Noise generation in the solid Earth, oceans and atmosphere, from nonlinear interacting surface gravity waves in finite depth

• Published in: Journal of fluid mechanics 2013-02, Vol.716, p.316-348
• Main Authors: Ardhuin, Fabrice, Herbers, T. H. C.
• Format: Article
• Language: English
• Subjects: Acoustic noise; Acoustics; Atmosphere; Atmospheres; Deep water; Earth; Earth sciences; Earth, ocean, space; Earthquakes, seismology; Exact sciences and technology; Fluid mechanics; Geophysics; Gravity; Gravity waves; Internal geophysics; Marine; Noise; Nonlinearity; Ocean waves; Oceans; Sciences of the Universe; Seismic waves; Solid-earth geophysics, tectonophysics, gravimetry; Surface acoustic waves; Water depth
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2012.548

Oceanic pressure measurements, even in very deep water, and atmospheric pressure or seismic records, from anywhere on Earth, contain noise with dominant periods between 3 and 10 s, which is believed to be excited by ocean surface gravity waves. Most of this noise is explained by a nonlinear wave–wave interaction mechanism, and takes the form of surface gravity waves, acoustic or seismic waves. Previous theoretical work on seismic noise focused on surface (Rayleigh) waves, and did not consider finite-depth effects on the generating wave kinematics. These finite-depth effects are introduced here, which requires the consideration of the direct wave-induced pressure at the ocean bottom, a contribution previously overlooked in the context of seismic noise. That contribution can lead to a considerable reduction of the seismic noise source, which is particularly relevant for noise periods larger than 10 s. The theory is applied to acoustic waves in the atmosphere, extending previous theories that were limited to vertical propagation only. Finally, the noise generation theory is also extended beyond the domain of Rayleigh waves, giving the first quantitative expression for sources of seismic body waves. In the limit of slow phase speeds in the ocean wave forcing, the known and well-verified gravity wave result is obtained, which was previously derived for an incompressible ocean. The noise source of acoustic, acoustic-gravity and seismic modes are given by a mode-specific amplification of the same wave-induced pressure field near zero wavenumber.