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
Geophysics
Final Report Abstract
The Cheb Basin, the 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 the International Continental Scientific Drilling Program (ICDP) project "Drilling the Eger Rift" in which several geophysical surveys are currently carried out in this area to image the topmost hundreds of meters to assess structural situation, as existing boreholes are not sufficiently deep to characterize it. Two large-scale (6.7 km and 5.5 km) and one medium-scale (1.6 km) geoelectrical surveys using a special dipole-dipole measurement strategy of electric resistivity tomography (ERT) were carried out between 2017 and 2019 in order to image fluid-relevant or fluid-influenced structures, as electrical resistivity is sensitive to the presence of conductive rock fractions such as clay minerals, liquid fluids and also metallic components. The 6.7 km long profile crosses the Hartoušov mofette field, which is the location for the ICDP fluid monitoring drill, the 5.5 km cross the U Mostku spring located at the PPZ and the MLF. A profile with a higher spatial resolution (length of 1.6 km and dipole spacing of 20 m) was placed across the trace of the MLF. For the large-scale experiments, we used permanently placed data loggers for voltage measurements in conjunction with moving high-power current sources for generating sufficiently strong signals that could be detected all along the profile with 100 to 150 m dipole spacings. After extensive processing of time series for voltage and current using a selective stacking approach, the pseudo-section is inverted which results in a resistivity model that allows reliable interpretations depths of up than 1000 m. The subsurface resistivity image, that covers the documented CO2 degassing zone Hartoušov reveals the deposition and transition of the overlying Neogene Vildštejn and Cypris formations very clearly, but also shows a significantly high conductive basement of phyllites and granites to depths of several hundreds of meters with a trough-shaped structure between PPZ and MLF. This is contrary to the previous assumptions of a “solid”, nearly unweathered basement, but instead supports the theory of deep weathering and alteration due to the physic-chemical alteration of the basement rocks due to the ascending magmatic fluids in the tectonically stressed basement. However, distinct, narrow pathways for CO2 ascent are not observed with this kind of setup due to limits in the resolution. This also hints at rather wide degassing structures over several kilometers within the crust instead. In case of the MLF, we can observe typic features of the fault which controls the eastern limit of the Cheb Basin. In general, the resistivity decreases and the chargeability increases, underlining the hypothesis of mineral alteration into clay minerals caused by the impact of fluids along a fault, which has been suggested before. In this case, clay minerals at fault planes might act like as a lubricant so hat tectonic stress might not build up at faults to critical levels, but can then be released via low magnitude earthquake swarms. The MDR-Fernsehen “Sachsenspiegel” reported about our survey in June 2017.
Publications
- (2017): Architecture and temporal variations of a terrestrial CO2 degassing site using electric resistivity tomography and selfpotential. Int. J. Earth Sci (Geol. Rundsch.). 106, 2915-2926
Nickschick, T., Flechsig, C., Meinel, C., Mrlina, J., Kämpf, H.
(See online at https://doi.org/10.1007/s00531-017-1470-0) - (2017): Drilling into an active mofette – pilot hole study of the impact of CO2-rich mantle-derived fluids on the geo-bio interaction in the western Eger Rift (Czech Republic). Sci. Drill, 23, 13-27
Bussert, R., Kämpf, H., Flechsig, C., Flores, H., Hesse, K., Nickschick, T., Vylita, T., Wagner, D., Umlauft, J., Wonik, T., Alawi., M.
(See online at https://doi.org/10.5194/sd-23-13-2017) - (2018) Influence of CO2 Degassing on the Microbial Community in a Dry Mofette Field in Hartoušov, Czech Republic (Western Eger Rift). Front. Microbiol. 9:2787
Liu Q, Kämpf H, Bussert R, Krauze P, Horn F, Nickschick T, Plessen B, Wagner D and Alawi M
(See online at https://doi.org/10.3389/fmicb.2018.02787) - (2018): A remote-control datalogger for large-scale resistivity surveys and robust processing of its signals using a software lock-in approach, Geosci. Instrum. Method. Data Syst.,7, 55–66
Oppermann, F. and Günther, T.
(See online at https://doi.org/10.5194/gi-7-55-2018) - (2019): Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an ICDP monitoring drill site to image fluid-related structures. Solid Earth, 10, 1951–1969
Nickschick, T., Flechsig, C., Mrlina, J., Oppermann, F., Löbig, F., and Günther, T.
(See online at https://doi.org/10.5194/se-10-1951-2019)
