| Abstract: |
The Upper Rhine Graben is a Tertiary Graben located in the northeastern part of France and western Germany. The local geological structure in the Soultzsous- Forêts region corresponds to a horst, where a 1400m thick sedimentary cover overlays the crystalline basement. The basement is made of altered and fractured granitic rocks, which are older than 330 My (Cocherie et al., 2004). The site was selected because of the well-known geothermal anomaly (Haas and Hoffmann, 1929). The Enhance Geothermal System (EGS) pilot project of Soultz-sous-Forêts started in 1987. The wells are drilled in naturally fractured and altered granites from 3200 to 5260m depth. After 20 years of research and development on the geothermal reservoir, a 1.5 MWe power plant has been designed, built and tested. The current phase of the project consists in the long- term testing and monitoring of the power plant, together with production of electricity. Another geothermal site is now in construction in Rittershoffen, close to Soultz-sous-Forêts. The aim of this second project is to produce 24Mwth using the high temperature of the natural water circulation around 2500m depth. Previous geodetic monitoring of geothermal production sites have shown large subsidence and horizontal displacement attributed to thermal contraction (Massonnet et al., 1997; Fialko and Simons, 2000), pressure reduction in the reservoir (Carnec and Fabriol, 1999) or changes in the local stress field (Motoyama et al., 1999). In order to monitor surface displacements around the Soutlz-sous- Forêts and Rittershoffen geothermal sites and investigate reservoir processes, we initiated a program of ground and space based geodetic observations (GPS, InSAR). This work presents the monitoring strategy and preliminary results. |