Effect of sea-surface roughness on sound transmission in the ocean mixed layer

Abstract

A series of 20 experiments on sound propagation in the mixed layer has been conducted in deep ocean areas around Australia. The results of the first eight experiments were reported at the 89th Meeting of the Acoustical Society [J. Acoust. Soc. Am. 57, S63(A) (1975)]. The results of the remaining 12 experiments are presented in this paper. This second set of experiments differs from the first set in that acoustic backscattering experiments were also conducted (in order that the scattering strength of the sea surface could be determined), and the sea surface roughness was monitored with a wave-rider buoy (in addition to visual estimates). These experiments have encompassed a wide range of oceanographic conditions: the mixed-layer thickness ranged from 20 m in one experiment, to 90 m in another; the swell height varied between 0.0 and 1.5 m; and the wind speed from 0.5 to 18 knots. The sound sources were small, shallow explosives, and the results have been analysed at frequencies of 4, 8, and 16 kHz. Results for propagation loss for receivers both in and below the mixed layer have been obtained, for horizontal ranges from 0.5 to 6 km. These results are compared with the normal-mode curves applicable to each experiment, in order to show directly the effect of any roughness in the sea surface. In addition, comparisons are made between the experimental results and curves obtained from a first-order scattering theory based on ray acoustics.