Low-frequency seismic properties of thermally cracked and argon-saturated granite

Abstract

Torsional forced-oscillation techniques have been used to measure the shear modulus and strain-energy dissipation on cylindrical specimens of a fine-grained granite, Delegate aplite. The specimens were subjected to thermal cycling and associated microcracking under varying conditions of confining pressure [Formula: see text] and argon pore-fluid pressure [Formula: see text] within the low-frequency saturated isobaric regime. Complementary transient-flow studies of in-situ permeability and volumetric measurements of connected crack porosity allowed the modulus measurements to be interpreted in terms of the density and interconnectivity of the thermally generated cracks. The modulus measurements indicate that newly generated thermal cracks are closed by a differential pressure, [Formula: see text], which ranges from [Formula: see text] for temperatures of [Formula: see text]. This suggests crack aspect ratios on the order of [Formula: see text]. The covariation of in-situ permeability [Formula: see text] and thermal crack density [Formula: see text] that we infer from the modulus deficit is consistent with percolation theory. There is a well-defined threshold at [Formula: see text], beyond which [Formula: see text] increases markedly as [Formula: see text], with [Formula: see text]. At lower crack densities, it is difficult to measure the sensitivity of shear modulus to variations of confining and pore pressures because pore-pressure equilibrium is approached so sluggishly. At temperatures beyond the percolation threshold, the modulus variation is a function of the effective pressure, [Formula: see text], with the value of [Formula: see text] increasing toward one with increasing crack connectivity.