A Study On The Characteristics Of Scattering From A Target In A Shallow-Water Wave-guide

With the appearance of the quiet submarine, active sonar is attached much more importance and play a more and more important role. The study on the characteristics of scattering from an underwater target is only the base that the active sonar works on but also the source which new ideas, new theories and new methods come from. For long-range detection, fish torpedo accurate control and guide, key strike, detection of mines and pipe laying, all these need an accurate model. So it is of great significance both for military affairs and civil affairs to study on the characteristics of scattering from a target in a shallow-water wave-guide. And the problem is one of the focuses in the field of underwater acoustic research both from home and abroad.There are many factors which can effect the target scattering in complex ocean environment, especially in shallow water, such as sea surface , sea bottom and sound speed profile in the water body. And this can lead to the obvious difference between the scattering field in the free space and that in a wave-guide. Likewise, the theories used to study the scattering problems in a wave-guide are correspondingly more complex. From the end of 1980s, academic researches have been done on the scattering in ocean environment, which include two aspects. One aspect is about the scattering near the boundary, the other is upon the scattering in a wave-guide. Recently, internal researchers also have started to study the scattering in ocean environment. But all these studies only refer to the scattering problems near the boundary, such as detection of mines buried in the sea bottom. The issue about the scattering in a wave-guide has not been referred by now. Some academic research has been done on thecharacteristics of scattering from a target in a shallow-water wave-guide, and the effect coursed by the factors in the ocean environment on the distribution of the scattering field and the structure of the echo signal is presented.In this paper, the scattering problem of a rigid sphere in a shallow-water wave-guide is studied on the base of Ingenito' s normal mode method. The KRAKENC, which is a program used to compute the local normal mode, is brought in Ingenito' s normal mode method, so the far-way assumption is avoided and in more conditions the method can be used. The method used to study the scattering field distribution can be improved to study the structure of the echo signal. In this paper, time- frequency analysis is used to separate different normal modes in the echo signal. The influences of the wave-guide depth, sound speed profile in the water body, sound speed and the attenuation in the sea bottom are discussed. The multi-way phenomena and coupling of normal mode caused by target are analyzed by numerical computations.The results show that the existence of the wave-guide can bring intense effect to the scattering field. Contrary to the case in free space, the direction distribution is range-dependent in a wave-guide. In the view of the normal mode theory, the reasons are as the following. There are several propagable normal modes excited by source in a wave-guide, and the scattering sound field is the sum of these modes. Every mode can be regarded as being consisted of up-going wave and down-going wave, each of them has different angle of incidence. The contribution of each mode is different versus the range, so at some distance the scattering field is in the representation of only one mode or the sum of several modes. So there is a significant connection between the normal modes in the wave-guide and the scattering field. The variables such as wave-guide parameters and sea bottom parameters, which can affect the normal modes,can also affect the scattering field. The echo signal is composed of mirror-reflecting wave and circle wave. When source and the receiving spot are on the same side of the target, the mirror-reflecting wave and circle wave are parted. When source and the receiving spot are on different sides of the target, the mirror-reflecting wave and circle wave are superposed partly. The multi-way phenomena and coupling of normal mode caused by target can lead to the superposition of normal modes, which lead to the obvious difference between echo wave form in the free space and that in a wave-guide. In addition, it can weaken the effect by the wave-guide to take the each normal mode alone for research.