Degenerate orthorhombic models

Abstract

SUMMARY We define the degenerate orthorhombic anisotropy models which have two symmetric singularity lines with constant phase velocity for S1 and S2 waves. Depending on the singularity line trajectory, we consider two types of degenerate models (VTI- and HTI-type). In addition to this singularity line, seriethere is always one isolated singularity point in one of non-essential symmetry planes. The degenerate orthorhombic model has seven independent parameters and can be parametrized by different combinations of the stiffness coefficients. Exploiting the fact that the second-order derivatives matrix computed from the Christoffel polynomial is degenerate, we also compute the group velocity image of this singularity line.