Abstract
Broadband (200 Hz–20 kHz) measurements of the vertical coherence of ambient noise in two well-surveyed, shallow-water channels with fluid, sedimentary seabeds are reported. The noise at one of the sites, the StrataForm natural laboratory off Eureka, northern California, was occasionally found to be exclusively wind generated, with negligible contributions from surface traffic and biological sources. Under these conditions, the theoretical coherence, computed using the known properties of the sediment at Eureka, closely follows the coherence data up to a frequency of 20 kHz. Subtle effects due to the finite depth of the sources, that is, the bubbles produced by wave breaking, are evident in both the theory and the data. At the second site, in Jellicoe Channel, New Zealand, some 64 km north of Auckland, wind noise again was a major factor, but much of the time local shipping also contributed to the overall noise field. Two sets of coherence data from the New Zealand site are examined, the first taken in near-isovelocity conditions, when the noise was due to wind plus a local ship, and the second recorded in the absence of shipping, when the channel showed a mildly linear, downward refracting profile. In both instances, the coherence data over a 5-kHz band compare well with theoretical curves computed using the known geoacoustic parameters of the sediment in the region.
Published Version
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