Vector acoustic observations of underwater ship noise from the 2017 seabed characterization experiment

Abstract

The Intensity Vector Autonomous Recorder (IVAR) measures acoustic particle velocity and pressure simultaneously. IVAR was deployed on the seabed at depth of 75 m during the 2017 Seabed Characterization Experiment (SBCEX) with objective to study sound propagation within underwater waveguides for which the seabed consists of fine-grained, muddy sediments. In this presentation, vector acoustic observations of ship generated noise recorded by IVAR from a cargo ship traversing the central region of the SBCEX are discussed. Pressure and particle velocity data as a function of frequency are converted into a bounded, non-dimensional form known as circularity, a quantity that is independent of the ship noise source spectrum and which can be interpreted as the normalized curl of active intensity. The frequency dependent pattern in circularity is sensitive to the sediment layering structure, and bathymetry. This pattern, as the vessel closes and opens in range, is inverted for sediment properties using a Bayesian framework, with inversion model space consisting of a low-compressional speed elastic layer, above an elastic half-space. Change in water depth between the location of the ship source and IVAR over the 20-min observation period is addressed using adiabatic mode theory. [Study supported by Office of Naval Research.]