Modal sensitivity of acoustic interactions with internal waves in shallow water

Abstract

Analyses of individual acoustic mode interactions with internal waves should permit both qualitative and quantitative understanding of the influences of internal waves on multimode signals in shallow water. In this report, some experimental results taken in the Yellow Sea during the summer with strong thermoclines, will be reported. Numerical simulations will be used to analyze the potential internal wave interactions suggested by these data. (a) A multimode signal obtained at a single hydrophone and the first acoustic mode obtained by a mode filter array, both from a fixed 442-Hz cw source, exhibit very different fluctuation characteristics. (b) The transmission loss (TL) measured with explosive sources indicates a strong source/receiver depth dependence. (c) TL is, generally, anisotropic and can be abnormally large over some frequency range. It has been shown that the acoustic mode-coupling induced by internal waves can be an important loss mechanism for sound propagation during the summer [Zhou etal., J. Acoust. Soc. Am. 90, 2042–2054 (1991)]. The observed fluctuations can be explained in terms of the propagation characteristics of individual modes to the presence of internal waves. In so far as the mode-coupling and both the intensity and arrival time fluctuations are concerned, certain acoustic modes are more sensitive to internal waves than others. The characteristics of acoustic-internal wave interaction strongly depend on signal frequency, thermocline profile, seabottom properties, source/receiver depths and distance. [Work supported by ONR.]