Vertical coherence of ambient noise in shallow water: Application to broadband inversions

Abstract

Broadband (200-Hz to 20-kHz) measurements of the vertical coherence of ambient noise have been made in two well-surveyed, shallow-water channels with fluidlike, sedimentary seabeds. The noise at one of the sites, the StrataForm natural laboratory off Eureka, northern California, was occasionally found to be almost exclusively wind generated. Under such conditions, the theoretical coherence, computed using the known properties of the sediment at Eureka, closely matches the data. Subtle effects associated with the depth of the sources (bubbles from breaking waves) are evident above 6 kHz in both the theory and the data. At the second site, in Jellicoe Channel, New Zealand, wind noise was also a major factor but shipping sometimes contributed significantly to the field. Again, theory and data matched well when the known geoacoustic properties of the bottom were used as inputs to the model. Based on these results, it appears that the vertical coherence of ambient noise in shallow water is predictable over a bandwidth as high as 20 kHz, and accordingly has potential as the basis of an inversion technique for obtaining the geoacoustic properties of the sea floor.