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EN
Magnetotelluric (MT) field data are usually presented as plots of amplitude and phase MT sounding data vs frequency. Visualization of resistivity changes of geoelectric complexes with the use of apparent resistivity and phase curves give qualitative results only. Quantitative interpretation of MT sounding curves is needed to get geoelectric parameters. For a 1D horizontally layered earth, amplitude curves (apparent resistivity curves) and phase curves can be transformed into apparent velocity curves versus depth of EM field penetration into the conducting earth. Apparent velocity curves can be approximated by straight-line segments corresponding to homogeneous geoelectric layer complexes. Each segment of the apparent velocity curve (with a given angle of inclination) is related with the resistivity and thickness of individual geoelectric complexes. For heterogeneous earth (2D or 3D) vertical component of the magnetic field is directly connected with boundary of geo-electric complexes. It can be used to express components of vectors of apparent velocity. For a 1D horizontally layered earth, a vector of apparent velocity has only the vertical component. For heterogeneous earth horizontal components of apparent velocity also are inducted. The angle of inclination of the total vector of velocity and its value depend on the geometry of studied structure.
PL
Podstawową wielkością prezentującą zmiany przewodnictwa elektrycznego górotworu z głębokością jest oporność pozorna jako funkcja częstotliwości pola magnetotellurycznego. Wizualizacja tych zmian, jak również granic struktur geologicznych, przez oporność pozorną ma charakter jedynie jakościowy, dalece przybliżony. Parametry przekroju geoelektrycznego uzyskujemy jedynie przez interpretację ilościową danych pomiarowych. W przypadku przekrojów geoelektrycznych 1D krzywe sondowań magnetotellurycznych możemy przetransformować w krzywe prędkości pozornej jako funkcje głębokości wnikania pola elektromagnetycznego w głąb badanego ośrodka. Krzywe te możemy aproksymować odcinkami linii prostych, a kąty nachylenia poszczególnych odcinków względem osi głębokości i ich punkty przecięcia są ściśle związane z opornościami i miąższościami poszczególnych warstw geoelektrycznych. W przypadku ośrodków niejednorodnych 2D i 3D wielkością bezpośrednio związaną z granicami kompleksów geoelektrycznych jest pionowa składowa pola magnetycznego. Przez tę wielkość możemy wyrazić składowe wektora prędkości pozornej. W obszarach 1D wektor prędkości redukuje się do składowej pionowej, natomiast w obszarach niejednorodnych generują się również składowe poziome. Kąt nachylenia całkowitego wektora prędkości do poziomu i jego długość są ściśle związane z geometrią badanej struktury.
EN
Determination of impedances is necessary in order to eliminate some shortcoming of our knowledge about structures of the exciting source fields and their fickleness. The experimental impedances for induction soundings result from the impedance boundary conditions or heuristic models. The simplified models give just a rough idea of their domain of applicability. Impedances can depend on many factors, including the exciting field structures of several source types which are present in the period range of the mantle soundings (10&sup4;-4×10&sup8; s). The problem in the mantle investigations arises if impedances measured by different methods have to be jointly inverted in order to essentially prolongate the analyzed period range and hence to increase the reliability and depth of induction soundings on land. The subject of our work is an analysis of the known magnetotelluric and magnetovariation impedances to suggest a physically substantiated approach for their joint inversions.
PL
Przedmiotem artykułu są wyniki reinterpretacji trzech regionalnych profili magnetotellurycznych zlokalizowanych w polskich Karpatach Zachodnich. Badania magnetotelluryczne w tym obszarze wykonało Przedsiębiorstwo Badań Geofizycznych w Warszawie w latach 1997-2002, Prace reinterpretacyjne poprzedzone zostały przez analizę danych pomiarowych i ich reprocessing dla wybranych sondowań. W reinterpretacji wykorzystano głównie automatyczną inwersję 2D oraz inwersję 1D przy użyciu algorytmów Occama i LSQ. Dwuwymiarowe przekroje oporności opracowane na podstawie wyników automatycznej inwersji 2D oraz interpretacji 1D wykorzystano do konstrukcji modeli geoelektryczno-geologicznych. W interpretacji geologicznej wykorzystano także informacje geologiczne i wiertnicze oraz dane sejsmiki refleksyjnej.
EN
The subject of the paper includes results of reinterpretation of three regional magnetotelluric profiles located in the western part of the Polish Carpathians. Magnetotelluric survey in the area was made by the Geophysical Exploration Company, Warsaw, in the years 1997-2002. Reinterpretation works were proceeded by analysis of measurement data and reprocessing for some soundings. Automatic 2D inversion and 1D inversion with the use of Occam and LSQ algorithms were employed in reinterpretation process. 2D resistivity sections obtained based on results of automatic 2D and 1D data interpretation were used to construct geoelectric and geologic models. Geological and borehole data, and reflection seismic data were also applied in geological interpretation.
PL
W prezentowanej pracy przedstawiono próbę konstrukcji modelu 3D rozkładu oporności wykonaną dla obszaru Lachowice - Zawoja w polskich Karpatach Zachodnich. Rejon ten przecinają odcinki czterech regionalnych profili magnetotellurycznych. Znaczne zagęszczenie sondowań w tym obszarze pozwala na podjęcie przestrzennej analizy rozkładu oporności. Wyniki inwersji 1D, według algorytmu Occama, posłużyły do opracowania map oporności w cięciach poziomych, ilustrujących przestrzenne zróżnicowanie tego parametru. Do konstrukcji wstępnego modelu 3D wykorzystano wyniki interpretacji 1D i 2D. Wstępny model trójwymiarowy weryfikowany był i poprawiany poprzez wielokrotnie powtarzane modelowanie proste 3D. Podkreślić należy, że jest to pierwsza w Polsce próba wykorzystania modelowania 3D do analizy wyników sondowań magnetotellurycznych.
EN
An attempt of construction of 3D model of resistivity distribution made for Lachowice-Zawoja area in western part of Polish Carpathians is presented in the paper. The study area is cut by sections of four regional magnetotelluric profiles. Significant density of MT soundings in the area allows to undertake of the spatial analysis of resistivity distribution. Results of 1D Occam inversion were used to elaborate resistivity maps on selected depth levels that illustrates spatial resistivity differentiation. The entrance 3D resistivity model was made based on results of 1D and 2D sounding interpretation. The starting spatial model was verified and corrected by repeatedly recurred forward 3D modelling. It should be stated that this is the first attempt of use of 3D modelling for analysis of magnetotelluric soundings in Poland.
EN
Qualitative interpretation of the magnetotelluric results, based on the calculated polar diagrams and impedance tensors, is presented. The experimental data have been collected in Sterea Ellas region in Greece. Our interpretation allowed us to localize two conductive faults in the study area. One of them is probably a continuation of the west margin of the well-known Anatolian Fault.
6
Content available remote Regionalne badania magnetotelluryczne w polskich Karpatach zachodnich
PL
W latach 1997-2002 Przedsiębiorstwo Badań Geofizycznych w Warszawie wykonało regionalne badania magnetotelluryczne w polskich Karpatach. Prace pomiarowe skoncentrowane zostały w dwóch wyodrębnionych regionach - zachodnim i wschodnim. W części zachodniej sondowania magnetotelluryczne wykonano wzdłuż sześciu profili poprzecznych do głównej osi orogenu i dwóch równoległych do niej. Prace pomiarowe prowadzono za pomocą systemu MT-1 w zakresie częstotliwości 300-0.0005 Hz. W interpretacji ilościowej sondowań wykorzystano automatyczną inwersję 1D przy użyciu algorytmów Bosticka, Occama i LSQ. Dwuwymiarowe przekroje oporności opracowano na podstawie wyników automatycznej inwersji metodą LSQ. Do ich konstrukcji wykorzystano również wyniki automatycznej inwersji 2D metodą RRI, informacje geologiczne i wiertnicze oraz dane sejsmiki refleksyjnej. Najważniejszymi wynikami interpretacji są przekroje opornościowe i mapy strukturalne horyzontów wysokooporowych, związanych ze stropami podłoża mezopaleozoicznego i prekambryjskiego.
EN
The Geophysical Exploration Company, Warsaw, conducted regional magnetotelluric survey in the Polish Carpathians in the years 1997-2002. The measurements were concentrated in the western and eastern part of the investigation area. In the western part, magnetotelluric soundings were conducted on six lines perpendicular to the major orogen axis and two parallel to it. The measurements were made with the MT-1 system over a frequency range 300-0.0005 Hz. 1D automatic inversion with the use of Bostick, Occam and LSQ algorithms was employed in the quantitative interpretation. 2D resistivity sections were obtained based on results of automatic LSQ inversion. Results of 2D RRI inversion, geological and borehole data, and reflection seismic data were also applied. As a result of interpretation, resistivity sections and maps of high-resistivity horizons related with the top of the Meso-Paleozoic and Precambrian basement were obtained.
EN
Semi-detailed magnetotelluric sur vey was carried out in the area between Kamienica Dolna, Gogołów and Łopuchowa in the Polish Outer Carpathians. Magnetotelluric soundings were made along three lines located in a zone of the tectonic loop in the Carpathians' basement, which is reflected by the distribution of flysch outcrops. The sounding sites along profiles were ca 1.5 km apart. Magnetotelluric data were measured with the use of MT-1 system. To eliminate the effects of electromagnetic noise, the magnetic field remote reference was applied. Measurement data were processed using standard procedures of remote reference processing. Quantitative data interpretation was made with the use of 1D LSQ inversion. Initial geoelectric models were constructed basing on geological cross-sections obtained from surface and borehole data. 2D resistivity cross-sections, obtained from MT data interpretation, allowed the general structure of the flysch cover and its basement to be identified. Two major high-resistivity horizons were related to the top of Meso-Palaeozoic and Precambrian basement. A low- resistivity layer, related to the Lower Palaeozoic sediments, was interpreted.
EN
This paper considers a number of important aspects of the modern theory of electrical conductivity in application to magnetotelluric and magnetovariation methods. Attention is paid to insufficient argumentation of the known attempt to carry out deep geomagnetic sounding by applying a truncated version of accurate impedance ratio, as it was done by Rytov and Leontovich for harmonic fields of natural and controlled origin. We propose a new approach to the problem of processing data from magnetotelluric and magnetovariational methods of studies that is based on the concept of scalar conditions of impedance type that enables us to simplify and unify an experiment in which the measurements are completely and unambiguously conducted in terms of input impedance.
EN
Near-surface inhomogeneities cause the so-called static shift of amplitude magnetoelectric sounding curves for the magnetic polarization mode, and thus disturb the quantitative interpretation of magnetotelluric data. A number of possible methods for removing the static shift are presented in the geophysical literature; however, they all require data from other geophysical methods. Having known the electromagnetic field distribution in the given geoelectric cross-section, one can determine quantitative relationships between specific resistivities of the near-surface layer and calculated apparent resistivities. This is also an efficient way of removing static shift effects. The presented geoelectric model comprises a non-homogeneous near-surface layer resting over a 1D horizontally layered earth with arbitrary number of layers and the insulating base. The electromagnetic field distribution in such a model was calculated by means of the Fourier transformation with Dirichlet's boundary conditions. A plane electromagnetic wave with normal incidence was the source of electromagnetic field. The solutions are analytic expressions for the electric field at the Earth's surface. Moreover, asymptotic expressions for high-frequency and low-frequency field variations were calculated. In a high-frequency range, the impedance behaves as in the case of a uniform half-space. In a low-frequency range, the impedance depends most of all on the longitudinal conductivity of the layered earth.
EN
The quality of natural geoelectrical field observations was studied with special attention to long period signals. Two different types of electrodes, Ag/AgCl electrodes designed in Germany and Cu/CuSO4 electrodes designed in Poland, were used simultaneously to estimate the accuracy of the measurements. The difference between the recorded signals was found to be 9-150 miV (including noises of amplifiers of about 4 miV) for periods from 5 s to 2 days, respectively. These values show that the electrical fields induced by daily geomagnetic variations (expected signal is about 1-3 mV/km) can be registered reliably by 200-m dipoles, at least during summer time, in mid-latitudes by both considered types of electrodes, if other possible hindrances like the temperature influence on amplifier or the bad quality of cables are excluded. Main experiments have been carried out at the geomagnetic observatory Belsk in Poland, situated at the margin of the East-European Platform. This position may be responsible for the observed considerable difference between the electrical field components along and across the platform boundary for the harmonics of daily variation.
EN
Two electromagnetic (magnetotelluric and geomagnetic) soundings have been made in the region of Bornholm Island (Denmark). The results of analysis suggest a very complicated, locally three-dimensional character of the conductivity distribution; however, we interpret the outcome of the soundings in terms of a two-dimensional model, additionally using other published results from the region under study. From the interpretation it follows that Bornholm belongs to the East European Platform, whereas from the south and from the west an influence of large sedimentary basins appears. The depth of the sedimentary basin adjoining the island to the south is probably much deeper than estimated so far. Also, geological structures at greater depth in this region of the Baltic Sea may be much more complicated than it could be assumed from a simple extrapolation of data obtained from the surface geology.
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