### ASA 126th Meeting Denver 1993 October 4-8

## 2pAO7. A higher-order stochastic model for surface backscatter in high sea
states.

**Kenneth E. Gilbert
Lucy J. Ameling
**

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*Appl. Res. Lab. and the Graduate Program in Acoust., Penn State Univ., P.
O. Box 30, State College, PA 16804
*

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A stochastic bubble-layer scattering model based on first-order
perturbation theory (plane-wave Born approximation) has recently been developed
[K. E. Gilbert, submitted to J. Acoust. Soc. Am. (1993)]. In the first-order
model, the scattering is treated as a perturbation on the plane-wave solution
for the bubble-free water and is written in terms of a ``geometric factor''
times the horizontal wave-number spectrum of the sound-speed fluctuations in
the bubble layer. For low sea states and higher grazing angles, this approach
is adequate. For high sea states and low grazing angles, first-order
perturbation theory is less accurate and one needs to account for the average
bubble density and compute backscatter due to fluctuations about the average.
In this paper results are compared for scattering computed with the ordinary
plane-wave Born approximation to that computed with the ``distorted-wave'' Born
approximation (DWBA). It is shown that for high sea states and low grazing
angles, distorted waves significantly enhance the backscatter at low
frequencies and consequently give better agreement with experiment. The effect
can be understood in terms of near-surface upward refraction that creates
greater insonification and larger grazing angles near the surface than that
obtained with plane waves. Finally, the effect of near-surface upward
refraction on rough surface scattering is briefly discussed.