Electrical structure beneath the eastern collapsed flank of Piton de la Fournaise volcano, Reunion Island: Implications for the quest for groundwater

Time domain electromagnetic (TDEM) and tensor audiomagnetotelluric (AMT) data have been acquired at several locations on the eastern flank of Piton de Ia Fournaise volcano (Reunion Island) within a depressed area called Grand Brule, interpreted as a collapse structure. The survey objectives were (1) to provide a geophysical estimate of the subsurface structure and (2) to evaluate the possibility of detecting aquifers in a volcanic environment not very known. The TDEM and the AMT data collected along two E-W traverses orthogonal to coastline on the northern and southern edges of Grand Brule were interpreted with one-dimensional layered models. From the surface downward, the geoelectrical sections reveal two major units: very resistive, young lava flows (dry) and a shallow conductor (<500 m) which is probably primarily attributable to a clayey, poorly permeable base. A notable exception to this pattern is seen at sites close to the coast, where we found three-layered structures. There is an intermediate layer of resistivity of about 100-200 ohm m between the top resistive layer and bottom conductive layer that represents a probable freshwater lens in the southern part and an alluvial fan with resistivities substantially higher (200 ohm m) in the northern part of Grand Brule. It is suggested that the 200 ohm m layer, interpreted as a buried paleoriver, corresponds to a drainage structure.