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 Pc and argon pore-fluid pressure Pf 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, Pc-Pf, which ranges from ~120 to 160 MPa for temperatures of 300-600°C. This suggests crack aspect ratios on the order of 10-3. The covariation of in-situ permeability k and thermal crack density ε that we infer from the modulus deficit is consistent with percolation theory. There is a well-defined threshold at εc~0.17, beyond which k increases arkedly as (ε-εc)v, with v~2. 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, Peff = Pc -nPf, with the value of n increasing toward one with increasing crack connectivity. © 2006 Society of Exploration Geophysicists.