3-D tomographic imaging of anomalous stress conditions in a deep US gold mine

In an effort aimed at locating highly stressed and destressed areas that may influence rockbursts, the US Bureau of Mines conducted an active 3-D seismic tomographic investigation between the 7100 and 7250 levels (2165 m to 2210 m depth) of the Homestake gold mine, Lead, South Dakota. Existing rock bolts were used to mount geophones and as strike points for introducing seismic energy (nominally 750 Hz) using an 8-lb sledge hammer. Approximately 2900 compressional wave travel time measurements were recorded spanning up to 155 m between drifts at several levels. Eight different reconstructions, computed using different uniform starting models, were averaged together to produce a robust final velocity model. The average velocity tomogram portrayed a heterogeneous structure (3-6.8 km/s). Regions of low-velocity correlated with known locations of drifts, stopes, ore shoots, and rockburst damage, while high-velocity regions were indicative of elevated levels of compressional stress. 3-D isoprobability surfaces were used to assess the degree of risk associated with anomalous regions; significant risk zones appeared adjacent to backfilled stopes and at the center of pillars indicating the likelihood of concentrated stress.