Ultra-Low Frequency Strain Measured by Distributed Acoustic Sensing in a Mountain Borehole

Rock mechanical properties are an important consideration for the siting, development, and management of geothermal reservoirs. Vertical Seismic Profiling (VSP) is a typical tool for estimating geomechanical properties with depth and, in high-temperature reservoirs, distributed acoustic sensing (DAS) is rapidly becoming the preferred method of measuring velocity profiles and changes to geomechanical properties over time. Here we report on an attempt to use DAS to measure strains at ultra-low frequencies using ambient earth forcings. We interrogated a fiber grouted 460 m deep into a borehole installed in the San Gabriel Mountains of Southern California. The rock is primarily gneiss and anorthosite and is at some depths brecciated and faulted. We collected two days of displacement along the fiber. After removing common-mode noise and drift, we found strains of a few tens of nanostrain about the mean at periods approaching diurnal. We attribute the variation in this strain to variations in rock strength (Youngs Modulus) with depth. Although we cannot entirely explain the mechanical responses observed, the results suggest that DAS may be useful for acquiring geomechanical responses to stress at frequencies 5-6 orders-of-magnitude below those used for VSP.