Acoustic Propagation in a Channel with Range-Dependent Sound Speed

Abstract

Ray geometry, travel time, and spreading loss are studied in a horizontal channel in which sound speed depends on range. Formulas valid for a general sound-speed function are developed, and then specialized to the class of linear functions. Rays reaching a fixed receiving point are then isolated and studied. Properties of corresponding arrivals at a fixed receiving point are compared for the cases of a range gradient channel and a constant sound speed channel. Differences in initial inclination angle, travel time, and spreading loss are found and bounds on these differences are obtained. For a CW source, the total field at a receiving point is then determined. It is demonstrated that the introduction of even a weak range gradient causes major changes in the amplitude and phase, including the occurrence of interference nulls accompanied by a sharp offset in phase. Finally, approximate formulas are developed for the phase shift of the total field, caused by a constant range gradient, relative to the phase in isospeed channels.