Study On Backscattering From Underwater Typical Elastic Bodies Based On Ray Approximation

As a branch of acoustics, underwater acoustics investigates the principle of propagation, radiation, scattering and reception of sound in water to solve all kinds of acoustic problems related to signal transmission process and how to detect the target in water. The purpose of studying the scattering is to find and summarize the rules and physical mechanisms of sound when it propagates in water. Typical scatters, such as submarine, torpedoes, mines et al., vary from geometric shapes, however, their shapes are either similar to spheres or cylinders. With the considering of convenient mathematical calculation, these scatters are usually treated as spheres or cylinders. This approximation trends to greatly simplify the arithmetic, and the calculated results are applicable to the actual scatters. So this work investigates the scattering of the fluid-loaded, simplest, nontrivial, three dimension elastic scatters, such as sphere, thin spherical shell, cylinder and cylindrical shell.This thesis reviews the history of underwater acoustics, and briefly introduces the research profile and numerical methods of backscattering for the typical elastic scatters. When calculating the scattering of the elastic spherical shell in the far field, usually use the ray approximation, which has more appropriate in the high frequency range.In this thesis, the form functions of sphere, spherical shell, cylinder, cylindrical shell for the case of backscattering are expressed by the relevant partial wave series representation. Inspection of these form functions suggests that the thin spherical shell has apparent enhancement phenomenon in a rather broad midfrequency range. However, the other elastic objects do not have such phenomenon. The midfrequency enhancement phenomenon for tone burst backscattering by thin spherical shells in water has been discussed by several investigators. In their works, it is found that this enhancement is mainly due to the lowest subsonic antisymmetric Lamb wave. The modified ray approximations are used to calculate the echo contributions from different Lamb wave, for example, s0 ,a 0 ? ,a 0 + wave. The numerical results show that the ratio of a/h and the dimensionless echo delayΔT 0l have nearly linear relationship with the frequency of greatest enhancement. Based on this property, two linear approximate equations are formulated to evaluate the radius and thickness of a thin spherical. A simple method is developed to estimate the frequency of greatest enhancement and the corresponding echo delay from a short tone burst echo with higher carrier frequency. The evaluated results show that the present method is effective on determination of the radius and thickness of a thin spherical shell in water.