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International audienceThe study is devoted to the monitoring of a geothermal reservoir by hybridgravimetry combining different types of instruments (permanent superconductinggravimeter, absolute ballistic gravimeter, and micro-gravimeters) and differenttechniques of measurements (both time-discrete and recording data collection).Using a micro-gravimetric repetition network around a reference station, which isregularly measured, leads to the knowledge of the time and space changes in surfacegravity. Such changes can be linked to the natural or anthropic activities of the reservoir.A feasibility study using this methodology is applied to two geothermal sites in theAlsace region (France) of the Rhine graben. We show the results in terms of gravitydouble differences from weekly repetitions of a network of 11 stations around thegeothermal reservoir of Soultz-sous-Forêts, separated into 5 loops during July–August2013 and 2014 as well as preliminary results from 2 stations near Rittershoffen (ECOGI).We point out the importance of a precise leveling of the gravity points for the control ofthe vertical deformation. A first modeling of surface gravity changes induced by realisticgeothermal density perturbations (Newtonian attraction) is computed in the frame of theexisting geological model and leads to gravity changes below the μGal level beinghence undetectable. However, and for the same case, borehole gravity modeling showeda significant anomaly with depth that can be used as a complementary monitoringmethod. We show that in the limit of our uncertainties (SD ~ 5 μGal), we do not detectany significant gravity change on the geothermal site of Soultz in agreement with thefact that there was indeed no geothermal activity during our analysis period. On thecontrary, the measurements near Rittershoffen show a signal above the noise level whichcorrelates in time with a production test but cannot be explained in terms of Newtonianattraction effects according to our basic numerical simulation

geothermal energy

Monitoring of a geothermal reservoir by hybrid gravimetry; feasibility study applied to the Soultz-sous-Forêts and Rittershoffen sites in the Rhine graben