Mechanical and Civil Engineering Seminar

Acoustic emission (AE) analyses that utilize piezoelectric sensors have been used for decades in rock mechanics testing, but because recording systems are not typically calibrated, the absolute sizes of dynamic microcrack growth and other physical processes responsible for the generation of AEs are poorly constrained. I describe efforts to calibrate such recording systems on both biaxial and triaxial rock deformation machines. The first calibration approach includes elastodynamic waveform modeling and benchtop sensor calibration experiments. The second is an empirical calibration technique for the AE recording system as a whole (sensors + amplifiers + digitizers + sample + loading frame) that uses the impact of a tiny ball as a reference source. Using these techniques, we find that tiny AEs that occur during shear loading of the saw-cut simulated fault have moment magnitudes ranging from M -5.5 down to at least M -8. Dynamic slip events that rupture the entire simulated fault surface (stick-slip events) radiate seismic waves similar to those from M -2.5 to -3.5 earthquakes. Absolute calibration allows the laboratory-generated earthquakes to be compared to mining-induced earthquakes and natural earthquakes of all sizes. The tiny events are consistent with observations of self-similar rupture growth and magnitude-independent stress drop.