Time-frequency analysis of backscattered echoes from absorbing spherical shells

Abstract

The backscattering of tone bursts by spherical shells immersed in water has been investigated by several researchers. For a very thin shell, the backscattering specular echo is followed by enhanced echoes. The mechanism of these enhanced echoes has been attributed to a subsonic surface guided wave from recent ray synthesis analysis [P. L. Marston and N. H. Sun, J. Acoust. Soc. Am. 96, 1862–1874 (1992)]. In this study, time-frequency analysis is performed on the scattering signal. A discrete Wigner distribution function is employed to express the information content in the scattered sound in relation to the size and material of the shell [G. C. Gaunaurd and H. C. Strifors, IEEE 84, 1231–1248 (1996)]. The simulation results support that the time delay between the echoes is determined by the outer radius of the shell as well as the shell material (with or without absorption), or the Rayleigh velocity of the shell material. Furthermore, the backscattered enhancement occurs at different ranges in frequency domain due to various shell materials, which provide a potential method to determine the shell material properties. The relationship of this work to target discrimination methods for sonar applications is highlighted. [Work supported by ONR.]