Investigations of the relations among residual strain, fabric, fracture and ultrasonic attenuation and velocity in rocks

Abstract

Residual strain measured by X-ray diffractometry, fabric, and ultrasonic velocity and attenuation in blocks of dry Charcoal Granite, Sioux Quartzite, and Berea Sandstone are investigated to determine their causes and effects and the degree to which each can be used to predict fracture anisotropy. The statistical trends of tensile fractures, induced by point-loading oriented discs, are reliably predicted from ultrasonic data in all three rocks; the attenuation data reflect some not sensitive to velocity. In the granite the fractures are compatible geometrically and probably genetically with the orientations and magnitudes of the residual strains (i.e. prestress) and with microfractures and exsolution lamellae. Fractures in the quartzite and sandstone are primarily oriented parallel to bedding. Those not parallel to bedding in the quartzite are compatible with the residual strains. Ultrasonic data for the bedded rocks do not correlate with any of the microscopic fabric elements studied. The tendency for tensile fractures in the sandstone and quartzite to propagate along grain boundaries moreso than for the granite suggests minute openings or flaws may exist at the boundaries and these may predominantly influence fracturing and acoustic properties.