Using Seismic Interferometry to Identify and Monitor Fluids in Geothermal Systems

Abstract

We use seismic interferometry in combination with waveform modeling 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, we observe that the 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, fluids move through the system and create measurable changes in the observed amplitudes.