Three years of monitoring using leveling and hybrid gravimetry applied to geothermal sites of Soultz-sous-Forêts and Rittershoffen, Rhine Graben, Fran

Here we report on some case histories of geodetic and gravity surveys for monitoring of geothermal sites. The monitoring of a geothermal reservoir by hybrid gravimetry combining different types of instruments (permanent superconducting gravimeter, absolute ballistic gravimeter and micro-gravimeters) is presented. A relative gravimetric network equipped with levelling benchmarks was designed and surveyed at annual frequencies (2013, 2014, 2015 and 2016 for gravity measurements, 2014, 2015 and 2016 for the leveling). Repetition of high precision relative gravity measurements on a network developed around a reference station, which is regularly measured with both relative and absolute gravimeters, leads to the knowledge of the time and space changes in surface gravity. The observed gravity 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 of Soultz-sous-Forêts and Rittershoffen in the Alsace region (France) in the Rhine graben (Hinderer et al. 2015).