Uniaxial stress and ultrasonic anisotropy in a layered orthorhombic medium

Abstract

Studies of orthorhombic anisotropy are becoming progressively essential, especially as many sedimentary rocks are considered to have orthorhombic symmetry. To study the effect of stress in a layered orthorhombic medium, a physical modeling study using intrinsically orthorhombic phenolic boards was conducted. The experiment was designed to simulate sedimentary reservoir rocks deposited in layers with inherent orthotropic symmetry and under the influence of stress due to overlying sediments. The study also explores which geologic phenomena dominate the contortion of anisotropy under different stress tenure. The phenolic boards were coupled together with the help of a pressure device and uniaxial stress was gradually increased while time arrival and velocity measurements were repeated. Results show maximum increase in compressional and shear wave velocities ranging from 4% to 10% in different directions as a function of increasing uniaxial stress. P and S wave dependent stiffness coefficients generally increased with stress. Anisotropic parameters (extension of Thomsen's parameters for orthorhombic symmetry) generally diminished or remained constant with increasing pressure and changes ranged from 0% to 33%. We observed anisotropic behavior a priori to both orthorhombic and VTI symmetries in different principal axes of the model. Polar anisotropy behavior is due primarily to layering or stratification and tends to increase with pressure. Certain anisotropic parameters however unveil inherent orthotropic symmetry of the composite model.