: Ultrasonic wave velocity measurements were made around one quadrant of a tunnel at Atomic Energy of Canada''s Underground Research Lab. The purpose of these measurements was to characterize the microcrack population induced during excavation of the tunnel at 420 m depth. An array of 1 m deep boreholes were drilled from the sidewall to a well-developed notch in the roof. Velocity measurements were made by pulsing on a source transducer at numerous points within a hole and on the tunnel surface, and then capturing the transmitted waves at receivers in the adjacent holes. The travel times of the waves between sources and receivers were used to compute the velocities and to construct a 2D tomographic image of the slowness (the inverse of velocity) and the anisotropy in this region. Velocities in the sidewall and midway to the notch are low and show significant anisotropy, indicating the presence of microcracks even as deep as 1 m from the tunnel wall. Velocities are high in the notch region with low anisotropy, indicating the rock there is relatively undamaged.
Studies of the physical properties of Lac du Bonnet granite from Atomic Energy of Canada''s Underground Research Laboratory (URL) show that the rock strength is reduced by the presence of microcracks (Martin & Read 1992, Martin & Chandler 1994). Because the velocity and amplitude of ultrasonic waves propagating through rock contain information about the density and orientation of microcracks, such data provide a means for characterizing the damage to material surrounding an underground excavation.