High frequency acoustic propagation in a range dependent guiding-to-antiguiding ocean channel transition

Variations in the ambient physical parameters in the ocean may be such as to change a depth dependent profile with weak range dependence from guiding to antiguiding. This paper examines conversion of an initially well-guided high frequency adiabatic mode into a non-guided sound field after passing through the transitional profile region. A theory is developed which smoothly patches a parabolic equation for the transitional domain onto trapped adiabatic mode fields on the guiding side and onto leaky modes on the non-guiding side. The parabolic equation is implemented numerically where it cannot be reduced to the simpler asymptotic forms. Numerical results for the dominant mode in a model profile confirm the analytical features of the theory, and reveal clearly the phenomenology of initial confinement in a surface duct, with subsequent detachment and "beaming" into deep water after passing through the transition to the antiguiding environment.