Research study for a geoelectrical pre-site survey of the drilling location within the Eger Rift - Investigation of the subsurface electrical conductivity distribution
Final Report Abstract
Within the frame of a feasibility study we demonstrated the potentials and constraints of the largescale Electrical Resistivity Tomography method applying in the Eger Rift / NW-Bohemia. The occurrence of fluids affects the electrical conductivity of the rocks and are often linked to fault zones. Hence, we propose the application of geoelectrical methods to determine significant subsurface fault-related fluid structures, which were suggested as conductive anomalies. The study result comprehend an essential potential concerning further research. The most remarkable outcome of the DC test is the large signal tracing in spite of the strong industrial noise level in that area. The voltage differences of the artificial square wave signals were successfully detected with maximum spacings between current injection dipole and potential dipole of approx. 15 km using sophisticated data processing routines. This wide spacing enables a target depth of investigation of 4-5 km. In case of sparse data-set obtained in the field test we developed an adapted inversion scheme called region concept based on BERT (Boundless Electrical Resistivity Tomography). This region concept allows to subdivide the inversion domain into a few structural units (parameter region blocks) based on a priori knowledge derived from previous supplemental geological and geophysical investigations. This inversion feature is now available also for large data sets. Thus, the calculated region model of NW-Bohemia will be a necessary basement for the design of actually large-scale DC investigations to optimize the field set-up in order to reach an appropriate resolution of the target structures. The present resistivity model is relatively coarse, but it can be improved by incorporating the results of further geoelectrical investigations and additional structural information received from seismic, seismology, geology etc. A second part of the research work was focused on the development of an applicable field set-up depending on the geological situation and environmental restrictions due to the field accessibilty. An existing test data set obtained in a large-scale experiment in Rittsteig/Bavaria with a 2D field setting (the geological situation is characterized by similar steeply dipping faults with strong resistivity contrasts and topography effects) were reinterpreted to gain accessory knowledge for a convenient array design. The DC-array design for further large-scale investigations has to represent a compromise between the required target depth / resolution (constrained by depth extension and width of faults), the field accessibility (restricted by infrastructure, agricultural usage and accessibility permissions) and logistical efforts (limiting factor for the number of current/voltage dipoles). Near surface DC-geoelectrical surveys across the main faults within the Cheb Basin (Marianske Lazne Fault and Pocatky-Plesna Fault zone) imaged the heterogeneous sediment structures down to a depth of 80 m. The 2-D and 3-D resistivity models represent the occurrence and width of the faults in the shallow subsurface by means of conductive anomalies, variations in sediment thickness and vertical layer displacement. Additional self-potential measurements support these fault models and confirm the link between the faults and the fluid activity along these permeable pathways.
Publications
- (2008): Ergebnisse tiefengeoelektrischer Untersuchungen in NW- Böhmen. XII. Seminar “Hochauflösende Geoelektrik”, Leipzig, 8.-10.10.2008
Schütze, C., Flechsig, Ch., Rücker, C.
- (2008): Erkundung struktureller Eigenschaften einer magmatischen Entgasungszone im Eger Rift/NW-Böhmen - Ergebnisse geoelektrischer und sedimentologischer Untersuchungen. 68. DGG-Jahrestagung 3.-6.3.2008, Freiberg
Schütze, C., Flechsig, Ch., Bussert, R., Rechner, J.
- (2008): Research study for a geoelectrical pre-site survey of the drilling location within the Eger Rift/W-Bohemia - Investigation of the subsurface electrical conductivity distribution. IODP/ICDP Kolloquium Hannover, 12.-14.03. 2008
Flechsig, Ch., Schütze, C.
- (2008): The Hartoušov Mofette Field in the Cheb Basin, Western Eger Rift (Czech Republic): a Comperative Geoelectric, Sedimentologic and Soil Gas Study of a Magmatic Diffuse Degassing Structure. American Geophysical Union, AGU 2008 Fall Meeting San Francisco, California
Flechsig, Ch., Schütze, C., Bussert, R.
- (2009): GIMLi – eine freie und flexible C++-Bibliothek zur Inversion und Modellierung in der Geophysik. 69. DGG-Jahrestagung, 23.3. - 26.3.09 Kiel
Rücker, C., Günther, T.
- (2009): Research study for a geoelectrical pre-site survey of the drilling location within the Eger Rift / NW-Bohemia - Investigation of the subsurface electrical conductivity distribution. ODP/ICDP Kolloquium Potsdam, 16.-18.03. 2009
Schütze, C., Rücker, C., Flechsig, Ch.
- (2009): The Cheb Basin, Western Eger Rift (Czech Republic): a Geophysical Approach to Investigate a Magmatic CO2 Degassing Structure. 69. DGG-Jahrestagung, 23.3. - 26.3.09 Kiel
Flechsig, Ch., Schütze, C., Rücker, C.
