Using ambient noise sonar (ANS) to probe the ocean environment in shallow water

The Ambient Noise Sonar system (ANS) is a passive sonar array that has been developed to study the ocean environment in coastal waters by measuring ocean noise global directivity using only six transducers in a sparse volume array of maximum size 2 m. The array output gives 3D images with thousands of independent pixels for any chosen look direction and so can provide both horizontal and vertical directivity patterns. This system uses the natural broadband characteristics of ambient noise using a multiplicative processing of the cross-correlations between the receivers, similar to Class I T.A.P. arrays. The sonar has already been used in a variety of experiments to probe the ocean environment in shallow waters off the coast of Florida. Although there have been many measurements made of ambient noise levels in shallow water there appear to be very few published results of the ambient noise horizontal directivity. More recently there have been a number of experiments on near-shore ambient noise and these have shown the importance of biological activity in low sea states and breaking waves in high sea states. The results found with the ANS indicate that in tropical waters and in low sea states the noise is highly anisotropic and is generated by biological noise sources and not breaking waves, contrary to the standard assumption that the noise is surface generated and isotropic. Colonies of snapping shrimp have been localized along the coast, clustered around man-made structures and natural reefs. Finally, by monitoring local boat traffic, the ANS used boat noise as a source of opportunity to probe the environment and obtain an estimate of the bottom reflection coefficient.