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)=c0zν, with c0=21.5 and ν=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.