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1 Acta Geophysica Polonica

1 2002

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Modeling of conductivity structures generating anomalous induction at the eastern margin of the Bohemian Massif and the West Carpathians

Cerv V., Kovacikova S., Pek J., Pecova J., Praus O.

Acta Geophysica Polonica

2002

Vol. 50, nr 4

527-545

Coordinated field campaigns were organized in close co-operation with the Polish and the Slovak colleagues to obtain geomagnetic variation data at 150 temporary field sites situated along profiles traversing the West Carpathians and the eastern part of the Bohemian Massif. Geomagnetic transfer functions and induction vectors (arrows) were calculated from the variation data. Two zones of the anomalous induction were suggested across the region. The West Carpathian anomaly (WCA), manifested by full reversals of strike-perpendicular induction arrows, is evidently a quasi-linear inductive feature with 2-D internal structure. The anomaly at the eastern margin of the Bohemian Massif (BMA) is defined by fast changes of induction arrow azimuths, but contrary to 2-D setting, by large strike-parallel induction arrows suggesting 3-D effects. Generating anomalous vertical magnetic field maps for the hypothetical primary field of different orientations, we found the NNE/SSW direction of hypothetical primary field, at which the strongest anomalous response appeared. To understand 3-D conductivity effects in the area of BMA at the eastern margin of the Bohemian Massif, possible distortions were analyzed. At long periods in quasi-static approximation, a thin sheet can replace anomaly source with anomalous conductivity and the equivalent current systems in thin sheet are calculated across the whole region. Equivalent current systems were obtained both for the thin sheet in free space and for the thin sheet buried in a medium with finite resistivity. The depth of the thin sheet corresponding to the top level of the anomaly source was estimated at 15-18 km and at 10-12 km for the WCA and BMA zones, respectively. Using minimum gradient support and maximum smoothness stabilizers for the Tikhonov functional in the inversion procedure, we computed a model of conductance distribution within the unimodal thin sheet located at a depth of 10 km and surrounded by a uniform medium.