Passive Seismic Tomography and Seismicity Hazard Analysis in Deep Underground Mines
Seismic tomography is a promising tool to help understand and evaluate the stability of a rock mass in mining excavations. Lab measurements give evidence that velocities of seismic wave propagations increase in high stress areas of rock samples. It is well known that closing effects of cracks under compressive pressures tend to increase the effective elastic moduli of rocks. Tomography can map stress transfer and redistribution and further forecast rock burst potential and other seismic hazards, which are influenced by mining. Recorded by seismic networks in multiple underground mines, arrival time of seismic waves and locations of seismic events are used as sources of tomographic imaging survey. An initial velocity model is established according to properties of a rock mass, then velocity structure is reconstructed by velocity inversion to reflect the anomalies of the rock mass. Mining-induced seismicity and double-difference tomographic images of rock mass in mining areas are coupled to show how stress changes with microseismic activities. Especially, comparisons between velocity structures of different periods (before and after rock burst) are performed to analyze effects of rock burst on stress distribution. Tomographic results show that high velocity anomalies form in the vicinity of rock burst before the occurrence, and velocity subsequently experiences a significant drop after the occurrence of rock burst. In addition, regression analysis of travel time and distance indicates that the average velocity of all the monitored region appears to increase before rock burst and reduce after them. A reasonable explanation is that rock bursts tend to be triggered in highly stressed rock masses. After the energy release of rock bursts, stress relief is expected to exhibit within rock mass. Average velocity significantly decreases because of lower stresses and as a result of fractures in the rock mass that are generated by shaking-induced damage from nearby rock burst zones. Mining-induced microseismic rate is positively correlated with stress level. The fact that highly concentrated seismicity is more likely to be located in margins between high-velocity and low-velocity regions manifests that high seismic rates appear to be along with high stress in rock masses. Statistical analyses were performed on the aftershock sequence in order to generate an aftershock decay model to detect potential hazards and evaluate stability of aftershock activities.