Seismometer deployments are often confined to near the Earth's surface for practical reasons, despite the clear advantages of deeper seismometer installations related to lower noise levels and more homogeneous conditions. Here, we describe a 3D broadband seismometer array deployed at the inactive Homestake Mine in South Dakota, which takes advantage of infrastructure originally setup for mining and is now used for a range of scientific experiments. The array consists of 24 stations, of which 15 were underground, with depths ranging from 300 ft (91 m) to 4850 ft (1478 m), and with a 3D aperture of ∼1:5 km in each direction, thus spanning a 3D volume of about 3:4 km3. We describe unique research opportunities and challenges related to the 3D geometry, including the generally low ambient noise levels, the strong coherency between observed event waveforms across the array, and the technical challenges of running the network. This article summarizes preliminary results obtained using data acquired by the Homestake array, illustrating the range of possible studies supported by the data. Electronic Supplement: A 3D image of the seismic array implemented at Homestake, along with the existing drifts and shafts in the mine, and the local topography.
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The authors thank the staff at the Sanford Underground Research Facility and Program for the Array Seismic Studies of the Continental Lithosphere (PASSCAL) for assistance, particularly the help of Tom Regan, Jaret Heise, Jamey Tollefson, and Bryce Pietzyk. Terry Stigall made important technical contributions to operate and maintain the array. The authors also thank C. Langston and an anonymous reviewer for comments. The seismic instruments used for this array were provided by the Incorporated Research Institutions for Seismology (IRIS) through the PASS-CAL Instrument Center at New Mexico Tech. Data collected are available through the IRIS Data Management Center. The facilities of the IRIS Consortium are supported by the National Science Foundation under Cooperative Agreement EAR-1261681 and the Department of Energy (DOE) National Nuclear Security Administration. This work was supported by National Science Foundation INSPIRE Grant PHY1344265.