Leveling and hybrid gravimetry monitoring applied to geothermal sites of Soultz-sous-Forêts and Rittershoffen, Rhine Graben, France

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 microgravimeters) is presented. A relative gravimetric network equipped with levelling benchmarks was designed and surveyed at annual frequencies (2013, 2014 and 2015 for gravity measurements, 2014 and 2015 for the levelling). 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).