The waveguide nature of a shallow water environment bounded below by a viscoelastic medium permits noise to couple into seismic waves. Geophone and hydrophone measurements have shown that below a threshold frequency of about 10 Hz in 100 m of water, there is a large increase in the measured noise levels, with a peak at approximately 0.25 Hz. A previously developed wave theory of distributed noise in a waveguide has been combined with a full wave solution technique for stratified elastic media and used for numerical modeling and analysis of this phenomenon. It is demonstrated that this low-frequency increase in noise level is only partly due to an increase in the source spectral level. At these frequencies, seismic interface waves become important propagation paths for the ambient noise, leading to a significant magnification of the observed noise levels. This strongly suggests that propagation effects have to be accounted for when evaluating source theories by comparison to experimental data.
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Schmidt et al. (1988) studied this question.