3-D TOMOGRAPHIC IMAGING OF ANOMALOUS CONDITIONS IN A DEEP SILVER MINE

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