Abstract
Sedimentary rock in the Earth’s crust is often layered and contains fractures which gives rise to two sources of seismic anisotropy, namely, textural and structural. These two sources of seismic anisotropy can have aligned or non-aligned symmetry axes depending on the orientation of the fractures relative to the layering. An acoustic wavefront imaging method was used on fractured transversely isotropic media to unravel the interaction between the fractures and the matrix layering. Wave-guiding theory for parallel fractures in transversely isotropic matrix shows that the delays of guided-modes are mainly affected by the fracture stiffness, signal frequency and fracture orientation with respect to the layering. From the experimental data, the apparent anisotropy was dominated by fractures at low stress because the energy was confined between the low stiffness fractures. At high stresses, the apparent anisotropy was controlled by the matrix because the fracture stiffness was sufficient to enable wave transmission across the fractures. The interpretation of the presence of fractures in anisotropic media can be determined if seismic measurements are made as a function of stress, which eliminates many fracture-generated guided-modes.
