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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Mar 2019

Research article | 06 Mar 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ICDP monitoring drill site to image fluid-related structures

Tobias Nickschick1, Christina Flechsig1, Jan Mrlina3, Frank Oppermann2, Felix Löbig1, and Thomas Günther2 Tobias Nickschick et al.
  • 1Institute for Geophysics and Geology, Leipzig University, Talstrasse 35, 04103 Leipzig, Germany
  • 2Leibniz Institute for Applied Geophysics, Stilleweg 2, 30655 Hannover, Germany
  • 3Institute of Geophysics CAS, Boční II 1401, 141 31 Praha 4, Czech Republic

Abstract. The Cheb Basin, a region of ongoing swarm earthquake activity in the western Czech Republic, is characterized by intense carbon dioxide degassing along two known fault zones – the N-S-striking Počatky-Plesná fault zone (PPZ) and the NW-SE-striking Mariánské Lázně fault zone (MLF). The fluid pathways for the ascending CO2 of mantle origin are subject of an International Continental Scientific Drilling Program (ICDP) project in which several geophysical surveys are currently carried out to image the near-surface geologic situation, as existing boreholes are not sufficiently deep to characterize the structures.

As electrical resistivity is a sensitive parameter to the presence of low-resistivity rock fractions as liquid fluids, clay minerals and also metallic components, a large-scale dipole-dipole experiment using a special type of electric resistivity tomography (ERT) was carried out in June 2017 in order to image fluid-relevant structures. We used static remote-controlled data loggers in conjunction with high-power current sources for generating sufficiently strong signals that could be detected all along the 6.5 km long profile with 100 m and 150 m dipole spacings. Extensive processing of time series and apparent resistivity data lead to a full pseudosection and allowing interpretation depths of more than 1000 m.

The subsurface resistivity image reveals the deposition and transition of the overlying Neogene Vildštejn and Cypris formations, but also shows a very conductive basement of phyllites and granites that can be attributed to high salinization or rock alteration by these fluids in the tectonically stressed basement. Distinct, narrow pathways for CO2 ascent are not observed with this kind of setup which hints at wide degassing structures over several kilometers within the crust instead. We also observed gravity/GPS data along this profile in order to constrain ERT results. Gravity clearly shows the deepest part of the Cheb Basin along the profile, its limitation by MLF at NE end, but also shallower basement with an assumed basic intrusion in SW part of profile. We propose a conceptual model in which certain lithological layers act as caps for the ascending fluids, based on stratigraphic records and our results from this experiment, providing a basis for future drills in the area aimed at studying and monitoring fluids.

Tobias Nickschick et al.
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Tobias Nickschick et al.
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Publications Copernicus
Short summary
An active CO2-degassing site in the western Eger Rift/Czech Republic was investigated by using a 6.5 km long geophysical survey using a specific, large-scale geoelectrical setup, supported by shallow geoelectrical surveys and gravity measurements. The experiment reveals unusually low resistivities in the sediments and basement below the degassing area and provides a base for a custom geological model of the area for a future 400 m deep research drilling in this area.
An active CO2-degassing site in the western Eger Rift/Czech Republic was investigated by using a...