Laboratory measurements of frequency-dependent seismic properties of cracked and fluid-saturated media

Abstract

The capability to perform laboratory measurements with both low-frequency forced-oscillation and high-frequency wave-propagation methods, under conditions of independently controlled confining and pore-fluid pressure, offers the prospect of new insight into the frequency-dependent seismic properties expected of cracked and fluid-saturated rocks of the Earths upper crust. An important step in the development of such broad-band capability has been the modification of existing laboratory equipment to newly allow flexural, as well as torsional, forced-oscillation testing of cylindrical rock specimens. Flexural oscillation tests on an experimental assembly containing a fused silica control specimen yield results indistinguishable from those of numerical modelling with both finite-difference and finite-element methods ? demonstrating the viability of the method. Both torsional and flexural oscillation methods along with complementary high-frequency wave propagation methods have been applied to specimens of dense polycrystalline alumina and quartzite, each thermally cracked to generate an interconnected network of cracks of low aspect ratio, and tested dry, and saturated with either argon or water. The shear and flexural moduli vary systematically with effective pressure ? providing clear evidence of pressure-induced crack closure. Similarities and differences between effective moduli measured under different conditions of pore-fluid saturation are tentatively interpreted in terms of the timescales for stress-induced redistribution of pore fluid.