Datensatz

Zusammenfassung

A better understanding of damage accumulation before dynamic failure events in geological material is essential to improve seismic hazard assessment. Previous laboratory tests revealed that such failure events are preceded by a phase of extending micro-cracks, leading to detectable changes in bulk seismic properties. We use seismic velocity estimates to measure changes in micro-crack populations and damage in intact actively and faulted Westerly granite samples. We use an array of 16 piezo-ceramic transducers to send and record ultrasonic pulses and to determine temporal seismic velocity changes. The coda-wave interferometry (CWI) and direct phase arrivals determine velocity changes.   The tests show that: 1) Higher confining pressures increase seismic velocities due to reduced pore space and no hysteresis effects during the unloading cycle. 2) During increasing differential stress, the crack growth before the onset of AE activity is aseismic and could be associated with a monitoring or deformation mechanism. However, the velocity measurement records the changes in a medium way before we observe AE or non-linearity in stress and strain data. 3) Direct waves exhibit strong anisotropy, increasing differential stress and accumulating damage before rock fracture. On the other hand, CWI produces velocity variations that are similar to P-wave velocity changes along the vertical axis. 4) Seismic interferometric measurements are thought to be highly sensitive to changes in the medium. However, we found that such measurements also depend on the type of rock damage (i.e., orientation and shape).