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 (AM T) data have been acquired at several locations on the eastern flank o f Piton de Ia Fournaise volcano (Reunion Island) within a depressed ar ea called Grand Brule, interpreted as a collapse structure. The survey objectives were (1) to provide a geophysical estimate of the subsurfa ce structure and (2) to evaluate the possibility of detecting aquifers in a volcanic environment not very known. The TDEM and the AMT data c ollected along two E-W traverses orthogonal to coastline on the northe rn and southern edges of Grand Brule were interpreted with one-dimensi onal layered models. From the surface downward, the geoelectrical sect ions reveal two major units: very resistive, young lava flows (dry) an d a shallow conductor (<500 m) which is probably primarily attributabl e to a clayey, poorly permeable base. A notable exception to this patt ern 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 a n alluvial fan with resistivities substantially higher (200 ohm m) in the northern part of Grand Brule. It is suggested that the 200 ohm m l ayer, interpreted as a buried paleoriver, corresponds to a drainage st ructure.