Seismic Wave Velocity Variations in Deep Hard Rock Underground Mines by Passive Seismic Tomography

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Virginia Tech


Mining engineers are tasked with ensuring that underground mining operations be both safe and efficiently productive. Induced stress in deep mines has a significant role in the stability of the underground mines and hence the safety of the mining workplace because the behavior of the rock mass associated with mining-induced seismicity is poorly-understood. Passive seismic tomography is a tool with which the performance of a rock mass can be monitored in a timely manner. Using the tool of passive seismic tomography, the advance rate of operation and mining designs can be updated considering the induced stress level in the abutting rock. Most of our current understanding of rock mass behavior associated with mining-induced seismicity comes from numerical modeling and a limited set of case studies. Therefore, it is critical to continuously monitor the rock mass performance under induced stress. Underground stress changes directly influence the seismic wave velocity of the rock mass, which can be measured by passive seismic tomography. The precise rock mass seismicity can be modeled based on the data recorded by seismic sensors such as geophones of an in-mine microseismic system. The seismic velocity of rock mass, which refers to the propagated P-wave velocity, varies associated with the occurrence of major seismic events (defined as having a local moment magnitude between 2 to 4). Seismic velocity changes in affected areas can be measured before and after a major seismic event in order to determine the highly stressed zones. This study evaluates the seismic velocity trends associated with five major seismic events with moment magnitude of 1.4 at a deep narrow-vein mine in order to recognize reasonable patterns correlated to induced stress redistribution. This pattern may allow recognizing areas and times which are prone to occurrence of a major seismic event and helpful in taking appropriate actions in order to mitigate the risk such as evacuation of the area in abrupt cases and changing the aggressive mine plans in gradual cases. In other words, the high stress zones can be distinguished at their early stage and correspondingly optimizing the mining practices to prevent progression of high stress zones which can be ended to a rock failure. For this purpose a block cave mine was synthetically modeled and numerically analyzed in order to evaluate the capability of the passive seismic tomography in determining the induced stress changes through seismic velocity measurement in block cave mines. Next the same method is used for a narrow vein mine as a case study to determine the velocity patterns corresponding to each major seismic event.



Passive seismic tomography, seismic velocity, rockburst, mining induced seismicity, induced stress distribution, hard rock mining, mining induced seismicity, major seismic events, seismic monitoring