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2009-05-21 – SEMINAR by Dr. Jan Vozar: Electromagnetic investigations in Central Europe and the numerical simulation of the induction soundings

21 May, 2009 (16:00 GMT), 5 Merrion Square, Dublin 2.

Speaker: Dr. Jan Vozar (DIAS).
Title: Electromagnetic investigations in Central Europe and the numerical simulation of the induction soundings.

Abstract:

During the years 2002-2006, the short-period and long-period magnetotelluric and magnetovariational data were collected and interpreted in order to accomplish the national MT CELEBRATION project and the international CEMES project in Central Europe.

Five 2D MT shallow profiles and ten deep 1D electromagnetic soundings composed from MT and magnetovariational data were performed and 3D image is presented as the result for Slovak region. The results from 1D inversions give the estimation of the lithosphere thickness from 100km to 160km for the investigated region of western Slovakia. The 2D modelling in the eastern Slovakia confirmed the existence of the strong conductive zones represented by the Carpathian Conductivity Anomaly (CCA) and the sedimentary basins with strong heat flow. The 3D image of geoelectrical structures obtained from these profiles show the border between Inner and Outer Carpathians.

In the frame of the CEMES project, five international teams were engaged in collecting the long-period magnetotelluric data at positions of eleven permanent geomagnetic observatories situated within the region of the longitude of a few hundreds kilometres along the south-west margin of the East European Craton. They estimated the magnetotelluric responses independently 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 the evidence that the electrical structure of the upper mantle varies between the East European Craton and the Phanerozoic plate of west Europe. There is a separating transition zone that coincides with the Trans-European Suture Zone.

The forward spherical 3D EM modelling of Central Europe and the numerical simulation of EM sounding methods is presented with incorporated improved Earth’s surface conductance map. On the ground of the obtained result, we have suggested to apply the horizontal spatial gradient method for sounding of inhomogeneous media instead of the old approach which was originally designated for the laterally homogeneous media. The advantage of this method (in comparison with magnetotelluric one and with the old approach) lies in the increased accuracy, as it has been established by the numerical simulations of responses for axially symmetric (spherical 2D) and 3D models with known mantle and surface properties. The responses were obtained from the modelled electromagnetic fields induced by the corresponding sources on the spherical Earth with the inhomogeneity in the upper mantle. The simulated apparent resistivities and the impedance phases were evaluated for the long periods from tenths up to 1024 days for different deep sounding methods. It was also found that the generalized magnetovariational method (GMV) produces the response functions correctly reduced to the magnetotelluric ones contrary to the traditional approach if only the Earth’s surface is homogeneous (“shift effect” is absent!). Three independent theoretical approaches are known for this GMV method [Gulielmi & Gokhberg, 1987; Shuman & Kulik, 2002; Schmucker 2003]. The first approach based on Rytov’s (1940) theory for the radio-waves is inaccurate for our long period variations. The third approach is not taken into consideration the “anomalous” field for estimation of divergence of the field that is rather far from reality (inhomogeneities are situated in the laterally homogeneous Earth).