A new shallow-ocean technique for determining the critical angle of the seabed from the vertical directionality of the ambient noise in the water column

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In the absence of interference from local shipping, the vertical directionality of ambient noise in the shallow ocean overlying a fast, fluid sediment is a stable feature of the noise field. It is controlled primarily by the bottom reflectivity, which is a time invariant property of the channel. The noise field shows a peak symmetrically placed around the horizontal whose angular width is 2αc, where αc is the critical grazing angle of the bottom. By measuring the noise power distribution in the vertical, αc can be determined and hence the compressional sound speed in the bottom sediment deduced. Measurements of the vertical directionality of ambient noise in shallow water have been made at six sites around the coast of the United Kingdom, using specially designed vertical-line-array sonobuoys. Estimates for the critical grazing angle and the sound speed in the bottom have been derived from the noise data for all six areas. Similar data from independent seismic surveys are available for only one of these sites, the Celtic Sea South. For this area the independent survey yielded a value of αc =28°, which compares closely with the value of αc =26.2° obtained using the new noise technique.