The Development and Evaluation of a Crosshole Seismic System for Crystalline Rock Environments

Included in Research as Applied Geophysics No. 27, Geophysics Laboratory, University of Toronto, Canada.Crosshole seismic techniques appear to be useful in the evaluation of the mechanical properties of inhomogeneous crystalline rock masses, particularly when near surface weathered zones prohibit the use of high resolution surface seismic methods. A project has been undertaken to develop a crosshole seismic system specifically designed to meet the criteria of the Canadian Nuclear Fuel waste Management Program. Because the rock masses to be studied are at depth, the system was required to operate in environments with high hydrostatic pressures of up to 15 mPa, and was required to be non-destructive, because of the high cost of the boreholes. The system that has been built utilizes piezoelectric ceramics in a wall-clamped accelerometer geometry as both receiving and transmitting transducers. Although the peak power of the transmitter is limited by the design criteria, the highly controllable nature of the piezoelectric ceramic allows the use of a continuous transmitted signal. The impulse response of the rock mass is then estimated using cross-correlation techniques. In this manner significant noise reduction is obtained and the detection of low level signals is enhanced. The received signals are digitized and processed in the field using a microcomputer system. Tests have been conducted which demonstrate that crosshole seismology is capable of evaluating the mechanical properties of rock masses. P-wave transmissions on paths of up to 300 m and S-wave transmissions on paths of up to so m nave been recorded, and show that both velocity and amplitude information in the data reflect geological and geophysical effects observed using other methods. The system has also been used to study tube wave phenomena, which can provide information on the shear modulus of the rock in the absence of direct shear arrivals. Although problems exist in accurately determining the position of the probes and in calibrating the amplitude response of the system, valuable information can be collected and interpreted with the present system. Future developments in instrument design and in methods of tomographic reconstruction hold great promise in several fields of application, including mining engineering and rock mass assessment.M.Sc