Toward the Construction of a 3D Velocity Model for Raft River Geothermal Field Using Ambient Noise

Abstract

Ambient noise from a geothermal field can provide useful initial information for high-resolution velocity model building. We extract surface-wave travel times from seismic ambient noise and invert these travel times for a 3D velocity model for the Raft River geothermal field. We calculate cross correlations of ambient noise data recorded by eight geophones distributed in the geothermal field. To measure surface-wave travel times from the Green’s functions that emerge from the noise cross correlations, we perform multiple-filter analysis of the cross correlations. Surface-wave travel-times are measured between 0.2 and 35 Hz. Using surface-wave travel times between stations, we first conduct tomographic inversions to construct 2D surface-wave velocity models at multiple frequencies. We are able to achieve a model resolution of approximately 300 m and a travel-time misfit reduction of larger than 80% at certain frequencies. These 2D surface-wave travel-time models are then used to construct a 3D velocity model of the Raft River geothermal field through 1D inversion of surface-wave dispersion at model-node locations. This 3D velocity model will be used as the first-stage, low-resolution background model in a multi-scale model building effort to construct a high-fidelity and high-resolution 3D velocity model for subsurface imaging and geothermal reservoir characterization.