Subsurface electrical imaging of anisotropic formations affected by a slow active reverse fault, Provence, France
Résumé
In seismic hazard assessment studies, near-surface geophysical prospecting is increasingly used to help in identifying active faults and to constrain seismic deformation parameters (fault offset, recurrence...). In the current study, electrical tomography was used (1) to locate the Trévaresse slow reverse fault (Provence, SE France), the source of the most important seismic event recorded in the 20th century in France and (2) to test electrical tomography's ability to detect faults and tectonic deformation in a complex geological environment. Our study is supported by a detailed geologic and geomorphic field study. In the investigated area, where the geology is made of a succession of folded marl and limestone layers, electrical images are strongly influenced by electrical macro-anisotropy which plays a critical role in this survey, depending on the investigation scale. When resolution (i.e. the smallest target that can be resolved) is larger than the layer thickness (at depth and/or for large electrode spacing), inversion is unable to retrieve the real structure. To successfully interpret these electrical images, in terms of tectonically induced dip variations, we constrain field data interpretation by comparing measured and inverted electrical data with synthetic modelling results and geological data. The synthetic models were hypothetical coarse geological structures determined on the basis of geological/geomorphologic information (e.g. outcropping layers). For shallow depths, when the resolution is smaller or equal to the anisotropy scale, an electrical tomogram yields straightforward interpretation of the subsurface structure. Coupled with numerical modelling, electrical results successfully helped in positioning a trench for fault characterization.