Three-Dimensional Seismic Velocity Structure in a High-Injection Region in The Northwest Geysers, California, from Standard and Double-Difference Seismic Tomography

The Northwest Geysers contains some of the highest-volume injection and production wells in the Geysers geothermal field. These wells coincide spatially with dense clusters of microseismicity with exception of a sub-region central to several injectors which has shown lower rates of seismicity over the past 10 years. This low-seismicity region is underlain by a cluster of deep seismicity extending up to 4.2km below sea level (b.s.l.). The low-seismicity region has been imaged to 610 m resolution using passive-source 3D seismic tomography and co-location of hypocenters. The results indicate a low-velocity (2.9 km/s) anomaly that extends from the surface to approximately 1.5km b.s.l. in both P- and S- velocity models. It lies just above and to the Northwest of the low-seismicity region. The high-injection/production region is bounded on the southeast by higher velocities (range 4.0 km/s to 5.3 km/s), although it is dominated by velocities in the 3.8 km/s range. The low-velocity feature persists over our 5-year study period from 2005 to 2010, but appears to diminish spatially in 2010. Mean velocity values vary nominally from year to year, as do the extent of high and low velocity regions, but it is yet unknown whether this effect is temporal, an artifact of topography, or related to differences in data quality during different monitoring periods. The the low-velocity feature is being confirmed and re-imaged using double-difference tomography with a node-spacing of 150 m, and the feature's evolution over time will be correlated with injection and production rates in the surrounding area.