As part of a paleoseismological investigation along the Bree fault scarp (western border of the Roer Graben), various geophysical methods [electrical profiling, electromagnetic (EM) profiling, refraction seismic tests, electrical tomography, ground-penetrating radar (GPR), and high-resolution reflection seismic profiles] were used to locate and image an active fault zone in a depth range between a few decimeters to a few tens of meters. These geophysical investigations, in parallel with geomorphological and geological analyses, helped in the decision to locate trench excavations exposing the fault surfaces. The results could then be checked with the observations in four trenches excavated across the scarp. Geophysical methods pointed out anomalies at all sites of the fault position. The contrast of physical properties (electrical resistivity and permittivity, seismic velocity) observed between the two fault blocks is a result of a difference in the lithology of the juxtaposed soil layers and of a change in the water table depth across the fault. Extremely fast techniques like electrical and EM profiling or seismic refraction profiles localized the fault position within an accuracy of a few meters. In a second step, more detailed methods (electrical tomography and GPR) more precisely imaged the fault zone and revealed some structures that were observed in the trenches. Finally, one high-resolution reflection seismic profile imaged the displacement of the fault at depths as large as 120 m and filled the gap between classical seismic reflection profiles and the shallow geophysical techniques. Like all geophysical surveys, the quality of the data is strongly dependent on the geologic environment and on the contrast of the physical properties between the juxtaposed formations. The combined use of various geophysical techniques is thus recommended for fault mapping, particularly for a preliminary investigation when the geological context is poorly defined.
This paper describes the application of geophysical prospecting techniques to locate the active faults in superficial quaternary sediments. Both electrical tomography and seismic refraction tomography presented here were part of a geophysical campaign performed in the Roer Graben area as a reconnaissance tool before trenching for paleoseismological observations. The aim of this investigation was, in the first place, to determine the exact position of the active fault in order to determine the best emplacement of a later trench. An additional objective was to image the fault zone at shallow depth, allowing both a comparison with trench data, and a downward extrapolation of these direct observations. Both methods proved to be successful but generated relatively smooth images with a poor spatial resolution that were prone to a subjective interpretation by the operators. In order to restrict the interpretation of tomographies with quantitative measures of gradients, image analysis techniques were tested. This paper presents some preliminary results obtained on the use of crest line location methods issued from mathematical morphology. Synthetic models of simple resistivity anomalies along a faulted section were used to check the reliability of the image processing method. #
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