Amplitude Modulation of an Acoustic Wave Propagating Near the Sea Surface

Abstract

Sixty Khz CW sound was propagated parallel to and near the ocean surface adjacent to an array of four thermistors, a salinometer, a sound velocimeter, a turbulent velocity probe, and a wave-height probe in order to investigate the statistics and environmental causes of amplitude modulation. The temporal variations of the sound amplitude modulation were studied during four twenty-minute runs over a range of 2 meters at varying depths from near the surface to 14 meters during sea-state one conditions in water of 16 meters-depth. Analysis indicated mean percentage amplitude modulation of approximately 5% with a variance of approximately 10 raised to the -3 power. Temporal correlation times were about 2 seconds except for a 7 second time near the bottom. The power spectral densities of the modulation show strong components corresponding to the predominant ocean wave frequencies as well as higher frequencies that may be attributed to bubble, temperature and/or salinity inhomogeneities. There is evidence that the microstructure dimensions are larger near the bottom than in the upper and middle depths.