Abstract
The caustics of high-frequency wave propagation may be classified using catastrophe theory. The wavefield in the vicinity of any caustic is described by the corresponding diffraction catastrophe. The singularity index, β, is a measure of the rate at which such a wavefield diverges as ω→∞ at the point where all control parameters and moduli are set equal to zero. It is shown that away from this point β also describes a balance between two different measures of the unfolding of the wavefield in each control direction,
β =
σ
n
ϱ
n
. The indices σ
n
and ϱ
n
describe, respectively, the rate at which individual ray arrivals separate in time and decay as a function of control parameter.
