Seismic anisotropy in granite at the Underground Research Laboratory, Manitoba

The Shear-Wave Experiment at Atomic Energy of Canada Limited's Underground Research Laboratory was probably the first controlled-source shear-wave sur vey in a mine environment. Taking place in conjunction with the excavation of the Mine-by test tunnel at 420 m depth, the shear-wave experiment was de signed to measure the in situ anisotropy of the rockmass and to use shear w aves to observe excavation effects using the greatest variety of raypath di rections of any in situ shear-wave survey to date. Inversion of the shear-w ave polarizations shows that the anisotropy of the in situ rockmass is cons istent with hexagonal symmetry with an approximate fabric orientation of st rike 023 degrees and dip 35 degrees. The in situ anisotropy is probably due to microcracks with orientations governed by the in situ stress field and to mineral alignment within the weak gneissic layering. However, there is n o unique interpretation as to the cause of the in situ anisotropy as the fa bric orientation agrees approximately with both the orientation expected fr om extensive-dilatancy anisotropy and that of the gneissic layering. Eight raypaths with shear waves propagating wholly or almost wholly through grano diorite, rather than granite, do not show the expected shear-wave splitting and indicate a lower in situ anisotropy, which may be due to the finer gra in size and/or the absence of gneissic layering within the granodiorite. Th ese results suggest that shear waves may be used to determine crack and min eral orientations and for remote monitoring of a rockmass. This has potenti al applications in mining and waste monitoring.