Modelling of ambient noise created by a shipping lane to prepare passive inversion, application to Ushant case

The Ushant thermal front is a seasonal phenomenon which occurs from May to October in a shallow water environment (100m) of the Iroise Sea (off the coast of the northwestern France). It corresponds to the boundary separating a well mixed inner shelf water from an open sea stratified water. To determine the dynamic of the front -or more basically the presence of a stratified or homogeneous water column- the possibility to use a shipping lane as a continuous acoustic source is studied. The originality of this work is to use a single receiver. Simulation results of sounds radiated by a shipping lane in a shallow water environment are presented, both for stratified and homogeneous water column. The corresponding pressure fields show a mean level difference in the frequency band 50-300 Hz. This feature will be used in the future as an observable to differentiate both environments, and thus passively detect the Ushant thermal front. One of the issue to get the mean level offset is to record the shipping lane noise without isolated ship interferences. As a consequence, an optimum mooring position to track the thermal front is suggested from the analysis of real acoustic data, acquired around the Ushant traffic lane during 2010, coupled with AIS (Automatic Identification System) data, and a MARS-3D oceanographic model output. Keywords: Passive acoustic inversion