ELECTROMAGNETIC INVESTIGATIONS OF AQUIFERS IN THE GRAND BRULE COASTALAREA 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 audiomagne totelluric (AMT) and time domain electromagnetic (TDEM) techniques, Th e aims of the present study were firstly to define the shallow geoelec trical structure, and subsequently to locate large bodies of ground wa ter with anomalous electrical properties, Most of the AMT sites are is otropic across the entire observational bandwidth (7500-10 Hz), except for a few sites where static-shift effects are present. Joint one-dim ensional interpretation of the TDEM and AMT data indicates basically t hree-layered structures with the following resistivity zones, progress ing from the surface downward: (1) a resistive unsaturated surface lay er 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 b oundary dividing the area into a southern zone where the aquifer seque nce is thin (60-100 m) and a northern zone corresponding to a relative ly thick aquifer (>150 m), Geoelectrical models are compared with the available geological information given by two geothermic exploration d rillings, The coastal drill hole gives a crucial information, i.e. the thick intermediate-resistivity layer in the northern part of Grand Br ule can be related to water-saturated alluvium deposits down to 200 m below sea level, AMT and TDEM soundings have revealed a major paleoall uvial drainage system buried under recent lava flows. A delimitation o f sectors with different hydrogeological characteristics can be propos ed.