Rays and modes in an acoustic channel with exponential velocity profile

High‐frequency sound propagation in a guiding channel is usually analyzed either in terms of a normal mode expansion or in terms of ray acoustics. These formulations lead to difficulties when the number of required modes or rays is excessively large. Moreover, ray acoustics requires corrections near caustics of the ray system, and actually becomes inapplicable in a surface duct where the ray fields may experience an arbitrarily large number of reflections. To alleviate these difficulties, we apply a newly developed theory that utilizes a judiciously chosen mixture of ray fields and modal fields to represent the signal due to an oscillating line source at various depths, observed at different depth and range intervals. The medium is modeled by an exponentially increasing sound velocity between the free surface and a homogeneous semi‐infinite bottom. This environment, which may be encountered in the ocean, combines surface duct and waveguide phenomena. The nature of the hybrid formulation for surface ducted fields and fields guided by top‐bottom interaction is studied in detail, as is the possibility of choosing the ray‐mode mixture so as to avoid corrections near caustics of the ray‐acoustic pressure field.