A CONTROLLED IN-SITU INVESTIGATION OF THE RELATIONSHIP BETWEEN STRESS, VELOCITY AND INDUCED SEISMICITY

A p-wave velocity survey was performed after the excavation of a cylindrical, horizontal tunnel through homogenous, visibly unfractured granite. An image of the velocity structure around the tunnel indicated a distinct zone of high velocity corresponding to a zone of increased elastic stresses, where a break-out notch formed. Perpendicular to this zone, a region of decreased velocity was found associated with a region of tensile elastic stress. The stress changes are believed to have preferentially opened and closed pre-existing micro-fractures in the rock, thereby decreasing and increasing the p-wave velocity. Additional velocity decreases are also attributed to creation of new fractures. The excavation-induced microseismicity is found to be generally associated with the high-velocity region adjacent to the notch, and appears to be located along the boundary between the fractured, low-velocity and high-velocity regions.