Observing Changes in Fluid Pressure at the Brady Geothermal Field using Seismic Interferometry

The Poroelastic Tomography experiment (PoroTomo) was conducted in March 2016 at Brady Hot Springs in Nevada. A key goal of the experiment is to understand how fluids travel from shallow aquifers, through faults and fractures, to deep geothermal reservoirs. As part of the effort, seismic, geodetic, and hydraulic technologies were tested and developed to fully characterize the rock mechanical properties.

During the experiment, a large seismic array was deployed and recorded more than two weeks of continuous data, active vibroseis sweeps, local traffic noise, and the ambient seismic wavefield. In this study, we use several methods of seismic interferometry to investigate the site. We focus on two techniques: sweep interferometry uses the energy from the vibroseis sweeps as sources of high frequency energy. Ambient noise correlation uses the energy of the ambient background field. In each case, the data recorded at one seismometer are correlated with the data recorded at another to obtain an estimate of the Green function between the two. The 238 geophones, concentrated over a 1.5 square-kilometer area, allow us to calculate nearly 30,000 paths, which we use to characterize the site and measure the localized wavefield.

In collaboration with Ormat, pressures were changed during four stages of operation, including shutdown, followed by increased injection and pulsing. These changes caused measurable differences in the material properties beneath the site, including the attenuation of seismic energy. We see fault-bounded variations in attenuation from one stage to the next, with areas undergoing subsidence becoming more seismically efficient.

We use two methods to study the changes in seismic attenuation at the site. The simplest is to measure the normalized amplitudes of the Green functions and to compare the values during each stage of operation. The second is to waveform model the data, separating out contributions of Qs and Qp. Because P and S have different sensitivity to fluids in fractures and pores, the ratio of Qs/Qp is highly sensitive to the fluid saturation. We see anomalously high values of Qs/Qp at depth at the injection site and following fault boundaries.