Imaging subsurface structures near wells in the northwest Geysers geothermal site

Vapor-dominated geothermal systems provide a reliable, low-carbon source of heat and electricity, but optimizing their exploitation requires high-resolution imaging of fracture networks and fluid pathways at the reservoir scale. We analyze a dense microseismic cluster in the northwestern Geysers (California), selecting 1,276 induced earthquakes recorded between 2006 and 2015. Using inter-event interferometry and fast-marching surface-wave tomography, we retrieve Rayleigh wave phase velocities on horizontal layers at 100 m spacing and jointly invert them with previously derived local group velocities to obtain a quasi-3D shear-wave (Vs) model at ~ 100 × 100 × 10 m blocks. The resulting Vs models reveal three main types of low-velocity anomalies: (I) fault-related zones associated with fracturing and hydrothermal alteration, (II) shallow steam-cap and normal-temperature-reservoir (NTR) boundary transitions spanning depths of ~ 900-1400 m, exhibiting sharp Vs contrasts due to thermal and fluid effects, and (III) injection/engineering-related anomalies characterized by localized or vertically elongated low Vs patches. Integrating these results with induced microseismicity provides valuable insights into fracture activity, fluid migration, and stress evolution within the geothermal reservoir.