Abstract
Ultrasonic wave velocities in rock are reduced significantly by the presence of microcracks. In general, these microcracks are not randomly orientated and the rock displays an elastic anisotropy determined by the shape and content of the cracks and by the crack orientation distribution function. This function gives the probability of a crack having a given orientation with respect to a set of axes fixed in the rock, and is used here to calculate the variation of elastic wave velocity with propagation direction. The coefficients, W lmn, of a series expansion of the crack orientation distribution function in generalized spherical harmonics can be obtained to order l = 4 from the angular variation of the ultrasonic wave velocity. This allows construction of microfracture pole figures, which may be compared with those obtained by petrofabric examination. The theory is applied to the measurements of Thill, Willard and Bur on Salisbury granite. Since the anisotropy in strength properties originates from the preferred orientation of microstructural defects, the prediction of the microfracture orientation distribution will have an important application in the field of rock fracture.
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