David E. Weston
White Laird, 77 Wyke Rd., Weymouth, Dorset DT4 9QN, England
Several sets of propagation problems in shallow-water acoustics are discussed, covering overlapping frequency ranges. At VLF (say below 300 Hz) the bottom material is important, because energy travels in the bottom or because of attenuation associated with shear-wave coupling. At LF (say above 30 Hz) the presence of gas in surficial sediments can change boundary conditions and bottom losses, and even allow a new type of interface wave. At MF (300 Hz to several kHz) dispersed fish can raise the attenuation, typically by 1 dB/km. Fish have also been observed to dominate the echo structure or reverberation, out to 100 km. At HF (say above several 100 Hz) the sound-speed profile becomes important, partly because of channeling. At VHF (usually above several kHz) the propagation mechanisms tend to change from boundary reflection to boundary scattering. In addition all these effects are greatly influenced by distance and by water depth.