ULTRASONIC-IMAGING AND ACOUSTIC-EMISSION MONITORING OF THERMALLY-INDUCED MICROCRACKS IN LAC-DU-BONNET-GRANITE

Concurrent ultrasonic tomography and acoustic emission monitoring were employed to study thermally induced microfracturing in an unconfined, 15-cm cube of Lac du Bonnet granite from Atomic Energy of Canada Limi ted's Underground Research Laboratory. An electrical resistance cartri dge heater, placed in a central vertical borehole, was used to cycle t he sample to progressively higher peak temperatures between 75-degrees -C and 175-degrees-C. Tomography data were collected, at room temperat ure, before and after each thermal cycle. Acoustic emission monitoring proceeded during both heating and cooling phases of each thermal cycl e. Microfractures opened above 80-degrees-C and eventually coalesced i nto a macroscopic fracture plane. The macroscopic fracture originated at the outer edges of the sample and then extended inward, parallel to the fast velocity direction, and eventually intersected the borehole. Both acoustic emission locations and slowness difference tomography c learly delineated the fracture plane. We attribute the development of the macroscopic fracture to a thermal gradient cracking mechanism acti ng upon a brittle, anisotropic medium.