In many long-range ocean propagation experiments the source and receiver are placed close to the depth of the waveguide axis. In this case, rays emerging from the source at sufficiently small angles intersect the sound-channel axis many times and form in its vicinity a large number of caustics with caustics cusps located repeatedly along the axis. In neighborhoods of cusped caustics there exists a very complicated interference pattern. Neighborhoods of interference grow with range and at long ranges they overlap. As a result, a complex interference wave (axial wave), that propagates along the waveguide axis, appears. The goal of this paper is to obtain the representation for the axial wave in the time domain and calculate its magnitude for a realistic model of a three-dimensional range-independent medium. Numerical computations are done for the average profile from the Acoustic Engineering Test (AET) experiment. The pulse center frequency of 75 Hz with 30-Hz bandwidth is used that corresponds to broadband acoustic signals which were transmitted during November 1994 in the eastern North Pacific Ocean as a part of the AET. The propagation range is 3250 km. The sound source is located on the waveguide axis, and the receiver is placed close to the depth of the axis. Through numerical simulation the dependencies of the magnitude of the axial wave on depth of the receiver and propagation range are studied.
Long-range undersea acoustic pulse propagation: A stochastic-mechanical approach