Experimental study of fracture size effect on elastic wave velocity dispersion and anisotropy

ABSTRACTWe have designed transversely isotropic models containing penny-shaped rubber inclusions, with the crack diameters ranging from 2.5 to 6.2 mm to study the influence of fracture size on seismic velocity under controlled conditions. Three pairs of transducers with different frequencies (0.5, 0.25, and 0.1 MHz) are used for P- and S-wave ultrasonic sounding, respectively. The P-wave measurements indicate that the scattering effect is dominant when the waves propagate perpendicular to the fractures. Our experimental results demonstrate that when the wavelength-to-crack-diameter ratio (λ/d) is larger than 14, the P-wave velocity can be described predominantly by the effective medium theory. Although the ratio is larger than four, the S-wave velocity is close to the equivalent medium results. When λP/d u003c 14 or λS/d is u003c 4, the elastic velocity is dominated by scattering. The magnitudes of the Thomsen anisotropic parameters ϵ and γ are scale and frequency dependent on the assumption that the transversely...