Ultrasonic monitoring of frictional interfaces

Acoustic_TransmissivityThere has been recent interest in ultrasonic monitoring as a means to probe the mechanics of contact interfaces during rock friction interfaces. Ultrasonic waves transmitted across sliding surfaces in rocks have been shown to be sensitive to the ‘state’ of the frictional interface, and these hold promise as meaningful probes of the deformation of microcontacts during frictional sliding. Such ultrasonic data have revealed a hitherto unknown shear stress dependence in fault friction. Our analysis of 1-D nucleation with this new rate-state formulation reveals that the introduction of the stressing rate dependence significantly alters the response to large velocity steps, and hence the style of nucleation.


Left to right: Shear stress and ultrasonic responses to 3 times, 30 times and 300 times velocity step increases plotted versus time (in units of 1000s). We simultaneously monitor P- and S-signals to better probe the rheology of the interface. The transmitted amplitudes and travel times both show sensitivity to sliding conditions.

In continuing collaboration with the Penn State Rock Mechanics laboratory, we have been trying to improve our understanding of the contact scale mechanics by simultaneously using P- and S-transmitted amplitudes and travel times. Based on even the simplest models of contact interfaces, one can argue that S- and P-transmissivities would exhibit differences in their sensitivities to the deformation of elasto-plastic contacts. When one considers additional complexities arising out of shear loading and slip across contacts, it is not unreasonable to argue that, when compared to earlier experiments using P-wave data alone, the combined use of P- and S-transmissivities might provide new information on the contact-rheology of the frictional interfaces in these experiments.