High-frequency backscattering enhancements from elastic cylindrical shells in water: Observations and ray theory

Abstract

Sometimes elastic features for acoustically illuminated targets in water are relegated primarily to low frequency bands. For empty spherical shells however, a pronounced high-frequency backscattering elastic-enhancement was observed and modeled using ray-theory [G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994)]. That phenomenon was termed a backwards wave enhancement because the wave phase and group velocities guided on the shell move in opposite directions and required a modified ray diagram. In the present research, this type of backscattering enhancement is demonstrated and modeled for empty cylindrical elastic shells in water. While the full ray-theory requires solving for complex roots descriptive of elastic waves on fluid-loaded shells, it was found helpful to first consider the properties of ordinary Lamb waves on plates whose thickness was selected to be that of the shell of interest. A search for different roots of Lamb’s Equation confirms that a root exists that exhibits opposing phase and group velocities in the frequency region of interest. As expected from the model, the associated backscattering enhancements are localized in frequency though they can be easily identified using tone bursts. [Work supported by Office of Naval Research.]Sometimes elastic features for acoustically illuminated targets in water are relegated primarily to low frequency bands. For empty spherical shells however, a pronounced high-frequency backscattering elastic-enhancement was observed and modeled using ray-theory [G. Kaduchak, D. H. Hughes, and P. L. Marston, J. Acoust. Soc. Am. 96, 3704–3714 (1994)]. That phenomenon was termed a backwards wave enhancement because the wave phase and group velocities guided on the shell move in opposite directions and required a modified ray diagram. In the present research, this type of backscattering enhancement is demonstrated and modeled for empty cylindrical elastic shells in water. While the full ray-theory requires solving for complex roots descriptive of elastic waves on fluid-loaded shells, it was found helpful to first consider the properties of ordinary Lamb waves on plates whose thickness was selected to be that of the shell of interest. A search for different roots of Lamb’s Equation confirms that a root exists ...