Measurements of anisotropy parameters with variable frequency and stress in synthetic fractured media

Abstract

VTI anisotropy parameters for a synthetic fractured medium were computed from ultrasonic velocity measurements in vertical and horizontal directions. The medium was a stack of plexiglass plates, pressed together with uniaxial normal stress. The ultrasonic measurements were repeated under different uniaxial stresses with and without circular rubber inclusions between the plates, and at low (90/120 kHz, long wavelength range: 12-23 mm) and high (431/480 kHz, short wavelength range: 1-6 mm) frequencies.The stress dependence of measured P-wave velocities was more pronounced 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 VTI medium whereas for the case with inclusions there is a significant difference between velocities of the two S-waves. For our experiments, Thomsen's anisotropy parameter γ is highly correlated to ε and η (with correlation coefficients of about 95% on average). For all measurements, the medium showed positive anellipticity (i.e. ε-δ > 0).