New insights into the hydrogeology of a basaltic shield volcano from a comparison between self-potential and electromagnetic data: Piton de la Fournaise, Indian Ocean

In order to investigate aquifers, several geophysical surveys have been car ried out in the Baril area of the southern flank of Piton de la Fournaise v olcano on Reunion in the Indian Ocean using audiomagnetotelluric (AMT), ver y-low-frequency (VLF) and self-potential (SP) methods. We present the resul ts with emphasis on a comparison between SP data and the findings of geoele ctric surveys. AMT soundings have indicated, from the surface downward, thr ee layers: (i) resistive volcanic rocks, (ii) an intermediate resistivity l ayer, and (iii) a conductive basement attributed to a seawater-bearing aqui fer. VLF measurements allow the mapping of the first layer apparent. resist ivity, and therefore its bottom, when the true resistivity is supposed to b e isotropic and homogenous. When this assumption does not hold, only the SP method permits the mapping of this bottom. Because of the good agreement b etween the SP and electromagnetic results, we propose the SP method as the first tool that should be used in studying shallow hydrogeological structur es in volcanic areas. (C) 1998 Elsevier Science B.V. All rights reserved.