Potential and kinetic energy of underwater noise measured below a passing ship and response to sub-bottom layering

Abstract

For underwater sound energy emitted by a passing vessel and received at a relatively steep angles, such as vessel directly overhead, there can be a significant excess in potential acoustic energy relative to kinetic energy with the opposite also occurring. Observations when expressed as a ratio of kinetic to potential energy in decibels are interpretable upon inspection, yielding an estimate of the sensor height above the seabed, and information on properties of the seabed. The effect was studied as part of the recently completed experiment on the New England Mud Patch. The R/V Neil Armstrong underwent controlled transits directly overhead a vector sensor, positioned 1.45 m above the seabed. A model for kinetic and potential energy developed from method of images combined with a layered seabed, is used to invert the data. The low-speed mud-layer is identified along with higher-speed transition layer separating the mud substrate from a sediment basement, with parameters consistent with other recent studies. Similar observations made in Puget Sound are briefly discussed with the higher-impedance seabed identified being consistent with thin Holocene sediments characteristic of region. Both examples illustrate the standing wave features of such data.