Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution February 1994
Description:
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 H z microseism peak is found to be nearly
invariant with depth and has a power level of 65 and 75 dB rel 1 (nm/ s2)2)/ H z
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
bandwidth variability in the noise field that is not monotonic with depth. Dispersion
calculations show normal mode Stoneley waves below 0.7 Hz and evidence of
higher modes above 0.8 Hz. A strong correlation between noise levels in the borehole
and local sea state conditions is observed along with clear observation of the
nonlinear frequency doubling effect between ocean surface waves and microseisms.
Particle motion analysis further verifies that noise propagates through the array as
Rayleigh/Stoneley waves. Polarization direction indicates at least two sources; distant
westerly swell during quiescent times and local surface waves due to a passing
storm.
Above 1.0 Hz the LFASE data shows little coherence and displays random polarization.
Because of this, we believe scattered energy is a significant component
of the noise field in the Blake Bahama Basin. A fully 3-D finite difference algorithm
is used to model both surface and volume heterogeneities in the ocean crust.
Numerical modeling of wave propagation for hard and soft bottom environments
shows that heterogeneities on the order of a seismic wavelength radiate energy into
the water column and convert acoustic waves in the water into small wavelength
Stoneley waves observed at the borehole. Sea floor roughness is the most important
elastic scattering feature of the ocean crust. Comparisons of 2D and 3D rough sea
floor models show that out-of-plane effects necessitate the use of 3D methods. The
out-of-plane energy that is present in the LFASE data comes from either heterogeneities
in the source field (i.e. mixed gravity wave directions) or, equally likely,
scattering of the source field from surface or volume heterogeneities in the sea floor.
Description:
This research was supported by Office of Naval Research grants N00014-89-C-0018, N00014-89-J-1012, N00014-90-C-0098, N00014-90-J-1493 and N00014-93-1-1352.
Keywords:
Microseisms
;
Ocean bottom
;
Seismology
;
Boundary layer noise
;
Underwater acoustics
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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