ABSTRACT
This chapter addresses the development of empirical models applicable to special propagation paths such as surface ducts, shallow water, and Arctic half-channels. Data support requirements for mathematical models of propagation are discussed and a select number of field experiments are described in order to highlight promising areas for future research and development. In shallow-water environments, the optimum frequency of propagation is often the result of competing propagation and attenuation mechanisms at either end of the frequency spectrum. The megaphone effect produces a gain in upslope propagation and a loss in downslope propagation. The processes involved in upslope propagation can be better understood by using the ray-mode analogy. In the case of downslope propagation, the transition is from a radiative to a trapped state. Jensen examined both upslope and downslope propagation using an appropriately modified parabolic equation model. The specification of sound-speed profiles for initialization of propagation models is very important, and the chapter elaborates on this topic.
