GEOELECTRICAL INVESTIGATIONS IN THE CHEB BASIN/W-BOHEMIA: AN APPROACH TO EVALUATE THE NEAR-SURFACE CONDUCTIVITY STRUCTURE

The Cheb Basin, located in the western Eger (Ohre) Rift, is part of the European Cenozoic Rift system. Although presently non-volcanic, it is the most active area within the European Rift with signs of recent geodynamic activity like emanations of mantle derived CO2, and the repeated occurrence of swarm earthquakes, which are common features in active volcanic regions. It is assumed that the fluids, uprising in permeable channels, play a key role for the genesis of these earthquake swarms. An image of the distribution of the electrical conductivity (resp. resistivity) in the upper crust can give information about the fluid distribution since the electrical patterns reflect pathways of fluids and fluid properties like ionic content. This was the motivation to start both a regional-scale direct current (DC) geoelectrical test covering the Cheb Basin area and several local-scale near surface investigations inside the basin at the seismically active faults Pocatky-Plesna Zone (PPZ) and Marianske Lazne Fault Zone (MLF) near Novy Kostel. It was the research idea to test electrical tomography's ability to detect faults and tectonic deformation in a complex geological environment and to identify characteristic features of these faults. The more methodically justified regional field test mainly aimed at clarifying the opportunity to trace DC electrical signals in an area with known high industrial noise in a sufficient quality. A field set-up with a range of 15-20 km is necessary for an investigation depth of 4-5 km in case of DC geoelectrics. A new developed inversion strategy for sparse electrical data sets allows for a first (coarse) model of resistivity distribution. The near surface investigations in prominent fault zones of the Cheb Basin give more detailed information about the structure of fault zones, divide the sedimentary units in different resistivity zones and detect vertical displacements in the quaternary formations.