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

4pUW11. Near-grazing underwater seismoacoustic wave backscattering from a trench.

Jacques R. Chamuel

Sonoquest Adv. Ultrasonics Res., P.O. Box 81153, Wellesley Hills, MA 02181-5339

Ultrasonic modeling results are presented on monostatic backscattering of broadband pulsed Scholte waves and grazing underwater acoustic waves from a trench at a liquid/solid interface where most of the Scholte wave energy is in the liquid. The backscattered waves are characterized by interference of diffracted waves reflected from the trench bottom and the trailing steep side. The backscatter amplitude has an inflection occurring near one fixed value of W(lambda)/d[sup 2] in the range d=3.9(lambda)--8.3(lambda), where W is the trench width, d is the trench depth, and (lambda) is the acoustic wavelength. The effects of trench width, trench depth, and sidewall slope on the backscatter waveform and amplitude are demonstrated. The ultrasonic modeling data reveal that the backscatter amplitude increases as the trailing side wall slope is slightly decreased. Waveforms from trench models at a water/glass interface are compared with waveforms from a water/air boundary with the same topography. The experimental results show the importance of converted Scholte waves on insonifying objects located in shadow zone. The basic findings pertain to the general problem of wave propagation in waveguides with outward boundary deformations as related to shallow-water underwater acoustics, bathymetric mapping, noise barriers, borehole geophysics, and nondestructive testing. [Work supported by ONR.]