Potential and kinetic energy of underwater noise measured at steep grazing angles over contrasting seafloors

Abstract

Observed near the seabed, broadband noise emissions from a source or vessel passing directly above the sensor exhibit frequency bands where potential acoustic energy is greater than kinetic energy while the opposite occurs in neighboring frequency bands. The condition where the dynamic and kinematic energy forms differ in this manner is characteristic to interference involving steep angles or near-normal incidence reflection from the seafloor. Recent experiments conducted on the New England Mud Patch (NEMP) and the neighboring (towards offshore) shelf break waters demonstrate the sensitivity to seabed structure, which evolves from the soft mud layers at the NEMP into the harder sediments that are exposed on the continental slope. Results are expressed as a ratio of kinetic to potential energy as a function of frequency, with resulting data series yielding information on seabed properties. Variance components as function of frequency are also isolated based on a singular value decomposition of a matrix constructed from the data series (known as singular spectrum analysis). The components are associated with different sub- bottom layering features. A model for kinetic and potential energy combined with candidate geoacoustic models is used to interpret both the data series and components.