SEISMIC CHARACTERIZATION OF A HIGHLY STRESSED ROCK MASS USING TOMOGRAPHIC IMAGING AND INDUCED SEISMICITY

A seismic tomography study was carried out at Falconbridge's Lockerby Mine, Sudbury, Canada, in April 1989. The objective was to determine the integrity of a rock mass at 1 km depth and to investigate the correlation between induced seismicity and P wave velocity structure. The tomographic image showed two distinct and significant P wave velocity anomalies: a low-velocity zone at the source region of a M(N) 2.3 mining-induced tremor recorded in 1985 and a high-velocity zone at the source region of a M(N) 2.6 mining-induced tremor recorded in 1988. Analysis of mining-induced microseismicity showed that the high-velocity zone was the focus for seismic activity prior to and after the 1988 tremor, in contrast to the low-velocity zone which remained aseismic for the same period. The seismic anomalies were diamond drilled, following the tomographic imaging, to verify the physical characteristics of the zones. The seismically active high-velocity zone showed severe core disking, indicative of a highly stressed strong rock, while the low-velocity zone was characterized by extensive fracturing and failed rock. The results showed that the 1985 tremor and rock burst sequence resulted in a major failure of the rock mass in the source region of the event, and stress was redistributed and concentrated in the source region of the 1988 tremor. The 1988 event and rock burst sequence did not result in failure of the source region, which therefore remains a seismically active high-velocity zone with a potential for significant seismic energy release and further rock bursting. The study has highlighted a correlation between P wave velocity structure and induced seismicity which can be used to characterize highly stressed rock masses in underground mines.