Laboratory measurement of seismic velocity dispersion in cracked quartzite

Abstract

Reversible fluid flow within low aspect ratio cracks is expected to cause seismic velocities in hard rock to be strongly frequency dependent. Experimental measurements are necessary to constrain theoretical velocity dispersion models in order to allow comparisons between laboratory measurements at megahertz frequencies, sonic logging at kilohertz frequencies and in-situ exploration seismic at typically 10-300 Hz frequencies, but are rare due to the complexity of low frequency measurements on core samples. Quartzite samples from Cape Sorell, Australia and Alberta, Canada are thermally cracked to induce ~2% crack porosity with aspect ratio <0.01. The shear and Young's moduli of the samples are measured at frequencies of 0.01-1 Hz and 1 MHz while the samples are dry, saturated with argon and saturated with water over effective pressures of 10-150 MPa. As anticipated, no dispersion is exhibited while the samples are dry. Similarly, no dispersion is observed while the samples are argon saturated as a result of the low viscosity and high compressibility of argon. Water saturation, however, causes significant dispersion in both the shear and Young's moduli of the samples between the low and high frequency measurements.