Acoustic evidence of mechanical damage surrounding stressed boreholes

Abstract

Abstract Laboratory experiments demonstrate that acoustic waveforms recorded in a borehole provide evidence of stress-induced mechanical damage in surrounding rock. In the experiments, external uniaxial stress was applied perpendicular to the borehole. Stress concentrations near the borehole wall caused velocities of refracted compressional-wave to vary with azimuth. Low velocities occurred in zones of tensile stress, and high velocities occurred in zones of compressive stress. Velocity variations are on the order of 10%. At high values of externally applied uniaxial stress, rock exceede its yield strength and permanent damage developed. This damage decreased the measured velocities by approximately 10%, especially in the zones of compressive stress concentration. The heterogenous nature of the velocities surrounding the borehole resulted in low-velocity channels parallel to the borehole wall, caused either by tensile stress concentrations or by mechanical damage. These low-velocity channels may be responsible for high-amplitude "bright-spots" that appear on variable density plots of azimuthal waveform scans. The amplitude increases can be on the order of 500% and are associated with low-velocity zones, not with decreased attenuation. The hypothezised mechanism is acoustic focusing, whereby velocity gradients refract acoustic waves back towards the borehole.