Velocity dispersion measurements on micro‐cracked and fluid‐saturated rock

Abstract

Seismic properties of saturated, cracked rock are expected to be strongly frequency dependent as a result of reversible fluid flow within the crack porosity at all scales caused by the oscillating stress induced by seismic waves. Laboratory measurements, typically made with frequencies on the order of MHz, must systematically overestimate in-situ seismic wave velocities that are typically measured with frequencies on the order of mHz-kHz, a range of frequencies applicable to earthquake teleseisms (< 10 Hz) through active exploration seismic and microseismic investigations (~10 to 300 Hz), mine seismology (~1 kHz) and finally geophysical logging (~ 10 kHz). Forced flexural and torsional oscillation of core samples in the laboratory allows measurement of seismic properties at lower frequencies (0.01-1 Hz) that are more directly comparable to typical in-situ frequencies. Here we describe progress in the development of flexural oscillation methods at the Australian National University (ANU) laboratory for use alongside the established torsional mode capability, as well as preliminary results from a thermally cracked synthetic sample of polycrystalline alumina and a thermally cracked core of Cape Sorell quartzite for comparison with ultrasonic measurements on the samples.