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1

New induction sounding tested in Central Europe

Semenov V., Ladanivskyy B., Nowożyński K.

Acta Geophysica

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2011

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Vol. 59, no. 5

815-832

EN

The hourly data of nine geomagnetic observatories situated in Central Europe have been analyzed using the generalized magnetovariation (GMV) method designed recently for induction soundings of inhomogeneous media. In this method, impedance is one of transfer functions in the differential relation between spectra of the magnetic components and their derivatives. The peculiarity of this impedance is its correspondence to the magnetotelluric one estimated from the linear relations. Three transfer functions have been estimated simultaneously for data of geomagnetic observatories, using three different routines working in the period range from three hours up to two days. Noises in the source field components have been compared with noise in the estimated plane field divergence. The multivariate errorsin-variables method was used to extract spatially and temporally coherent geomagnetic field structure from the partially incoherent geomagnetic variations. This method allows estimating reliably impedances and gradient tippers for each observatory, taking into consideration the Earth’s sphericity. The obtained responses have been used for induction soundings and for detecting a deep inhomogeneity in the region.

2

Impedances for induction soundings of the Earth's mantle

Semenov V., Shuman V.

Acta Geophysica

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2010

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Vol. 58, no. 4

527-542

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.

3

Electrical structure of the upper mantle beneath Central Europe: results of the CEMES project

Semenov V., Jóźwiak W., Pęk J., Adam A., Ladanivskyy B., Logvinov I., Pushkarev P., Vozár J.

Acta Geophysica

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2008

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Vol. 56, no. 4

957-981

EN

In the years 2001–2003, we accomplished the experimental phase of the project CEMES by collecting long-period magnetotelluric data at positions of eleven permanent geomagnetic observatories situated within few hundreds kilometers along the south-west margin of the East European Craton. Five teams were engaged in estimating independently the magnetotelluric responses by using different data processing procedures. The conductance distributions at the depths of the upper mantle have been derived individually beneath each observatory. By averaging the individual cross-sections, we have designed the final model of the geoelectrical structure of the upper mantle beneath the CEMES region. The results indicate systematic trends in the deep electrical structure of the two European tectonic plates and give evidence that the electrical structure of the upper mantle differs between the East European Craton and the Phanerozoic plate of west Europe, with a separating transition zone that generally coincides with the Trans-European Suture Zone.

4

New evidence of the anomalously conductive mantle beneath the Tornquist-Teisseyre zone in Poland

Semenov V., Jankowski J., Jóźwiak W.

Acta Geophysica Polonica

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2002

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Vol. 50, nr 4

517-526

EN

In the summer of 2000, ten deep magnetotelluric soundings were carried out in northeastern Poland. These observations were organized along the 250-km profile crossing the East European Craton (EEC) and the Tornquist-Teisseyre zone (TTZ) up to the Holy Cross Mountains. These data extended by the ultralong period ones obtained at the geomagnetic observatories Minsk (at the EEC) and Belsk (at the TTZ) have allowed estimating the mantle conductivity down to 1000 km depths. The satisfactory analysis of these data has been made by means of the 1D spherical inversions in the context of the reliable conductance distributions. On the background of relatively low conductances of sediments (10-600 S) along the profile it has been revealed that the upper mantle conductivity is increasing from the EEP towards the TTZ. The main conductive property bas been detected at the asthenosphere depths of about 150-300 km beneath the TTZ. A good correlation exists between the present result and the previous one obtained beneath the parallel profile situated 150 km towards the northwest.

5

Geoelectrical structure of the Baltic shield and its vicinity through the lithosphere and mantle

Zhamaletdinov A., Semenov V., Shevtsov A., Tokarev A., Bobachev A.

Acta Geophysica Polonica

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2002

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Vol. 50, nr 4

583-606

EN

The structure of the regional "normal" model of electrical conductivity of the Earth's crust and upper mantle for the Baltic shield and its vicinity has been studied by combination of several methods of electromagnetic sounding using both controlled sources: DC and frequency methods, soundings with the use of Extra Low Frequency (ELF) antenna "Zevs" and with impulsive magneto-hydro-dynamic (MHD) generator of 80 MW power; and natural electromagnetic sources (magnetovariation and partly deep magnetotelluric soundings). Such a combination gave, for the first time, a possibility to analyze the regional responses in the ultra wide period range, from 10(-6) s up to 14 months. These data have allowed us to construct a regional 1D model of conductivity distribution in the lithosphere and mantle, in the depth range from several tens of meters up to 2000 km. The primary peculiarity of the obtained section is the very high value of the Earth crustal electrical resistivity (until 5x10(5) Om) and the high transversal resistance of the lithosphere as a whole (more than 10(10) Om2). Against of this background, an intermediate layer of relatively low resistivity (about l0(4) Om) is revealed in the depth range of 3-9 km. The resistivity of the upper mantle reduces gradually and becomes seven orders of magnitude less down to the 2000 km depth. Against this gradient background, another intermediate conductive layer has been detected at about 150-200 km depth. This layer, with conductance of 2-4 kS, could be associated with the asthenosphere. The mid-mantle conductive layer has 450 kS conductance at about 650-km depth, which is in agreement with the known data of global geomagnetic sounding. The very high resistance of the lithosphere accompanied by the thin and ill-conductive sediments can make abnormal lateral influence on magnetotelluric data from both crustal anomalies of electrical conductivity and sea basins surrounding the Baltic shield.

6

Tests of equipment for electric field observation in deep magnetotelluric soundings

Semenov V., Hempfling R., Junge A., Marianiuk J., Schmucker U.

Acta Geophysica Polonica

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2001

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Vol. 49, nr 3

373-388

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.

7

Reconnaissance electromagnetic soundings on Bornholm

Jankowski J., Ernst T., Jóźwiak W., Lewandowski M., Abrahamsen N., Semenov V.

Acta Geophysica Polonica

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2001

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Vol. 49, nr 3

361-372

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.

8

Deep geoelectrical structure estimation in the West Antarctic

Jóźwiak W., Semenov V.

Acta Geophysica Polonica

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1998

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Vol. 46, nr 1

61-67

EN

Geomagnetic data from the Polish Antarctic Station Arctowski were analyzed to estimate the geoelectrical mid mantle structure for the West Antarctic. Response functions were obtained at periods from 11 to 65 days by using Dst variations. The original inversion in the spherical geometry was applied to establish the 1D structure. This first deep geoelectrical sounding in the Antarctic has shown the existence of a mid mantle conductive layer, of 850 kS conductance, at a depth of about 700 km. This result has confirmed the idea of the global character of that layer over the Earth.