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

Abstract

A 15 kHz cross-hole seismic tomography survey was completed between two boreholes that cross a well-known subvertical fracture called the "Room 209 fracture" on the 240 Level of the Atomic Energy of Canada Limited Underground Research Laboratory. This survey measured in situ P- and S-wave velocity and amplitude relations across the fracture between the two boreholes. A velocity anisotropy of about 1.5% (peak-to-peak) exists in the rock mass for both P and S waves. Consistently faster velocities were observed for rays oriented roughly parallel to the Room 209 fracture. This anisotropy is consistent with geological and geotechnical models of microcrack populations for the region. Tomographic velocity images for both the P- and S-waves show a distinct change across the fracture. Higher velocities occur east of the fracture, whereas lower velocities occur to the west. The velocity differences indicate minor changes in the rock structure from one side of the fracture to the other which correlate with observed differences in alteration and fracture frequency. There is a slight reduction in velocity at the fracture itself. A study of seismic amplitudes was also completed. A weak anisotropy also exists in the amplitude data, with higher amplitudes for rays that parallel the fracture direction. The P-wave attenuation ranges between 0.03 and 0.7 dB/m across the survey panel. The region of the fracture shows a subtle increase in the P-wave attenuation of about 0.04 dB/m above the local background. These measurements provide a useful demonstration of the sensitivity of the technique. The results correlate well with geological models and simple physical models for seismic wave propagation in sparsely fractured granite.