Abstract
The effect of ocean bottom and surface boundaries on the generation and reception of underwater acoustic noise fields is investigated for the frequency range 20 to 2500 Hz. The persistent noise fields in this frequency interval are dominated by distant shipping and by local meteorological disturbances. The generation and characteristics of these noise fields are described. Straight forward theoretical calculations are used to describe the effect of proximate ocean surface and sloping bottoms on the propagation of noise fields and the Parabolic Equation method is employed to parameterize effects of the ocean bottom physical properties and bathymetric blockage. These theoretical developments are used to interpret data collected in extensive field measurements which illustrate the separable effects of shipping and wind, the effect of the ocean surface and sloping bottom on noise generation and propagation and the effect of the ocean surface and intervening topography on the near-surface and depth dependent characteristics of ambient noise fields. Results show that the observed ambient noise level decrease as the ocean surface is approached can be accurately modeled as a surface image interference phenomenon when the noise field vertical directionality is known; topographic blockage results in a decrease in level of the ambient noise field at all depths but with a sharper rate of decrease at depths greater than the depth of the relevant bathymetric feature; and the presence of shipping in the vicinity of the upward sloping perimeters of ocean basins result in increase in the ambient levels in the acoustic sound channel.