Prospection of faults in the Southern Erftscholle with Refraction Seismics and Electrical Resistivity Tomography

As part of the Lower Rhein Embayment (LRE), the Southern Erft block is characterized by a complex tectonic setting that may influence hydrological and geological conditions on a local as well as regional level. The area presented in this study is located near Euskirchen in the south of North Rhine-Westphalia and traversed by several NW-SE-oriented fault structures. Past studies based on the lithological description of borehole cores and hydrological measurements stated that the present faults affect the local groundwater conditions throughout the targeted area. However, since the tectonic structures were located based on a sparse foundation of geological borehole data, the results include considerable uncertainties. Therefore, it was decided to re-evaluate and refine the assumed fault locations by conducting geophysical measurements.

Seismic Refraction Tomography (SRT) as well as Electrical Resistivity Tomography (ERT) was performed along seven measurement profiles with a length of up to 1.1 km. To allow a sufficient degree of model resolution, the electrode spacing was set to 5 m and halved for areas proximate to assumed fault locations. The geophone spacing was set to 2.5 m for all conducted seismic surveys. A large portion of data processing and inversion was performed with the open-source software package pyGIMLi (Rücker et al., 2017). In addition to compiling individual resistivity and velocity models for all deduced measurements, both ERT and SRT datasets were jointly inverted using the Structurally Coupled Cooperative Inversion (SCCI). This algorithm strengthens structural similarities between velocity and resistivity by adapting the individual regularizations after each model iteration.

This study emphasizes the benefit of multi-method geophysics to detect small-scale tectonic features. The surveys allowed to identify the fault locations throughout the area of interest, provided that the vertical displacements are large enough to be detected by the measurements. Previously assumed locations of the tectonic structures diverge from the new evidence based on ERT and SRT surveys. Especially in the western and eastern parts of the research area, differences between the survey results and formerly assumed locations are in the order of 100 m. Seismic and geoelectric measurements further indicate a fault structure in the southern part of the area, which remained undetected by past studies. The joint inversion provides minor improvements of the geophysical models, as most of the individually inverted datasets already provide results of good quality and resolution. Therefore, the effect of the SCCI algorithm is limited to underlining lithological and hydrological boundaries that are already present in the individually inverted ERT- and SRT-models.

 

References

Rücker, C., Günther, T., Wagner, F.M. (2017). pyGIMLi: An open-source library for modelling and inversion in geophysics, Computers and Geosciences, 109, 106-123, doi: 10.1016/j.cageo.2017.07.011.