A controlled <i>in‐situ</i> investigation of the relationship between stress, velocity and induced seismicity

Abstract

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.