Electromagnetic investigations of aquifers in the Grand Brule coastal area of Piton de la Fournaise volcano, Reunion Island

A geophysical survey was carried out on the eastern flank of Piton de la Fournaise volcano in the Grand Brule area using combined audiomagnetotelluric (AMT) and time domain electromagnetic (TDEM) techniques. The aims of the present study were firstly to define the shallow geoelectrical structure, and subsequently to locate large bodies of ground water with anomalous electrical properties. Most of the AMT sites are isotropic across the entire observational bandwidth (7500-10 Hz), except for a few sites where static-shift effects are present. Joint one-dimensional interpretation of the TDEM and AMT data indicates basically three-layered structures with the following resistivity zones, progressing from the surface downward: (1) a resistive unsaturated surface layer of subaerial basaltic flows with a 2000 to 10,000 ohm-m resistivity, its base being approximately at mean sea level, (2) an intermediate-resistivity layer (100-250 ohm-m) 50-300 m thick, interpreted as being fresh-water-saturated basaltic formations, and (3) a low-resistivity layer of less than 15 ohm-m probably due to more saline water at depth and/or higher clay content in crushed materials. In the center of the study area, the results have also shown the presence of a prominent boundary dividing the area into a southern zone where the aquifer sequence is thin (60-100 m) and a northern zone corresponding to a relatively thick aquifer (> 150 m). Geoelectrical models are compared with the available geological information given by two geothermic exploration drillings. The coastal drill hole gives a crucial information, i.e. the thick intermediate-resistivity layer in the northern part of Grand Brule can be related to water-saturated alluvium deposits down to 200 m below sea level. AMT and TDEM soundings have revealed a major paleoalluvial drainage system buried under recent lava flows. A delimitation of sectors with different hydrogeological characteristics can be proposed.