Estimations of P- and S-wave velocities and anisotropy from measurements on artificial fractured samples

Abstract

Based on the assumptions of the Hudson theory, we built nine physical models, eight of which contain penny-shaped inclusions used to simulate fractures. The fracture density of the eight fractured models ranges from 2% to 12%. In the case that the fracture size was smaller than the wavelength, P- and S-wave transducers with three different ultrasound frequencies were used to estimate the P- and S-wave velocities of the matrix and inclusions from the traveltime picks and distance measurements. The experimentally obtained velocities were substituted into the Hudson model to calculate the theoretical velocities and anisotropy of fractured models, which were then compared with the experimental values of velocities and anisotropy. In general, with the variation of fracture density, the experimental results agreed well with the theoretical estimations within the frequency range from 0.10 to 0.50 MHz. The experimental values of P- and S-wave velocities and anisotropy showed conspicuous dispersion. With the decrease of frequency, P- and S-wave velocities decreased, whereas the anisotropy increased. Besides, the interaction of fractures in the case of high fracture density can affect the propagation of the slow S-wave. When the ratio of S-wave length to fracture diameter ([Formula: see text]) was lower than five, a train of interference wave with small amplitude appeared before the first arrival of slow S-wave, which will possibly have influence on the identification of the slow S-wave.