The polarization of acoustic particle motion in the ocean.

Acoustic particle velocity is rarely measured in the ocean even though it is a fundamental property of the sound field. The Marine Physical Laboratory’s Swallow floats are capable of simultaneously measuring both acoustic particle velocity and pressure in the 0.5- to 25-Hz band. Previous work has focused on the relationship between the pressure and the particle velocity measurements. The purpose of this talk is to discuss the properties of the particle velocity cross spectral matrix in terms of the polarization of the motion and the relationship of these properties to the energy flow in the sound field. The measurements indicate that the infrasonic background sound field in the midwater column can be modeled as the superposition of three mutually uncorrelated, rectilinearly polarized waves. This is because the particle velocity cross spectral matrix is purely real since the spatial inhomogeneity of the sound field is negligible. However, the particle motion can be strongly elliptically polarized for sound fields generated by discrete sources. Particular attention will be focused on the polarization of the motion generated by a near-range, monotone source towed over an ocean bottom containing gas hydrates. [Work supported by ONR.]