ACOUSTIC EVIDENCE OF MECHANICAL DAMAGE SURROUNDING STRESSED BOREHOLES

Laboratory experiments demonstrate that acoustic waveforms recorded in a borehole provide evidence of stress-induced mechanical damage in su rrounding rock. In the experiments, external uniaxial stress was appli ed perpendicular to the borehole. Stress concentrations near the boreh ole wall caused velocities of refracted compressional-wave to vary wit h azimuth. Low velocities occurred in zones of tensile stress, and hig h velocities occurred in zones of compressive stress. Velocity variati ons are on the order of 10%. At high values of externally applied unia xial stress, rock exceeded its yield strength and permanent damage dev eloped. This damage decreased the measured velocities by approximately 10%, especially in the zones of compressive stress concentration. The heterogeneous nature of the velocities surrounding the borehole resul ted in low-velocity channels parallel to the borehole wall, caused eit her by tensile stress concentrations or by mechanical damage. These lo w-velocity channels may be responsible for high-amplitude ''bright-spo ts'' that appear on variable density plots of azimuthal waveform scans . The amplitude increases can be on the order of 500% and are associat ed with low-velocity zones, not with decreased attenuation. The hypoth esized mechanism is acoustic focusing, whereby velocity gradients refr act acoustic waves back towards the borehole.