Abstract:
The Sanford Underground Research Facility (SURF) has repurposed older workings of a long-lived gold mine to host scientific experiments. The present laboratories are developed primarily in Precambrian-age amphibolite host rock, but Tertiary-age rhyolite intrusives are also present. A portion of the pillar in the vicinity of the new physics laboratories on the 4850 level (1.5 km below the surface) was chosen for examination using seismic tomography to evaluate the degree and location of potential pillar degradation. Geophones and sources were placed on opposite sides of a pillar formed by the intersection of two drifts, and an acoustic wave was passed between them to produce multiple paths along which the P-wave velocities were calculated. The resulting tomographic velocity map of the pillar adjacent to the intersection shows a well-developed pattern of decreasing velocities toward the acute end of the triangle due to a decreasing pillar cross-section. Higher seismic velocities are associated with areas where the stresses are greater but where the rock has not failed, such as adjacent to well-maintained drifts.
Introduction
The Sanford Underground Research Facility (SURF) is being developed as an underground scientific laboratory in a former gold mine in northwestern South Dakota, USA (Heise, 2015; Lesko et al., 2011). The underground workings are extensive and have a history that extends over 125 years. The facility is a host for a number of existing and planned physics experiments that require shielding from cosmic radiation along with a lesser number of geological, geomechanical, and biological experiments. Although it is still indeterminate, a design life of 30 years is not unreasonable for this type of facility. Given the age of much of the underground infrastructure and the expected long operational life of the facilities, it is important to have monitoring programs that can assess the condition of rock making up the underground infrastructure. Such programs will allow evaluation of the necessity for mitigation of degradation of rock conditions. The purpose of the present study was to image the conditions of the rock in a pillar at the 4850 Level of the facility (1.5 km below the surface) near a critical part of the underground infrastructure.
The geology of the mine and of the rocks that constitute SURF was studied extensively during the 125 years when it was an active gold mine. Caddy, et al., 1991, and Bachman and Caddy, 1990, provide the most recent summary of the geologic environment in terms of the overall geology. More recent studies of the geotechnical characteristics of the rock in the vicinity of the physics laboratories on the 4850 Level were performed as part of the scoping for early versions of the laboratories (Hladysz et al, 2011). Geotechnical work is continuing as the experiment areas are expanded and developed.
