f Shear-wave point singularities in sedimentary basins

Shear waves may not behave uniformly for all directions of propagation in sedimentary basins. Splitting of seismic shear-waves (bi-refringence) is observed in many sedimentary basins, and appears to be caused by propagation through a combination of two phenomena: 1) the azimuthal isotropy of fine horizontal layering or Iithology, leading to transverse isotropy with a vertical axis of symmetry, and velocity anisotropy of anything up to 30 or 40%; and 2) the azimuthal anisotropy of stress-aligned fluid-fiIled cracks, microcracks, and preferentially oriented pore-space, leading to transverse isotropy with a horizontal axis of symmetry, and much weaker velocity anisotropy (often less than 5 %). The combination of the two transverse isotropies with orthogonal axes of summetry leads to an orthorhombic symmetry system.