PRELIMINARY ANALYSIS OF SHEAR-WAVE SPLITTING IN GRANITE AT THE UNDERGROUND RESEARCH LABORATORY, MANITOBA

The excavation of a test tunnel at a depth of 420 m in the Mineby Experiment at the Atomic Energy of Canada Limited’s Underground Research Laboratory, Manitoba, Canada, has allowed shear-wave propagation to be examined in accurately controlled conditions in in-situ granite. This is a preliminary report of shearwave splitting in 4.2 kHz signals from controlled shear-wave sources at distances between 7.5 m and 63 m over a wide range of azimuths and angles of incidence. Preliminary analysis indicates that about 78% of the data show the three-dimensional pattern of behaviour expected from propagation through uniform distributions of EDA cracks [(E)xtensive-(D)ilatancy-(A)nisotropy crack: stressaligned, fluid-filled microcracks] aligned relative to the measured stress field. Some 17% of the data with raypaths near the tunnel have anomalous behaviour which is attributed to the effects of excavation damage around the tunnel opening. This damage appears to be more extensive around the advancing face of the tunnel than around the length of the tunnel. The remaining 5% of data have anomalous behaviour which has not yet been interpreted. EDA cracks are the most compliant elements of the rock mass, and the sensitivity of shear waves to this internal crack geometry demonstrated by this experiment shows that controlled-source shear-wave experiments may be used for monitoring cracking, including induced cracking, and excavation damage in radioactive waste repositories, mines, geothermal reservoirs and other subsurface operations.