M. Courteaud

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 (AMT) data have been acquired at several locations on the eastern flank of Piton de la Fournaise volcano (Reunion Island) within a depressed area called Grand Brule, interpreted as a collapse structure. The survey objectives were (1) to provide a geophysical estimate of the subsurface structure and (2) to evaluate the possibility of detecting aquifers in a volcanic environment not very known. The TDEM and the AMT data collected along two E-W traverses orthogonal to coastline on the northern and southern edges of Grand Brule were interpreted with one-dimensional layered models. From the surface downward, the geoelectrical sections reveal two major units: very resistive, young lava flows (dry) and a shallow conductor (<500 m) which is probably primarily attributable to a clayey, poorly permeable base. A notable exception to this pattern 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 an alluvial fan with resistivities substantially higher (200 ohm m) in the northern part of Grand Brule. It is suggested that the 200 ohm m layer, interpreted as a buried paleoriver, corresponds to a drainage structure.

Debris avalanches on the western flank of Piton des Neiges shield volcano (Reunion Island)

The Saint-Gilles breccias, on the western flank of Piton des Neiges volcano, are clearly identified as debris avalanche deposits. A petrographic, textural and structural analysis of the breccias and inter-bedded autochthonous lava flows enables us to distinguish at least four successive flank slides. The oldest deposit sampled the hydrothermally-altered inner parts of the volcano, and has a large volume. Failure was favored by the presence of a deep intensely-weathered layer. The younger deposits are from superficial sources, as their products are rarely hydrothermalized and are more vesicular. The breccia formation, and especially the progressive breaking up occurring during the debris avalanche displacement, indicates the existence of high speed transport. In the Cap La Houssaye coastal area, abrasion and striation of the underlying lava formation, as well as the packing features observed in the breccia, are considered to be deceleration structures.

New geological and hydrogeological implications of the resistivity distribution inferred from audiomagnetotellurics over La Fournaise young shield volcano (Reunion Island)

Abstract An audiomagnetotelluric survey has been performed along an inactive flank of La Fournaise volcano massif in Reunion island, to study the subsurface resistivity structure. One-dimensional modelling of the AMT data at each site revealed an extensive low-resistivity (less than 10 ω m) zone at a few hundred meters below the surface. The significance of this unexpected conductive substratum is discussed in relation with the proposed impact of volcano-tectonic processes (caldera and landslides collapses), and a new interpretation of the geological structure is proposed. Moreover, it is likely that these conductors are poorly permeable argilaceous materials; then they coincide with a limit in permeability and determine groundwater behavior.

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

Abstract In order to investigate aquifers, several geophysical surveys have been carried out in the Baril area of the southern flank of Piton de la Fournaise volcano on Reunion in the Indian Ocean using audiomagnetotelluric (AMT), very-low-frequency (VLF) and self-potential (SP) methods. We present the results with emphasis on a comparison between SP data and the findings of geoelectric surveys. AMT soundings have indicated, from the surface downward, three layers: (i) resistive volcanic rocks, (ii) an intermediate resistivity layer, and (iii) a conductive basement attributed to a seawater-bearing aquifer. VLF measurements allow the mapping of the first layer apparent resistivity, and therefore its bottom, when the true resistivity is supposed to be isotropic and homogenous. When this assumption does not hold, only the SP method permits the mapping of this bottom. Because of the good agreement between the SP and electromagnetic results, we propose the SP method as the first tool that should be used in studying shallow hydrogeological structures in volcanic areas.

Audiomagnetotelluric prospecting for groundwater in the Baril coastal area, Piton de la Fournaise Volcano, Reunion Island

Surface geophysics has been used for a number of years to solve a variety of major groundwater exploration problems in coastal and island regions. The electrical properties of near‐surface rocks are highly dependent on porosity, degree of saturation, and pore fluid resistivity (Keller and Frischknecht, 1966). Because the resistivity of the formations decreases with increasing salinity of the water content, electrical methods, such as vertical electrical soundings, time‐domain electromagnetic soundings, and audiomagnetotellurics (AMT), are well‐suited for mapping changes in groundwater salinity and, in particular, for detecting a fresh water‐salt water interface.