Ray stability in weakly range-dependent sound channels
  Ray stability is investigated in environments consisting of a range-independent background sound-speed profile on which a range-dependent perturbation, such as that produced by internal waves in deep ocean environments, is superimposed. Numerical results show that ray stability is strongly influenced by the background sound speed profile. Ray instability is shown to increase with increasing magnitude of alpha := I omega^{prime} / omega, where 2 pi / omega(I) is the range of a ray double loop and I is the ray action variable. The mechanism, shear-induced instability enhancement, by which alpha controls ray instability is described. 
F.J. Beron-Vera and M.G. Brown