Fluid-dependent shear-wave splitting in fractured media

Azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock. A shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal axes of the medium. For vertical propagation of shear waves in a horizontally layered medium containing vertical fractures, the shear-wave splitting depends on the shear compliance of the fractures, but is independent of their normal compliance. If the fractures are not perfectly vertical, the shear-wave splitting also depends on the normal compliance of the fractures. The normal compliance of a fluid-filled fracture decreases with increasing fluid bulk modulus. For dipping fractures, this results in a decrease in shear-wave splitting and an increase in shear-wave velocity with increasing fluid bulk modulus. The sensitivity of the shear-wave splitting to fluid bulk modulus depends on the interconnectivity of the fracture network, the permeability of the background medium and on whether the fracture is fully or partially saturated.