Temporal coherence of broadband acoustic signals in the presence of internal waves in shallow water

Abstract

Ocean medium fluctuations reduce the temporal coherence of acoustic signals. At low (e.g., 500 Hz) frequencies, temporal coherence limits the time duration of a replica field used for matched-field source localization in a random ocean. At mid (e.g., 10 kHz) frequencies, temporal coherence of the acoustic signal dictates the update rate for the channel equalizer for a phase coherent communication scheme. In this paper, the temporal coherence of broadband acoustic signals is studied at low- and mid-frequencies in a shallow-water environment which includes both the linear and nonlinear (solitary) internal waves. The internal wave model is constructed based on the measured (average) sound-speed profile and sound-speed variance collected during the SWARM-95 experiment. A broadband pseudorandom (PRN) signal is used at low- and midfrequencies. The signals are propagated using a PE model through the internal wave perturbed sound-speed profiles which evolve in time according to the internal wave equation of motion. Temporal coherence at 400 Hz is shown to agree with experimental observations based on the SWARM-95 measurements. Implications for midfrequency communications will be discussed. [This work is supported by the Office of Naval Research.]