Passive Seismic Tomography and Seismicity Hazard Analysis in Deep Underground Mines Xu Ma Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Mining Engineering Erik Westman, Chair Mario Karfakis Kramer Luxbacher Norman Dowling 9 December 2014 Blacksburg, VA Keywords: Mining, Seismic Imaging, Double-Difference Tomography, Velocity Change, Stress Distribution, b-value, Aftershock Passive Seismic Tomography and Seismicity Hazard Analysis in Deep Underground Mines Xu Ma ABSTRACT Seismic tomography is a promising tool to help understand and evaluate the stability of a rock mass in mining excavations. It is well known that closing of cracks under compressive pressures tends 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, the arrival times of seismic waves and locations of seismic events are used as sources of tomographic imaging surveys. An initial velocity model is established according to the 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 a rock burst) are performed to analyze effects of a rock burst on stress distribution. Tomographic results show that high velocity anomalies form in the vicinity of a rock burst before the occurrence, and velocity subsequently experiences a significant drop after the occurrence of a rock burst. In addition, regression analysis of travel time and distance indicates that the average velocity of all the monitored regions 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 stress relief and as a result of fractures in the rock mass that are generated by shaking-induced damage from nearby rock burst zones. The 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 demonstrates 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. Acknowledgements I would like to thank my advisor, Dr. Erik Westman, for his guidance and assistance during all stage of research. I would like to thank committee members, Dr. Karfakis, Dr. Luxbacher and Dr. Dowling for their input and assistance on my research. I would like to thank Denis Thibodeau, Allan Punkkinen, Mike Yao, and for their help with data collection and helpful comments. I would also like to thank researchers in Geomechanics Observation & Imaging Research Group, Brent Slaker, Ben Fahrman, Andrew Russell, William Thomas, Will Conrad, Kyle Brashear, Yuncong Teng, Jeff Kerr, Dan Sadtler. I thank my family and friends for their support. This work was supported by the Canadian Mining Industry Research Organization and the US Department of Energy – National Energy Technology Laboratory. iii Contents List of Figures .............................................................................................................................................. vi List of Tables ............................................................................................................................................. viii 1 Introduction ........................................................................................................................................... 1 2 Literature Review .................................................................................................................................. 3 2.1 Seismicity and Rock burst Potential ............................................................................................. 3 2.2 Seismic Energy ............................................................................................................................. 7 2.3 Aftershock ................................................................................................................................... 12 2.4 Tomography ................................................................................................................................ 16 2.5 Summary ..................................................................................................................................... 23 Chapter 2 References .............................................................................................................................. 24 3 Seismic velocity change due to large magnitude events in deep underground mines ........................ 29 3.1 Abstract ....................................................................................................................................... 29 3.2 Introduction ................................................................................................................................. 29 3.3 Velocity and seismicity investigation ......................................................................................... 30 3.4 Velocity fit using mining-induced seismicity ............................................................................. 31 3.5 Case Study .................................................................................................................................. 32 3.6 Summary and discussion ............................................................................................................. 39 Chapter 3 References .............................................................................................................................. 41 4 Imaging of Temporal Stress Redistribution due to Triggered Seismicity at a Deep Nickel Mine ...... 43 4.1 Abstract ....................................................................................................................................... 43 4.2 Introduction ................................................................................................................................. 43 4.3 Data and methods ........................................................................................................................ 44 4.4 Results ......................................................................................................................................... 48 4.5 Summary and discussion ............................................................................................................. 54 Chapter 4 References .............................................................................................................................. 57 5 Stress redistribution with the change of frequency of mining-induced seismicity ............................. 59 5.1 Abstract ....................................................................................................................................... 59 5.2 Introduction ................................................................................................................................. 59 5.3 Data and methods ........................................................................................................................ 60 5.4 Results ......................................................................................................................................... 64 5.5 Summary and discussion ............................................................................................................. 73 Chapter 5 References .............................................................................................................................. 75 6 Passive tomographic study on velocity changes in underground mines ............................................. 77 iv 6.1 Abstract ....................................................................................................................................... 77 6.2 Introduction ................................................................................................................................. 77 6.3 Large magnitude events .............................................................................................................. 78 6.4 Average velocity analysis in tabulation areas ............................................................................. 80 6.5 Summary and discussion ............................................................................................................. 87 Chapter 6 References .............................................................................................................................. 89 7 Statistical analyses of mining-induced seismicity from deep hard-rock mines .................................. 91 7.1 Abstract ......................................................................................................................................