### ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

## 1pUW1. High-frequency forward scattering from Gaussian spectrum, pressure
release, corrugated surfaces: Measurements of twinkling exponents and the
dependence of the second moment on distance from surface and pulse length.

**J. S. Stroud
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P. L. Marston
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*Phys. Dept., Washington State Univ., Pullman, WA 99164-2814
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**K. L. Williams
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*Univ. of Washington, Seattle, WA 98105
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A single realization of a Gaussian spectrum surface (rms roughness 1.5 cm,
correlation length 10 cm) was manufactured out of Styrofoam. This surface
provided a pressure release, corrugated surface for an underwater,
forward-scattering investigation. Omnidirectional source and receiver were used
in the frequency range of 100--300 kHz. Short pulses were used to allow
isolation of individual contributions to the scattered field. These individual
contributions were then classified using catastrophe theory [K. L. Williams, J.
S. Stroud, and P. L. Marston, J. Acoust. Soc. Am. 96, 1687--1702 (1994)]. The
frequency dependence of the mth higher-order intensity moments I[sub m] was
measured and compared to predictions [M. V. Berry, J. Phys. A 10, 2061--2081
(1977)] that I[sub m] is proportional to k[sup (nu)[inf m]] (for m>2), where
(nu)[sub m] is a twinkling exponent. Also, the dependence of I[sub 2] on
distance from the surface was examined at a single frequency utilizing various
pulse lengths. It is known that far from a surface the wave field will obey
Gaussian statistics (I[sub 2]=2). For short pulses, however, the statistics of
the wave field are strongly dependent upon individual reflections. For longer
pulses this is the case near the surface but as one moves away it is shown that
the Gaussian limit is approached. [Work supported by ONR.]