Ambient Seismic Noise Tomography of a Deep Geothermal Area in the Upper Rhine Graben from the EstOF Dense Regional Array

Abstract:

Passive imaging from ambient noise correlation has proven to be a robust tool to probe the internal structure of the Earth. Assessing the potential of this technique for the exploration of deep geothermal reservoirs was the main motivation of the EstOF project. In September 2014, we deployed a regular grid of 288 Zland nodes over a 30km wide disc (1.5km inter-node spacing) in the Outre-Forêt region (Bas-Rhin, France) encompassing the geothermal sites of Soultz-sous-Forêts and Rittershoffen.

The correlation of one month of ambient noise data provides thousands of usable correlations in the 0.2-5Hz frequency range. We clearly observe body waves as well as the fundamental and first harmonic modes of the Rayleigh wave. The latter phases have been used to retrieve group and phase velocity maps using the Eikonal tomography thanks to the high density of the array. Depth inversion of these maps for various frequencies allows us to construct a 3D model of shear wave velocities of the region down to 5km depth. This model, having a lateral resolution of about 2km, appears to be in good agreement with our geological knowledge of the region. This case study validates the use of ambient noise recorded by spatially dense arrays at regional scale as a cheap and robust technique for the exploration of geothermal areas.