Measuring and modelling ambient noise in three-dimensional ocean environments

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Underwater signal processing and signal detection schemes require well-characterized spatial and temporal ambient noise field properties to operate effectively in tasks such as detecting ships, submarines, marine mammals, and other underwater sound sources. Additionally, a detailed understanding of the noise field in the ocean has led to the development of passive methods for querying the physical properties of the ocean, seabed and sub-bottom as well as investigating the physics and statistics of noise source mechanisms such as air-sea interaction, surface wave breaking, rainfall, geophysical processes, and bedload sediment transport. More recently, underwater noise has been proposed as a monitoring tool to study the ecological health of the oceans. The objective of this research program is to develop, validate, and apply new analytic and computational models that accurately predict ambient ocean noise spectral, temporal and spatial properties in arbitrary, realistic, and complex 3D domains. This will be achieved by using a parabolic equation (PE) propagation model, while model validation will be achieved by comparison with field measurements of the depth-dependence of ambient noise made on passive acoustic profilers. This combination of methods will be used to study the effect of inhomogeneities (internal waves, fronts), ocean dynamics and non-uniform surface noise processes (rain storms, ice fields) on the noise field, to develop inversion techniques for estimating sea-ice thickness and geophysical properties of the seabed, and to better detect and localize marine mammals, track ecosystem health, and quantify the role of anthropogenic noise on the marine soundscape. The ability to estimate the relative contributions of anthropogenic, environmental, and biological sound to the overall noise field has applications in acoustical oceanography, such as the estimation of precipitation and wind speeds in shallow water environments and in the presence of persistent shipping and biological noise. A portable computational model has the potential to be an operational tool for underwater security and defence, as well as research in the localization and tracking of marine mammals. The research may be applied in performing acoustic measurements of environmental parameters in remote regions of the ocean, particularly the Arctic, where weather buoys are too easily destroyed by sea ice and storms. An instrument development and field program will support this modelling research. The main objective will be to measure the spatial properties and power spectral density of noise and their relationship to oceanographic and atmospheric conditions, bathymetry and seabed properties, and the physical processes that act as sources. A full-ocean depth vertical profiler will be developed and used to investigate the depth-dependence of power spectral density, and vertical and horizontal coherence.