Direct current resistivity measurements can often provide significant contributions in the investigation of geological structures. In order to reach large penetration depths, e.g. for imaging fault zones, we carry out large-scaled dipole-dipole experiments with a strong current source and self-developed data loggers.
Large-scale resistivity investigations have been a unique feature of the institute by tradition. Since the 1970ies, lots of Schlumberger soundings with half-spread up to 5km have been measured for different geological problems. Alternatively, one can carry out large-scale dipole-dipole experiments in order to obtain 2D/3D images of the subsurface resistivity. Both current injection and potential measurements are done with dipoles of limited length, but large distances. For current injection we use a self-developed high-current source that can, with a maximum power of 40kVA and maximum voltage of 1500V, drive up to 50A current into the ground so that the signals can be detected in large distances and thus penetration depths.
The potentials are measured continuously with data loggers, in former time using MT data loggers. For better control of the measurements and improved data quality we developed a low-cost, remote-control data logger with three measuring channels and sampling rates up to 1kHz. For the analysis of the signals we use, additionally to the stacking and Fourier transform methods, a robust method based on the Lock-in approach (Oppermann & Günther, 2018).
For generating 2D/3D models we apply modern modelling and inversion methods based on Finite Element computations on unstructured meshes. Synthetic models and sensitivity studies insight into the anomalies and the resolution properties is gained. Special consideration is to regard the apparent resistivity as tensor property, derived from each two current and potential directions. The institute cooperates closely with the Universities of Leipzig and Frankfurt and carries our joint experiments. In several projects and campaigns the workgroup contributed to 2D and 3D imaging which is summarized in the following:
The first campaign was done in spring 2006 above the Cuxhaven-Bremerhavener buried valley in the frame of the Burval project with the aim of imaging the sediment distribution inside and outside of the quarternary valley. At 21 stations three-channel signals of 30 current injections were registered. At that time we used a correlation approach for determining the resistance. Result is a distribution of clay, freshwater and saltwater far beyond the penetration depth of the airborne EM (Schünemann et al., 2017).
Similar arrays were used 2006 on the water reservoid of the Kinzig (between Fulda and Frankfurt) with the aim of imaging fault zones between Vogelsberg and Spessart mountains. For the first time, underwater stations have been operated. The results are subsumed in the PhD thesis of Agricola (2017).
Vogelsberg is the only shield volcano and at the same time the biggest volcanic dome in Europe. Aim of the compaign in 2008 was to map a possible endogene lava dome at the boundary of the caldera. 21 receiver stations with 2-3 measuring directions were used along with 13 current stations with each two directions and currents of up to 30A. For the first time, a present scientific borehole (300m deep) could be used for current injections. Results (Agricola et al., 2017) showed valuable insight into the different volcanic events.
In the frame of the geothermal project gebo-G2, we carried out large-scale ERT and seismics at the eastern fault of the Leine grabel in 2010 and 2011. For the first time, seismic reflections have been introduced as structural information into the inversion process, yielding a significantly improved subsurface image (Günther et al., 2011).
In the frame of the BFBF-funded projects SaMoLEG (Saltwater Monitoring with Long Electrode Geoelectrics), in 2013 and 2014 we carried out two 3D experiments on a 4x3km large area, using a lot of steel-cased boreholes as long electrodes (Günther et al., 2015). As result we could image the saltwater intrusion in the Briesen are three-dimensionally (Ronczka, 2015). This was the first real application of the self-developed data logger (Oppermann & Günther, 2018).
In the frame of the junior research group subrosion we carried out a 5km long 2D profile in the subrosion basin Esperstedter Ried with seismics and large-scale ERT, in order to image the southern Kyffhäuser fault and the sediments influences by saltwater rise.
For pre-investigating the test area Schleiz of the BMBF-funded project DESMEX, we carried out two 4.5km long large-scale ERT profiles in 2015 und 2016, using each 36 electrodes spaced by 125m. Both measurements were then combined to a 7.5km long profile (Günther et al., 2017).
Aim of the DFG-funded 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" (EgerERT) is to image fluid-relevant structures in the area of the Pozatky-Plesna fault zone, together with the University of Leipzig. In 2017 a 6km long profile was measured that is planned to be analyzed along with seismic structural information. In 2018, another profile is planned.