ANOMALOUS PARABOLIC EQUATION RESULTS FOR PROPAGATION IN LEAKY SURFACEDUCTS

Surface ducts are formed by wind mixing at the sea surface and are a c ommon feature in many of the world's oceans. These surface ducts have the effect of channeling acoustic energy for long ranges. This paper, however, focuses on the energy that leaks out of the surface duct-path s that are typically neglected by ray models. It is found that the lea kage energy at lower frequencies can be surprisingly strong so that a receiver in a ray-theory shadow zone is actually well insonified. Furt hermore, the leakage energy is refracted back into the duct where it m ay dominate the ducted paths. Inside the duct, these two arrivals add up constructively or destructively resulting in anomalously high or lo w acoustic levels in the surface duct relative to a prediction that ne glects the leaky arrival. Thus, a full-wave model is needed; however, certain types of parabolic equations (PEs) fail for these problems bec ause they scramble the phases of the two dominant arrivals. The same m echanism that causes the PEs to fail makes the problem very sensitive to environmental data. For instance, a change in the mean duct speed o f 0.5 m/s can also produce large changes in the acoustic levels.