Effects of point singularities on shear-wave propagation in sedimentary basins

Abstract

SUMMARY In most directions of propagation in anisotropic solids, seismic shear waves split in regular and predictable ways that, in principle, can be directly related to the degree of anisotropy and the anisotropic symmetry of the rockmass. In all anisotropic solids, however, there are directions of propagation, known as shear-wave singularities, where the split shear-waves have the same phase-velocities. For directions of propagation near the commonest type of singularity, the point singularity, the relationship between the phase and group-velocities may undergo rapid variations for small changes in direction. This results in shear-waves along rays (propagating at the grozq-velocity) behaving anomalously, with irregular polarizations and amplitude changes as if they were propagating near cusps, although the degree of anisotropy may be too small to cause conventional cusps on the group-velocity wave surfaces. The effects of propagation near such point singularities have been identified in sedimentary basins where they are features of the well-established phenomenon of azimuthal isotropy (transverse isotropy with a vertical axis of symmetry) caused by horizontal lithology, or by fine layering (PTL anisotropy), combining with the more recently recognized azimuthal anisotropy, caused by distributions of near-parallel near-vertical fluid-filled inclusions (EDA anisotropy). This paper demonstrates these irregular effects by calculating synthetic shear waves in directions near a point singularity in a material simulating a possible sedimentary basin. Such anomalies may be important in exploration seismology as point singularities can occur along nearly vertical ray paths in sedimentary basins. If not identified correctly, the effects of such point singularities could be mistakenly attributed to structural irregularities, and if correctly identified, the directions of such singularities can place tight constraints on possible combinations of PTL and EDA anisotropy in sedimentary basins.