Shear-speed gradients and ocean seismo-acoustic noise resonances

Measurements of infrasonic seismo-acoustic ambient noise using an ocean bottom seismometer in shallow water have uncovered an unusual phenomenon: the noise spectrum of the horizontal component of seabed velocity shows several prominent peaks:in the frequency range 0-8 Hz, whereas the noise spectra of both the acoustic pressure and the vertical component of seabed velocity show very weak or nonexistent features at-the-same frequencies. This structure is interpreted theoretically as resonances of shear waves-of vertical polarization in the upper sediment layer, excited by the diffuse infrasonic sound field in the water. Independent interface wave dispersion studies at the site have revealed an approximate power-law profile of shear speed versus depth, having the form c(z) = c(0)z(v), with c(0) = 21.5 and v = 0.60 (SI units). The theoretical development concentrates on exact analytic solutions for-the resonance frequencies and wave field for power-law profiles and on the WKB and more advanced asymptotic solutions in the more general case of smooth shear-speed profiles with a power-law singularity. The experimental observations are interpreted in light of these analytic results, and are consistent with the previously determined power-law shear speed-profile. (C) 1999 Acoustical Society of America. [S0001-4966(99)04110-7].