Rays, modes, wavefield structure, and wavefield stability

Sound propagation is considered in range-independent environments and environments consisting of a range-independent background on which a weak range-dependent perturbation is superimposed. Recent work on propagation in both types of environments, involving both ray- and mode-based wavefield descriptions, have focused on the importance of α, a ray-based "stability parameter," and β, a mode-based "waveguide invariant." It is shown that, when β is evaluated using asymptotic mode theory, β= α. Using both ray and mode concepts, known results relating to the manner by which α (or β) controls both the unperturbed wavefield structure and the stability of the perturbed wavefield are briefly reviewed.