VERTICAL COHERENCE OF AMBIENT NOISE IN SHALLOW-WATER OVERLYING A FLUID SEABED

Broadband (200 Hz-20 kHz) measurements of the vertical coherence of am bient noise in two well-surveyed, shallow-water channels with fluid, s edimentary seabeds are reported. The noise at one of the sites, the St rataForm natural laboratory off Eureka, northern California, was occas ionally found to be exclusively wind generated, with negligible contri butions from surface traffic and biological sources. Under these condi tions, 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 sou rces, that is, the bubbles produced by wave breaking, are evident in b oth 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 majo r factor, but much of the time local shipping also contributed to the overall noise field. Two sets of coherence data from the New Zealand s ite 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, d ownward 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. (C) 1997 A coustical Society of America. [S0001-4966(97)02012-2].