Deep-water acoustic coherence at long ranges: Theoretical prediction and effects on large-array signal processing

This paper presents results of combined consideration of sound coherence an d array signal processing in long-range deep-water environments. Theoretica l evaluation of the acoustic signal mutual coherence function (MCF) of spac e for a given sound-speed profile and particular scattering mechanism is pr ovided. The predictions of the MCF are employed as input data to investigat e the coherence-induced effects on the horizontal and vertical array gains associated with linear and quadratic beamformers with emphasis on the optim al ones. A method of the radiation transport equation is developed to calcu late the MCF of the multimode signal under the assumption that internal wav es or surface wind waves are the main source of long range acoustic fluctua tions in a deep-water channel. Basic formulations of the array weight vecto rs and small-signal deflection are then exploited to examine optimal linear and quadratic processors in comparison with plane-wave beamformers. For ve rtical arrays, particular attention is paid also to evaluation of the ambie nt modal noise Factor. The numerical simulations are carried out for range- independent environments from the Northwest Pacific for a sound frequency o f 250 Hz and distances up to 1000 km, It was shown distinctly that both sig nal coherence degradation and modal noise affect large-array gain, and thes e effects are substantially dependent on the processing technique used. Rou gh surface sound scattering was determined to cause the most significant ef fects.