Body Wave Seismic Noise Tomography and Subsurface Characterization for Geothermal Exploration, the Haute-Sorne EGS project, Switzerland

A new enhanced geothermal system (EGS) is currently being engineered in the Haute-Sorne region in the Swiss Jura. Passive seismic imaging is conducted in the framework of the project as a viable basement-resolving and affordable technique to study subsurface structures at depths where active seismic surveys face limitations. We deployed 686 three-component seismic nodes with an inter-station spacing of approximately 300 m. This network spans a radius of about 15 km around the well and recorded the seismic ambient wavefield for one month in February 2024.

This study focuses on the reconstruction of diving P-waves from the recorded seismic ambient wavefield. We analyze data from 8 random days and apply the cross-coherency approach to estimate the impulse response between station pairs within the frequency band of 0.5–2.5 Hz. During processing, polarization characteristics are utilized to separate and enhance the P-wave signal from Rayleigh waves. Subsequently, we apply a bin-stacking method to compute a 1D P-wave retrieval with improved signal-to-noise ratio. We use this 1D P-wave as an example to apply a selected filter to individual traces and isolate the empirical Green’s functions containing the P-wave energy. 3D P-wave arrivals are automatically picked based on their coherent moveout.

Using these phase arrivals, we perform a travel time 3D tomography to estimate the P-wave velocity model in the Haute-Sorne region. The estimated P-velocity model reveals detailed information about shallow subsurface structures. A comparison with the S-wave velocity model, well-log data, and known geological structures demonstrates an agreement with the available complementary data and encourages the applicability of the P-wave tomography as a viable tool for geothermal prospecting. In the next phase, the combined analysis of P- and S- velocities will enable the estimation of the P-to-S velocity ratio, a key parameter for characterizing the geothermal reservoir.