. We developed our approach for determining the approximate one-dimensional (1D) model of background conductivity based on the formal transformation of the TEM experimental data and the mathematical analysis of continuous functions. Our algorithm, which allows the 1D model’s parameters to be obtained in terms of a layer’s thickness and resistivity, widely utilizes the numerical differentiation of experimental curves as well as of transformed ones. Since the noise level increases with time in the attenuating TEM signals and differentiation even enhances it, special procedures are required to calculate the derivative values. We applied the piecewise cubic spline approximation to solve this problem. In that case, the derivatives are obtained using polynomial coefficients which are available for each node. The application of the created facilities is demonstrated using real experimental data of the TEM soundings.
SUMMARY The generalized magnetovariation soundings were carried out around the Trans European Suture Zone(TESZ) using twenty four magnetic stations situated mainly in Poland and partly in Czech Republic, Hungaryand Ukraine. Data of nearby geomagnetic observatories of western countries were used too. Theimpedances were obtained in the period range from 4 up to 48 hours. Due to the new sounding approach itwas possible estimating the gradients of impedances also. They were transformed to the traditional inductionarrows which allow doing areal modeling of the deep electrical conductivity distributions at the CentralEurope region by two independent algorithms developed in Prague and Warsaw. The REBOCC inversionwas applied to distinguish conductivity features in the upper mantle along a chosen profile crossing TESZ onPolish territory. The projections of the experimental induction arrows onto the profile line were used as inputdata for the inversion. The subsurface conductance of shallow layers was accounted also. The obtainedmodel reaches depth of 400 km. The results with induction arrows, areal conductance distributions and crosssection beneath chosen profile are presented and discussed.
В работе анализируются результаты магнитовариационного зондирования (МВЗ) в геомагнитных обсерваториях, входящих в список международных центров данных: Львов (LVV) -Восточные Карпаты, Сурлари (SUA) -Южные Карпаты, Пана гюриште (PAG) -Балканиды и режимной геофизической станции (РГС) Нижнее Селище (NSl) -Восточные Карпаты (рис. 1, а).На каждом пункте выполнена 1D инверсия кривых МВЗ, сочленен ных с данными магнитотеллурического зондирования (МТЗ), и получен геоэлектрический разрез от глубины порядка 1 км до 1200 км.Данные о распределении проводимости (σ) вещества мантии по результатам МВЗ с использованием многолетних (3
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