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

P. L. Marston

Phys. Dept., Washington State Univ., Pullman, WA 99164-2814

K. L. Williams

Univ. of Washington, Seattle, WA 98105

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.]