Helicopter borne time-domain electromagnetic surveys: An advanced tool for the investigation of volcanic islands' groundwater resources
Résumé
The demographic explosion of many volcanic islands all over the world, with associated drinking water pressure, requires an urgent investigation of their subsurface in order to provide sustainable solutions to these increasing water demands. Hydrogeological studies in volcanic islands are challenging due to (1) difficulties in accessing the study sites (steep slopes, dense vegetation and a limited access), (2) the geological complexity of volcanic deposits (lateral and vertical lithological variability with superimposed intense weathering), and (3) the lack of boreholes enabling detailed hydrogeological characterizations, which limits the detailed validation of the different approaches and methods. The SkyTEM dual-moment Time Domain Electromagnetic system was used for the first time in a volcanic context in 2006 over the Galapagos Islands by UPMC-Sorbonne University and Aarhus University, then in 2009 by BRGM over Mayotte Island (with Aarhus University as partner) and in 2013 over Martinique and Guadeloupe Islands (Lesser Antilles). The SkyTEM TDEM system allows detailed mapping of weak conductivity contrasts in the shallow layers concurrently with those at depth and provides a detailed insight to understand the internal geological structure of the upper 200 m of these volcanic islands. In this communication we will present the main geological and hydrogeological structures highlighted, playing key roles on the hydrogeological functioning of such islands. For instance, the presence of a perched aquifer, potentially strategical for the island water supply, is suspected on Santa Cruz Island. On Mayotte Island a buried valley also clearly plays a key role on the hydrogeological functioning in the south of the island. Afterwards, the influence of successive phases of volcanic construction and erosion on the hydrogeological functioning of Mayotte Island's has been highlighted and correlations between age, resistivity and permeability have been put in evidence. Preliminary results in Martinique and Guadeloupe are meanwhile also promising.