Présentations Colloques

Time-lapse gravimetry is known to be a powerful non-invasive integrative tool to monitor mass redistributions near the Earth’s surface and particularly water storage changes (WSC) within the critical zone. WSC are important variables in highly heterogeneous hard-rock aquifers, as they provide insights in recharge discharge mechanisms. Hybrid gravimetry relies on the combined use in a specific study of different types of instruments measuring the earth’s gravity changes i.e. relative spring gravimeters and superconducting gravimeters, as well as ballistic absolute gravimeters. Hybrid gravimetry is able to provide new constraints on underground WSC and we will focus here on two hard-rock aquifers with distinct topography, climate and hydrological settings to illustrate the interest of the technique. **The first site is the Ara catchment near Djougou (Benin) in West Africa. This catchment with low topography is located in the humid zone and was investigated in the frame of the GHYRAF (Gravity and Hydrology in Africa) project. The space-time patterns of seasonal WSC as revealed by EOF decomposition is the result of lateral redistribution processes occuring mostly in the saturated zone, as confirmed by water table depths and space-time soil moisture patterns. Such observations bring additional information on recharge-discharge processes.**The second site is the Strengbach catchment in the Vosges Mountains (NE France) with large topographical changes (880-1150m). One issue that is investigated is the role played by the topography on WSC. Another point is to compare the WSC inferred from surface gravity observations to the ones deduced from hydrological measurements such as soil moisture content, piezometric data, RMP exploration for inferring the behavior of the saturated zone.**