3-D surface wave tomography of the Soultz-sous-Forêts Enhanced Geothermal Site using seismic noise correlations

The Soultz-sous-Forêts Enhanced Geothermal Site (EGS) provides a natural laboratory for developping monitoring systems of the geothermal sites. Seismic passive methods represent a useful tool to characterize and study seismic velocity structures before, during, and after the exploitation of the geothermal wells. In this work we show the first 3D seismic velocity model of the Soultz site using the technique of the noise cross-correlations tomography. We reconstructed Rayleigh waves for 9 dominant periods comprised between 1 and 5 seconds from cross- correlations of 15 months (from March 2010 to May 2012) of ambient seismic noise recorded by 22 seismological surface stations (13 temporary and 9 permanent stations). A semi-automatic procedure allowed the reliable estimation of the Rayleigh waves emergence even for the noisy cross-correlations. Then, we calculated the dispersion curves using the travel times and their errors. The results are inverted for two-dimensional group velocity maps and finally for a 3-D S-wave velocity model of the geothermal reservoir. Our results clearly show a low velocity body spatially delimited at the depth of the reservoir and an alternation of high and low anomalies in the shallowest parts of the study region. This study shows the possibility to apply the noise cross-correlation tomography as a passive method to study complex small size regions as geothermal reservoirs.