Broadband laboratory measurements of dispersion in thermally cracked and fluid-saturated quartzite and a synthetic analogue

In the field, the seismic waves used for active-source imaging typically contain frequencies from 10 to about 100 Hz, with corresponding wavelengths of tens of meters. This contrasts greatly with the ultrasonic (∼ 1-MHz) wave-speed measurements carried out in the laboratory, with millimeter wavelengths. The purpose of the laboratory measurements is, of course, to provide insight into seismic wave speeds in situ. However, with the presence of a pore fluid, velocity measurements are sensitive to the frequency at which velocity data are collected. A study focuses on such fluid-flow-related dispersion by performing a broadband measurement in the laboratory from millihertz (mHz) to megahertz (MHz) frequencies on a natural quartzite and on a synthetic sintered glass-bead sample. Thermal cracks that have small aspect ratios of about 10^-4 to 10^-3 were introduced in both samples, which are of low porosities (1% to 2%) even after thermal cracking. A seismic-frequency forced-oscillation method is combined with a high-frequency ultrasonic technique, providing access to a wide frequency range. Under water-saturated conditions, the observed seismic wave speeds display substantial variations between seismic and ultrasonic frequencies in the cracked quartzite. A systematic increase in shear modulus, attributed to the suppression of fluid flow, has been monitored on the cracked glass-bead specimen with both argon and water saturation at ultrasonic frequency.