SURFACE-WAVE CAUSTICS

The geometrical theory of surface wave propagation on a laterally hete rogeneous earth model diverges at caustics, where neighbouring rays cr oss. The caustics on a spherical earth are degenerate focal points at the source and its antipode; lateral heterogeneity removes this degene racy and transforms the caustics into multiply cusped and folded curve s. We investigate the geometric nature of these antipodal and source c austics using both linear ray-perturbation theory and exact ray tracin g. The regions occupied by the R1-R2 and R2-R3 caustics extend as far as 20-degrees from the antipode and 30-degrees from source, respective ly, even on a relatively smooth earth model such as M84A.