Three case studies are described where an integrated interpretation of field data from tomographic imaging and mining-induced seismicity experiments has provided important insights into the in situ correlations between seismic velocity, stress and seismicity. Seismic imaging and microseismic monitoring are techniques which have been used, usually separately, to characterize rock masses. This study highlights the potential usefulness of an integrated interpretation of seismic velocity images and low-magnitude induced seismicity (Mn -4 to 0). Correlations between seismic velocity and the spatial density of mining-induced microseismicity has been used to interpret the state of stress and rockburst potential of underground rock masses. The seismic velocity images described are some of the few in the literature which have been verified by borehole drilling and excavation.
Seismic tomographic imaging has been carded out in Canadian hard-rock fftines for several years in an attempt to delineate rockburst prone rock masses. The redistribution of stress due to mining results in measurable differences in seismic velocity and this has been used in conjunction with induced seismicity for rock mass characterization. Three types of imaging surveys can be carried out in underground mines; active, passive and sequential and these can be either 2D or 3D surveys. Active images are computed from tomographic surveys using controlled explosion sources. Passive images are computed from the induced seismicity itself by a simultaneous inversion procedure for source location and velocity structure. Also, because it is possible to carry out repeat surveys, temporal as well as spatial variation of seismic parameters can be studied using sequential active or passive imaging methods.
Anomalous stress conditions are a safety concern in underground mining because of their significance in rockburst phenomena. Velocity increases arise from the closure of microcracks, joints and discontinuities. Velocity decreases occur in regions of failed or dilated rock masses. In both these situations, the change in seismic velocity is an indirect consequence of the change in stress due to mining and as a result of this dependence, seismic tomography has been used to map stress related phenomena in mines. Several authors have demonstrated the applicability of 2D active tomographic imaging for the investigation of fracturing and stress induced effects in rock masses, for example Wong et al (1983) and Cosma (1983). The work of Kormendi et al (1986) and Young et al (1990) provides examples of 2D sequential active studies relating to mining-induced phenomena.
The 3D active and passive imaging case studies presented in this paper are the first high- resolution images obtained in underground hard-rock mines. Induced microseismicity can also be used to monitor rock masses. In mines, several hundred events per day provide an ideal 3D remote monitoring of the rock mass' response to mining. Low-magnitude seismicity (Mn -4 to 0) induced by some physical process, for example mining (Talebi and Young, 1990 and Gibowicz et al, 1991), hydraulic fracturing (Batchelor et al, 1983) and fluid injection/extraction (Talebi and Comet, 1987) can be used to map the redistribution of stresses/fracturing in a rock mass. These authors have all shown the application of earthquake analysis procedures to low-magnitude microseismicity and the advantages of this type of localized monitoring of rock mass behavior. Recent developments in the instrumentation and software for high frequency imaging and microseismic monitoring have extended the resolution and usefulness of both techniques.
The case studies described in this paper hi