Using Seismic Interferometry to Identify and Monitor Fluids in Geothermal Systems

Seismic interferometry in combination with waveform modeling is used to measure the seismic properties within geothermal systems, illuminating changes that occur as water moves through the subsurface. We apply "virtual earthquake" methods ( ambient noise correlation and active source interferometry ) to obtain seismic Green functions (GF) which are then used to calculate material properties, in particular VS, VP, QS and QP. Individually, these properties have complex sensitivity to geologic fabric, composition and temperature, but in combination they can be used to highlight fractured media through which fluids are transported. In particular, seismic attenuation is highly sensitive to the contrast along fluid filled fractures. Mapping the ratio of the attenuation of P wave energy to S wave energy (QP/QS) illuminates the fluid paths in the geothermal system. When pressures change, such as during plant operations, fluids move through the system and create measurable changes in the observed amplitudes. The key goal of this study is to understand how fluids travel along faults and fractures through geothermal reservoirs. We focus on two geothermal sites in Nevada: the Brady geothermal field, which was the subject of a multiphysics poroelastic tomography experiment (PoroTomo) (Feigl et al., 2017) and the Blue Mountain geothermal field which has been the subject of several geophysical studies over the last decade, including gravity, magneto-telluric, seismic reflection and refraction surveys. We discuss the use of seismic interferometry to image the physical properties of the subsurface. These images help capture the detailed changes in velocity and attenuation that may indicate permeable zones and potential drilling targets.