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WAVEFORM ANALYSIS OF SCHOLTE MODES IN OCEAN
SEDIMENT LAYERS

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Guust Nolet, Department of Geological and Geophysical Sciences,
Princeton University, Princeton, NJ 08544, USA.

LeRoy M. Dorman, Scripps Institution of Oceanography, UCSD,
9500 Gilman Drive, La Jolla, CA 92093-0215, USA.

SUMMARY:
In an effort to determine the characteristics of seismic noise on the
ocean bottom and its relationship to the structure of the seafloor,
we have adapted the method of nonlinear waveformfitting to
accommodate multi-dimensinal models (shear velocity
$beta$ and shear damping Q _{ S })
and applied it to invert
several records of interface waves (Scholte, 1959) from the THUMPER
experiment off Southern California.
Waveform fitting is a very powerful tool to determine the S velocity
in the top few meters of the sediment. Starting from $beta$= 30 m/s
at the top clay layer, the S velocity increases with a gradient
of 2.8 m/s/m over the first 150 m of sediment.
A theoretical estimation of the source strength gives coherent
estimates of Q _{ S } as a function of depth for ranges between
400 and 1070 m from the source. The Q _{ S } models are characterized
by very low values (10-20) in the top three meters, but values in
excess of 100 below that level.
The results confirm
the identification of the noise as harmonics of
interface waves. In the area of this experiment the
largest noise amplitudes belong to the fundamental mode and penetrate
to a depth of about 20 m into the sediment. The overtone energy
can be appreciable too, and will be noticeable to about 80 m depth.
The Q _{ S } structure confirms the strong influence that the seafloor
structure exerts on the noise spectrum. The high attenuation at
frequencies above 3-4 Hz suppresses noise propagation and produces
low noise at higher frequencies. (Similarly, high attenuation in the
asthenosphere suppresses noise propagation below 0.1 Hz.)

Geophys J. Int. (1996) 125, 385-396.

##### Last Revised: 5 May 1996