Large-scale geoelectrical survey in the Eger Rift zone (W-Bohemia) at proposed PIER-ICDP fluid monitoring drill sites to image fluid-related resistivity structures

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The western Eger rift area in West Bohemia/Vogtland (Central Europa) is characterized by persistent geodynamic activity in the intra-continental lithospheric mantle, that take place in absence of any presently active volcanism at the surface. Beside other geodynamic phenomena are repeated intra-plate earthquake swarms and high CO2 flow of mantle origin are the most characteristic features. In order to get a better understanding of the underlying processes especially the earthquake-fluid interactions, an array of observation drillholes of appr. 500 m depth will be placed and serve for seismological, fluid and geophysical monitoring. Our proposed geoelectrical pre drilling research project concentrates on drill holes locations with documented CO2 degassing along nearby fault zones, which are prioritized for fluid observation. We propose the application of electrical resistivity tomography on four profiles to determine the subsurface resistivity distribution down to the maximum depth of the planned drillholes (~500 m).The electrical resistivity is very sensitive to the presence to low-resistive fluids and fluid-rock interaction zones, which were suggested as conductive anomalies. Results together with joint geophysical interpretation and constraint inversion with structural information of seismic investigations aim to image fluid pathways and the feeding system of the CO2 degassing areas, to image the main resistivity characteristics of tectonic features, and to give a drill site characterization. The resistivity model can also be used as reference model for future measurements to monitor resistivity changes caused by fluids.