Monitoring Granitic and Sedimentary Rocks at the Utah FORGE Site Using Borehole Distributed Acoustic Sensing Ambient Noise

Distributed acoustic sensing (DAS) has great potential for subsurface monitoring in high-temperature, high-pressure environments such as enhanced geothermal systems (EGSs). A 1 km DAS fiber was installed in an observation well (78-32) at the Frontier Observation for Research in Geothermal Energy (FORGE) site in Utah to continuously monitor induced seismicity during fracture stimulation in an injection well (58-32). We analyze 48 hours of continuously recorded borehole DAS ambient noise data to infer elastic properties of geologic formations. We calculate root-mean-squares (RMS) amplitudes of ambient noise for each DAS channel to construct noise RMS depth profiles. The DAS ambient noise profiles show noise peaks at some depths and quiet zones at others. We compare the DAS RMS profiles with wireline logging data and find apparent correlations between major RMS peaks and highly fractured low-velocity layers bounded by the structural interfaces. We apply time-lapse and time-frequency analysis and find that the noise RMS amplitudes vary with time for quiet zones. We also perform spectral analysis and find that the noisy channels contain both blue and white noises. Our results show that borehole DAS ambient noise can be used as a source-free logging tool to monitor temporal changes in subsurface rock properties.