ABSTRACT:

Mine development and rock mass excavation impose stress changes in the surrounding rock that typically introduce mechanical failure, causing seismicity or acoustic emission and rock degradation, and consequently altering velocity inside the rock mass. Methods of detecting these changes include both passive and active monitoring techniques. Passive microseismic monitoring utilising the acoustic emission energy and provides insight into the rock failure. Ultrasonic velocity tomography, on the other hand, is a velocity imaging technique to characterise the rock deformation in the damage zones and thus provides better evaluation of the integrity. Based on these two monitoring methods, we introduce a seismic imaging technique that is able to detect the velocity variation of the rock mass using both passive seismic and active ultrasonic sources. In this paper, we first demonstrate the performance and advantages of this imaging technique in locating microseismic events and recovering velocity variations using synthetic data. We then apply the technique in two different scenarios:

  1. induced seismicity monitoring during the caving development of Lift 2 of Northparkes mine and

  2. active ultrasonic survey in an Excavation Damage Zone (EDZ) in the underground nuclear facility Onkalo.

We show that the fracture evolution and dynamic velocity variations inside the rock mass is well detected by the tomographic velocity images. Time-lapse images could be produced to reveal the stress evolution at different development and production phases. With the passive sources covering the seismogenic zone and active survey illuminating the aseismic zone, this technique can fully utilize all the ray paths available and provide enhanced images of the rock damage and degradation.

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