The use of passive seismic noise to constrain seismic velocities in mountainous geothermal systems

Geothermal hot-spots are often located in mountainous or volcanic areas, where strong heterogeneity and rough topography are expected. Medium-to-deep and deep geothermal projects require monitoring of the local seismicity, in order to reduce the seismic hazard associated with the operations. The location uncertainty of seismic events depends on many parameters, including the accurate knowledge of the subsoil in terms of seismic velocities. In this work passive seismic interferometry has been applied to one-year seismic ambient noise data recorded by a microseismic monitoring network close to Yverdon-les-Bains, Switzerland, where the development of some geothermal doublets is being considered. The proposed workflow is based on the retrieval of empirical Green's functions from station pair correlations, on the estimation of phase velocities, and on their depth inversion. The adopted workflow considers and corrects for possible biases given by rough topography and non-homogeneous distribution of noise sources, an important source of errors in urbanized areas. The obtained quasi-3D seismic velocity model was used to relocate three seismic events in the vicinity of the geothermal project. The results show that seismic interferometry is a cost-effective method to infer a site-specific shear-wave velocity cube for the frequency range of interest for seismological applications. Passive data were also used to estimate the resonance behaviour of the study area, that reflects the great complexity of both subsurface structures and topography.