Characterization of the Body Wave Anisotropy of an Interbedded Sandstone-Shale at Multi Orientations and Interlayer Ratios

Characterizing the elastic response from body waves propagating through an interbedded shale-sandstone geological formation with multi-orientation joints and various interlayer ratios is extremely challenging. A laboratory composite model of three layers, i.e., sandstone-shale-sandstone, with multi-angled joint orientations, i.e., 30 degrees, 45 degrees, 60 degrees, and 90 degrees, provides an expressive approach to evaluate the anisotropic behavior of body wave velocities. In this study, body wave velocities were obtained from tests using the Portable Ultrasonic Non-Destructive Indicating Tester and the Free-Free Resonant Column. The anisotropic dynamic responses of the composite model are characterized in comparison to the intact shale and the intact sandstone. Empirically, as the joint orientation approached a perpendicular angle, both velocities of the P-wave (V-P) and the S-wave significantly decreased. In comparison, the wave velocity values were reduced by 35% and 66% for V-P and V-S, respectively, as were the increment angles of joint orientation from 30 degrees to 90 degrees. The anisotropic behavior of wave velocity for the composite model appears to be influenced by the joint orientation rather than the bedded shale interlayer. The increase in elastic modulus and shear modulus values with respect to the increase in interbedded angles indicates that interbedded orientation has a significant influence on the stiffness behavior of the composite model. This result was supported by the decrement of Poisson's ratio, measured as the increment of the interbedded angle of orientation. The experimental data is then utilized to develop a semi-empirical model to predict the wave velocity at any angle of joint orientation and at any interlayer ratio.