A 15 kHz cross-hole seismic survey across a fracture at the AECL Underground Research Laboratory

摘要

A 15 kHz cross-hole seismic tomography surveywas completed between two boreholes that cross a well-knownsubvertical fracture called the "Room 209 fracture" on the 240Level of the Atomic Energy of Canada Limited UndergroundResearch Laboratory. This survey measured in situ P- and S-wavevelocity and amplitude relations across the fracture between thetwo boreholes. A velocity anisotropy of about 1.5% (peak-to-peak) exists in the rock nass for both P and S waves. Consistentlyfaster velocities were observed for rays oriented roughly parallelto the Room 209 fracture. This anisotropy is consistent withgeological and geotechnical models of microcrack populationsfor the region. Tomographic velocity images for both the P- andS-waves show a distinct change across the fracture. Highervelocities occur east of the fracture, whereas lower velocitiesoccur to the west. The velocity differences indicate minorchanges in the rock structure from one side of the fracture to theother which correlate with observed differences in alteration andfracture frequency. There is a slight reduction in velocity at thefracture itself. A study of seismic amplitudes was also completed.A weak anisotropy also exists in the amplitide data, with higheramplitudes for rays that parallel the fracture direction. The P-wave attenuation ranges between 0.03 and 0.7 dB/m across thesurvey panel. The region of the fracture shows a subtle increasein the P-wave attenuation of about 0.04dB/m above the localbackground. These measurements provide a useful demonstrationof the sensitivity of the technique. The results correlate well withgeological models and simple physical models for seismic wavepropagation in sparsely fractured granite.