Basement depths of 3D basins, estimated from 1D magnetotelluric inversion

Abstract

Detailed magnetotelluric soundings along the Hungarian section of the CEL-7 seismic profile (SW Hungary, where a series of very deep 3D sedimentary basins is known from various geophysical-geological investigations) enabled us to produce magnetotellurics-based estimations for the topography of the high-resistivity basement. Both TM and TE modes were used for 1D inversion, and the resulting depth values were compared to the depths, taken from the “Pre-Tertiary Basement Contour Map of the Carpathian Basin” by Kilényi and Šefara (1989), called as K-S depths.

The first conclusion is that the observed depth differences do not depend significantly on the sediment resistivity. At the same time, there is a significant difference between the magnetotelluric- and the K-S depths over the Balaton line (the most important tectonic (fracture) line crossing the MT profile). The TE mode-based inverted depth values are smaller, the TM mode-based depth values are larger than the K-S depth values. As the adjustment distance in case of the TE mode is, of course, larger than in case of the TM mode, the distortion in TE mode appears in a larger area.

Apart from this distortion, the magnetotelluric depth values, especially the TM mode-based ones, are surprisingly very well approximate the K-S depth values. In case of the TM mode 76 percents of the differences are less than 600 m. (The average depth is nearly 4000 m). In case of the TE mode such a small deviation is obtained only in 52 percents of the total sounding sites, due to the significant TE effect of the Balaton line (a deep high-conductivity tectonic line).