Low-frequency acoustic propagation loss in shallow water over hard-rock seabeds covered by a thin layer of elastic–solid sediment

Abstract

Shallow-water seabeds are often varied and complex, and are known to have a strong effect on acoustic propagation. Some of these seabeds can be modeled successfully as fluid or solid half-spaces. However, unexpectedly high propagation loss with respect to these models has been measured in several regions with rough, partially exposed, hard-rock seabeds. It is shown that the high propagation loss in these areas can be modeled successfully by introducing a thin layer of elastic–solid sediment over the hard-rock substrate. Propagation loss predictions using the safari fast-field program exhibit bands of high loss at regularly spaced frequencies. Normal-mode calculations show resonance phenomena, with large peaks in the modal attenuation coefficients at these same frequencies, and with rapid changes in the mode wavenumbers. Bottom reflection loss calculations indicate that the high propagation loss is due to absorption of shear waves in the sediment layer.

Propagation in a waveguide with range-dependent seabed properties

“…4 of Ref. 21 where the modeled propagation null at 9 Hz is much narrower than the measured data. One likely explanation is range-dependence of the layer.…”

“…Modeling showed that shear losses in the layer, due to a shear resonance did explain the frequency dependence, confirming the prediction by Vidmar. Hughes et al 21 later examined several more data sets with a wider frequency range, sufficient to observe the resonance null and also showed that the high propagation losses could only be explained by shear losses in the sediment layer.…”

“…4 of Ref. 21 where the mean thickness reported as 5 m. For the null at about 9 Hz, roughly estimating ⌬f ϳ 8 Hz, the thickness variability from Eq. ͑12͒ is d ϳ 2 m and including shear speed depth dependence from Eq.…”

Research report

“…Specifically, it is shown that shear wave resonance's in a thin sediment layer over a rock substrate have a profound effect on acoustic reflection loss and shallow water acoustic transmission loss [ Hughes et al, 1990]. Measurements of infrasonic seismo-acoustic noise using an OBS in shallow water have uncovered an unusual phenomenon: the noise spectrum of the horizontal component of seabed velocity shows several prominent peaks in the frequency range 0-8 Hz, whereas the noise spectra of both the acoustic pressure and the vertical component of seabed velocity show very weak or non-existent features at the same frequencies.…”

The effects of sediment porosity on acoustic reflection and transmission at the seafloor

“…The fluid model has been sufficient in fact to successfully predict various measures of a propagating acoustic field: most commonly transmission loss. Some of the experimental results [e.g., Ingenito and Wolf (1976), McDaniel and Beebe (1980), Chapman and Ellis (1984), Beebe and Holland (1986), Hughes et al (1990)], however, indicated that the effect of the sediment rigidity could be significant for various environments. Modeling efforts [Fryer (1978), Vidmar (1980), Holland (1985)] that considered the sediments as viscoelastic attempted to determine the bounds of validity for the fluid approximation as a function of frequency and seafloor environment.…”