Three-dimensional distributed acoustic sensing at the Sanford Underground Research Facility

Abstract

Distributed acoustic sensing (DAS) has proven to be a valuable tool for monitoring seismic signals by providing high spatial and temporal resolution sensing of strain along fiber optic cable. Furthermore, every DAS channel records the seismic wavefield essentially synchronously at each sampled time because the interrogator is sampling each channel at the speed of light in the fiber. Unlike traditional seismic sensors, DAS has an intrinsic directional sensitivity to the axial strain change along the cable, leading to difficulties when using standard seismic analysis and interpretations that rely on three-dimensional particle velocity sensing. Additionally, cable deployments on the surface can be dominated by high-amplitude wind or urban noise, impeding the detection of low-amplitude distant seismic sources. Here we investigate the capabilities of a unique three-dimensional array with spiral-like portions in the Sanford Underground Research Facility (SURF), the former Homestake mine, between 1250 m (4100 ft) and 1488 m (4850 ft) depth for detecting local, regional, and teleseismic sources of ground vibrations. Our pilot array finds that DAS records high-frequency (above 5 Hz) vibration sources, including mine activities and local and regional blasting events. Furthermore, we find that our deployment method (fiber resting on the surface with rocks placed every meter or so) may contribute to low-frequency noise that contaminates the interpretation of teleseismic waves, particularly lower-frequency S-wave arrivals Nevertheless, this three-dimensional DAS array provides significant data for future analysis as well as the basis for improving and expanding the array in SURF.