Buried valleys revealed by helicopter borne transient electromagnetic and hydrogeological implications : example of the volcanic island of Mayotte
Résumé
Volcanic aquifers are complex in terms of geological structure and spatial distribution of hydrodynamic parameters. Their hydrogeological functioning and geometry are mostly not well known and understudied. Only basaltic islands have hydrogeological schemes: the Hawaiian model (low-lying basal aquifer and perched aquifer) and the Canary Island model (continuous basal aquifer) and these conceptual models don't fit with most volcanic or basaltic island. Mayotte Island, a small French overseas territory (374 km²) of the Comoros archipelago in the Indian Ocean, is formed by at least two shield volcanoes. Main formations that outcrop are Miocene-Pleistocene basaltic and phonolitic lavas and pyroclastic projections. Its population has quickly increased during last decades and the island is now very densely populated (>500 inh/km²) with substantial water needs. At the end of dry seasons, water cuts mark the daily routine of the population. Understanding the hydrogeological functioning of the island is thus fundamental in order to run appropriate drilling programs. The innovative SkyTEM helicopter borne transient electromagnetic method (TDEM) has been performed in October 2010 in order to obtain a quasi-3D resistivity map of the island. Average helicopter flying speed was 65 km/h and the flight altitude of the transmitter frame and receiver coil was 40 m. Flight lines were oriented North-South with an average spacing of 200 m and 3000 km have been covered. Good conductivity contrasts (from 1 to 600 ohm.m) between the different rocks types allow to define the geometry of the principal geological units, up to 200 m depth. Comparison between TDEM data, borehole and pumping tests data have provided new evidence about the hydrogeological scheme of the island, which seems different from the two previously mentioned. The Mayotte Island is not marked by high permeability lava flows with a large extension and doesn't host any low-lying or continuous basal aquifer. Their structure and groundwater flow seems mainly controlled by imbricate buried valleys filled by lavas or pyroclastic projections. Some low permeable layers, resulting from weathering processes (which transforms primary minerals into clay-rich materials) are also suspected to play an important role as capacitive layers.