Geophysical imaging of frictional contacts and processes in shaly sandstone rock joints

We use a direct shear apparatus with embedded ultrasonic transducers to correlate the macroscopic frictional response with the microscopic contact processes occurring between two blocks of shaly sandstone. At constant normal load, we observe stable sliding at low velocities and oscillatory stick-slip at high velocities. For slow sliding, or during the stick phase of the stick-slip cycle, variations in the transmitted compressional P-wave amplitude show the existence of healing processes occurring at the joint, e.g. associated to the increase in contact area with contact time. Moreover, the transmitted shear S-wave amplitude is sensitive to other processes with opposite velocity dependence. The interplay between these processes, displaying velocity weakening (VW) and velocity strengthening (VS) respectively, explain the observed maximum in the steady state shear response as function of shearing rate. We also observe that the wave velocity is sensitive to gouge formation at the joint. This mechanism is induced by sliding and enhanced with shearing rate. At low shearing rates, in the VS region, small amounts of debris are formed, slightly strengthening the joint. At high shearing rates, in the VW region, the wear material significantly increases the mean separation between the two surfaces, resulting in a weaker joint.