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1 2002

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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

2002

Vol. 50, nr 4

583-606

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.