Measurements of seismic anisotropy and fracture compliances in synthetic fractured media

Abstract

Ultrasonic velocities were measured on a stack of synthetic materials (Plexiglass plates), at low (90/120 kHz, long wavelength range: 12–23 mm) and high (431/480 kHz, short wavelength range: 1–6 mm) frequencies. The Plexiglass plates were pressed together with uniaxial normal stress. The ultrasonic measurements were repeated under different uniaxial stresses with and without circular rubber inclusions between the plates. The stress dependence of measured P-wave velocities is more pronounced (about 200 m s−1 difference) at low frequency compared to the high frequency measurements. S-wave velocity measurements in the vertical direction (perpendicular to plates) and horizontal direction with polarization perpendicular to the plates show that, without rubber inclusions, the model can be approximated as a vertical transverse isotropy (VTI) medium (difference of about 20 m s−1between the two S-wave velocities for stresses higher than 6 MPa), whereas, for the case with inclusions, there is a significant difference (about 350 m s−1 for stresses higher than 6 MPa). The difference in vertically propagating and horizontally propagating vertically polarized S-wave velocities indicates that the medium is inhomogeneous at these wavelengths. Ultrasonic velocity measurements were used to calculate fracture compliances and VTI anisotropy parameters. For both experiments (with and without rubber disc inclusion), fracture compliances and also absolute values of VTI anisotropy parameters, decrease with increasing stress and frequency. For our experiments, Thomsen's anisotropy parameter γ is highly correlated to ε and η (with average correlation coefficients of about 95 per cent). For all measurements, the medium showed positive anellipticity (i.e. ε–δ > 0). In all cases, the tangential compliance was greater than the normal compliance. For experiments without inclusions, the normal to tangential compliance ratios show an increasing trend with increasing normal stress. For experiments with inclusions, these ratios show a decreasing trend, especially at higher frequency ranges.