The effect of internal waves on the ambient noise vertical directionality in deep ocean

Abstract

Vertical array measurements of low-frequency ambient noise in the mid-latitudes indicate that distant shipping noise has a flat angular distribution concentrated around broadside, e.g., [Wales and Diachok, JASA, 1981]. Ships cannot directly excite the lowest modes (associated with broadside angles) since those modes are trapped near the sound channel axis. Dashen and Munk [JASA, 1984] considered three mechanisms that could transfer shipping noise into the low modes: (1) downslope conversion on the continental slope, (2) volume scattering due to internal waves, and (3) direct excitation of the low modes at high latitudes where the sound channel intersects the surface. Dashen and Munk conclude that the first mechanism is the most likely since the second requires impractically large propagation ranges and the third requires unrealistic shipping densities at high latitudes. A limitation of Dashen and Munk's internal wave analysis is that it does not consider the effect of bottom loss. This talk adapts the transport theory approach of Colosi and Morozov [JASA, 2009] to study the effect of internal waves on the vertical directionality of distant shipping noise in the presence of seafloor attenuation. Initial results indicate that internal waves may have a significant effect at shorter ranges than previously thought.