Very low frequency seismo-acoustic noise below the sea floor (0.2-10 Hz)

Abstract

Ambient noise in the sea has been observed for over 100 years. Previous studies conclude that the primary source of microseisms is nonlinear interaction of surface gravity waves at the sea surface. Though this source relationship is generally accepted, the actual processes by which the wave generated acoustic noise in the water column couples and propagates to and along the sea floor are not well understood. In this thesis, the sources and propagation of sea floor and sub-sea floor microseismic noise between 0.2 and 10 Hz are investigated. This thesis involves a combination of theoretical, observational and numerical analysis to probe the nature of the microseismic field in the Blake Bahama Basin.Surface waves are the primary mechanism for noise propagation in the crust and fall into two separate groups depending on the relative wavelength/water depth ratio. Asymptotic analysis of the Sommerfeld integral in the complex ray parameter plane shows results that agree with previous findings by Strick (1959) and reveal two fundamental interface wave modes for short wavelength noise propagation in the crust: the Stoneley and pseudo-Rayleigh wave. For ocean sediments, where the shear wave velocity is less than the acoustic wave velocity of water, only the Stoneley interface wave can exist. For well consolidated sediments and basalt, the shear velocity exceeds the acoustic wave velocity of water and the pseudo-Rayleigh wave can also exist. Both interface waves propagate with retrograde elliptic motion at the sea floor and attenuate with depth into the crust, however the pseudo-Rayleigh wave travels along the interface with dispersion and attenuation and "leaks" energy into the water column for a half-space ocean over elastic crust model. For finite depth ocean models, the pseudo-Rayleigh wave is no longer leaky and approaches the Rayleigh wave velocity of the crust. The analysis shows that longer wavelength noise propagates as Rayleigh and Stoneley modes of the ocean+crust waveguide. These long wavelength modes are the fundamental mechanism for long range noise propagation.During the Low Frequency Acoustic Seismic Experiment (LFASE) a four-node, 12-channel borehole array (SEABASS) was deployed in the Blake Bahama Basin off the coast of eastern Florida (DSDP Hole 534B). This experiment is unique and is the first use of a borehole array to measure microseismic noise below the sea floor. Ambient background noise from a one week period is compared between an Ocean Bottom Seismometer (OBS) and SEABASS at sub-bottom depths of 10, 40, 70 and 100 meters below the sea floor. The 0.3 Hz microseism peak is found to be nearly invariant with depth and has a power level of 65 and 75 dB rel 1 (nm/s 2 ) 2 )/Hz for the vertical and horizontal components respectively. At 100 m depth, the mean microseismic noise levels above 0.7 Hz are 10 dB and 15-20 dB quieter for the vertical and horizontal components respectively. Most of this attenuation occurs in the upper 10 m above 1.0 Hz, however higher modes in the spectra show narrow ban...

Are deep‐ocean‐generated surface‐wave microseisms observed on land?

“…The physics of wave propagation over the 0.1–0.5 Hz frequency band for typical ocean depths (100–5500 m) spans the transition between solid earth seismology and ocean acoustics. The various types of seismic waves that propagate in a model consisting of a fluid layer over a homogeneous, solid half‐space are well known: direct acoustic waves, compressional and shear head (body) waves, acoustic modes in the fluid layer, pseudo‐Rayleigh waves (pRg), and Scholte waves [ Roever et al ., ; Strick , , ; Ewing et al ., ; Brekhovskikh , ; Biot , ; Cagniard , ; Scholte , , ; Tolstoy , ; Bradley , ]. There are two general oceanic crustal cases: “soft” bottoms with shear speed less than the fluid sound speed, and “hard” bottoms where the shear speed is greater than the fluid sound speed.…”

“…For pressure sources (as from wave‐wave interaction), much of the DF energy propagates in the water column (not in the solid seafloor as is the case for Rg) [ Latham and Sutton , ; Harmon et al ., ; Ardhuin and Herbers , ]. Following a long tradition, we refer to the surface waves under the oceans at DF microseism frequencies as “pseudo‐Rayleigh waves” (pRg) [ Roever et al ., ; Strick , , ; Ewing et al ., ; Brekhovskikh , ; Biot , ; Cagniard , ; Scholte , , ; Tolstoy , ; Bradley , ; Okal , ]. When the water‐layer depth approaches zero, pRg becomes indistinguishable from the free‐surface Rayleigh wave (FSRW).…”

Ocean Seismic Network Pilot Experiment

“…Observations and models of ambient noise near the seafloor indicated a preponderance of interface waves, perhaps generated by scattering from seafloor heterogeneities [ Bradley , 1994; Bradner et al , 1965; Dougherty and Stephen , 1988; Duennebier et al , 1987a; Latham and Nowroozi , 1968; Latham and Sutton , 1966; Orcutt et al , 1993a; Orcutt et al , 1993b; Schreiner and Dorman , 1990; Webb , 1992]. Also large eddies in the ocean near the seafloor induce pressure fluctuations which tilt the seafloor and generate a seismic signal at frequencies below a few Hertz [ Webb , 1988].…”

“…In shallow water studies, experiments had shown that burying the seismometer reduced ambient noise levels [ Duennebier et al , 1991; Sutton and Duennebier , 1988; Trevorrow et al , 1989a; Trevorrow et al , 1989b]. In deep water, a number of studies had also observed that ambient noise at frequencies above the microseism peak was consistently quieter for both vertical and horizontal sensors in boreholes than at the seafloor [ Adair et al , 1984; Bradley , 1994; Bradley et al , 1997; Duennebier et al , 1987a; Hedlin and Orcutt , 1989; Stephen et al , 1994]. In one study where the VLF noise was not quieter in the borehole than on the seafloor, the sensor had been clamped at only 190m depth into soft sediment [ Carter et al , 1984].…”