ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

5pUW1. Bistatic scattering of underwater sound from a porous solid sphere: A comparison of theory and experiment.

Martin E. Pace

Theodore W. L. Huskey

Steven R. Baker

Phys. Dept., Naval Postgraduate School, Monterey, CA 93943-5000

Steven G. Kargl

Univ. of Washington, Seattle, WS 98105

Raymond Lim

Coastal Systems Station, Panama City, FL 32407-7001

Measurements were made of the bistatic scattering of underwater sound from a porous solid sphere. Two sample spheres were employed composed of bonded glass beads of 100- and 500-micron nominal grain diameter, respectively. The diameter of each sphere was approximately 6.8 cm. The scattering into the rearward hemisphere was measured over the frequency range 13 to 150 kHz, corresponding to values of ka from approximately 2 to 20. Over this frequency range the viscous penetration depth varies from approximately 5.0 to 1.5 microns, so that the water in the pores can move freely. The experimental results were compared to numerical computations based on a theoretical Biot model [S. G. Kargl and R. Lim, 1527--1550 (1993)]. The physical parameters required by the model were obtained by independent measurements on the sample spheres and on similar cylindrical samples. Reasonably good agreement was found between the experimental and theoretical results for the 100-micron sphere, especially at the lower frequencies, except for a consistent deficit in the measured backscatter. Agreement between the results for the 500-micron sphere was poor, probably due to sample inhomogeneity.