Acoustic Propagation in an Isospeed Channel with Uniform Tidal Current and Depth Change

Abstract

Ray geometry, spreading loss, boundary effects, and travel time of individual CW arrivals are examined in a uniformly moving acoustic channel with horizontal boundaries and a constant sound speed. The properties of corresponding arrivals at a fixed receiving point are compared for the cases of a stationary channel, a uniformly moving channel, and a channel with slowly time-varying current. The total field at a fixed receiving point is examined and it is shown that, under rather general conditions, the amplitude is virtually independent of current changes while the phase can vary significantly. The effects on the total field of spatially uniform, tidally related depth variations are then investigated. For a channel with large range-to-depth ratio, it is found that changes in tidal current cause most of the phase variation while amplitude variation is more dependent upon depth changes. Depth changes are shown to generate the phase offset-amplitude fade phenomenon described in earlier investigations. Phase variations associated with current changes are computed for a 33-km×40-m channel and compared with experimental data. Close agreement between theory and experiment is obtained.