Large-scale ERT in the Eger rift zone (W bohemia) at proposed ICDP fluid monitoring boreholes for imaging fluid-relevant structures
The western Eger rift area (NW-Böhmen) is characterized by ongoing magmatic processes in the intra-continental lithospheric mantle. These lead to repeated earthquake swarms focused in the area of the crossing between the Marianske Lazne fault and the Pocatky-Plesna fault zone. Related to this is also uprise and degassing of CO2-rich fluids into so-called Mofettes. The ICDP-Initiative "Drilling the Eger Rift" tries to get insight into the origin of the earthquakes and the uprise of the CO2 by several up to 500m deep boreholes.
Accompagnying, there are several detailed projects, among them geophysical investigations with magnetotellurics (GFZ Potsdam), seismics (TU Freiberg) and electrical resistivity tomography (Uni Leipzig + LIAG) that aim at understanding the (near-surface) geology and its relevance for the fuild pathways.
In the project
Large-scale geoelectrical survey in the Eger Rift zone (W-Bohemia) at proposed PIER-ICDP fluid monitoring drill sites to image fluid-related conductivity structures
partners from University of Leipzig and LIAG carry out large-scale geoelectical measurements with a planned depth penetration of ca. 1km.
In 2017 and 2018, two campaigns with large-scale dipole-dipole experiments have been carried out. The profile 2017 lead in east-west direction (perpendicular to the Pocatky-Plesna fault zone) with the midpoint at the mofette in Hartousov, where also the ICDP drilling is located, to the Marianske Lazne fault at Kacerov. The profile 2018 leads in southwest-northeastern direction (perpendicular to the Marianske Lazne fault), from Devin to Kopanina (s. Fig.).
We deployed unpolarisable Ag/AgCl probes in a distance of 100m and registered voltage time series with data loggers. We used self-developed three-channel data loggers (Oppermann & Günther, 2018) as well as Reftek Texan-125A. At almost all dipoles we injected currents (in the mean 10A) using the high-current source of LIAG and Uni Leipzig. The time series were processed with the method of selective stacking. The resulting currents and voltages were inverted with the software BERT into 2D subsurface models (s. Fig.) and interpreted along with borehole information and small-scaled ERT (Nickschick et al. 2017).
As the MLFZ was hardly covered, we carried out another profile crossing the MLFZ centrally in Kopanina, using a ABEM Terrameter LS2 instrument and electrode distances of 20m. The 2017 experiment and its results were described in detail by Nickschick et al. (2019). The other data are not yet fully interpreted and will be published in the future.
Bussert, R., Kämpf, H., Flechsig, C., Flores, H., Hesse, K., Nickschick, T., Vylita, T., Wagner, D., Umlauft, J., Wonik, T., Alawi., M. (2017): Drilling into an active mofette – pilot hole study of
the impact of CO 2 -rich mantle-derived fluids on the geo-bio interaction in the western Eger Rift (Czech Republic). Sci. Drill, 23, 13-27. https://doi.org/10.5194/sd-23-13-2017.
Nickschick, T., Flechsig, C., Meinel, C., Mrlina, J., Kämpf, H. (2017): Architecture and temporal variations of a terrestrial CO 2 degassing site using electric resistivity tomography and self-
potential. Int. J. Earth Sci (Geol. Rundsch.). 106, 2915-2926. https://doi.org/10.1007/s00531-017-1470-0.
Flechsig, C. & Günther, T. (2019): 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. Final report, DFG, 15p.