3-D tomographic imaging of anomalous conditions in a deep silver mine

Mining-induced stress-field changes pose both safety and economic hazards. In an effort aimed at developing technology for mitigating such hazards, the Bureau of Mines together with Hecla Mining Company conducted an active 3-D seismic tomographic investigation of anomalous rockmass conditions in a large underground, high-grade, remnant ore pillar, at the Lucky Friday mine near Mullan, ID. Roughly 2400 P-wave traveltime measurements, were simultaneously inverted to obtain a velocity distribution. The resulting velocity structure appears extremely heterogeneous (1.5-6.0 km/s) and well correlated with mechanical models indicating the transfer of stress in direct response to mining. Regions of anomalous ground (intense fracturing or high-stress) were identified using threshold probabilities; the minimum velocity regions surrounding drifts indicate a zone of stress relief that extends up to three drift diameters into the rockmass, while regions of maximum velocity above and below mined-out portions of veins indicate the likelihood of concentrated stress. Active tomographic imaging provides the engineer with a flexible tool for routine underground 3-D monitoring of mechanical conditions in large mine structures.