The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium

As part of a paleoseismological investigation along the Bree fault scarp (w estern border of the Roer Graben), various geophysical methods [electrical profiling, electromagnetic (EM) profiling, refraction seismic tests, electr ical tomography, ground-penetrating radar (GPR), and high-resolution reflec tion seismic profiles] were used to locate and image an active fault zone i n a depth range between a few decimeters to a few tt ns of meters. These ge ophysical 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 a nomalies at all sites of the fault position. The contrast of physical prope rties (electrical resistivity and permittivity, seismic velocity) observed between the two fault blocks is a result of a difference in the lithology o f the juxtaposed soil layers and of a change in the water table depth acros s 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 tomog raphy and CPR) more precisely imaged the fault zone and revealed some struc tures that were observed in the trenches. Finally, one high-resolution refl ection seismic profile imaged the displacement of the fault at depths as la rge as 120 m and filled the gap between classical seismic reflection profil es and the shallow geophysical techniques. Like all geophysical surveys, th e quality of the data is strongly dependent on the geologic environment and on the contrast of the physical properties between the juxtaposed formatio ns. The combined use of various geophysical techniques is thus recommended for fault mapping, particularly for a preliminary investigation when the ge ological context is poorly defined.