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
S. Boubekraoui et al., 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, J APP GEOPH, 40(4), 1998, pp. 165-177
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.