Studies On The Spatial Structure Of Wind-generated Ambient Noise And The Inversion Of Bottom Parameters In Shallow Water

In shallow water, Sound emanating from a noise source undergoes multiple reflections between the sea surface and sea floor. The ocean ambient noise field is a stochastic process of many such noise sources and the respective interactions of their wave fields with the environmental boundaries. As a consequence of the multi-path interaction, the time-averaged ambient noise exhibits spatial structure that is determined by the environmental parameters. Especially in shallow water, the spatial structure is largely determined by the characteristic of seabed. The ambient noise field is one of interference fiend of underwater sound-channel, its spatial structure is one of major factors affecting the performance of underwater sound systems. And furthermore, the ambient noise field is useful information. Some environmental parameters can be inversed by its spatial structure. As relative stable characteristics, vertical directionality and vertical coherence of ambient noise field can exhibit sufficiently the spatial structure of ambient noise field. In this thesis, the vertical directionality, the vertical coherence and the inversion of seabed parameters using vertical coherence are studied based on the range-independent wind-generated ocean ambient noise model in shallow water. When calculating the cross-spectral density of ambient noise field, the normal-mode representation is used. This can not only directly explain the noise problem by the physical map of modes, but expand the noise expression to the case of range-dependence. The part of continuous spectra, which greatly contributes to the noise field, will be lost when the part of discrete normal modes is considered only. To include this part, an artificial layer is added at a certain depth in the bottom. The integration of branch cut is approximately calculated by the summation of the complex modes. Firstly, the vertical directionality of ambient noise field is discussed under different environmental conditions, such as the area of surface noise sources, source directional pattern, sound speed profile, bottom type, the design and placement of receivers and frequency. Secondly, the relations between environmental parameters and vertical coherence are analyzed, including on vertical coherence the effects of bottom, noise source depth, sound speed profile, depths and interval of two receivers. Finally, based on the discussions above, through numerical simulation, the bottom parameters are inversed by matching the "observed value" and "real value" of vertical coherence of ambient noise field and a posteriori probability distributions of the model parameters inversed are estimated. In the process of inversion, Genetic Algorithm is used. Through the works that have been done, the conclusions are drawn as follows: 1. The vertical directionality can exhibit sufficiently the characteristics of soundchannel in ambient noise field. The effects of environmental parameters, which including the area of surface noise sources, source directional pattern, sound speed profile, bottom type, the design and placement of receivers and frequency, on sound propagation and the discrete and continuous spectra of ambient noise field can be seen expressly through vertical directionality. In order to get better vertical directionality, the length of the vertical linear array and intervals of hydrophones should be increased.2.Among the environmental parameters of ambient noise field, noise source depth and the geo-acoustic parameters affect the vertical coherence in different ways, especially compressional speed, shear speed and depth of sediment do. So it can be considered to inverse seabed parameters by matching the "computing value" and "observed value" of vertical coherence.3.The results of numerical simulation shows that the parameters inversed converge on their "real value" in ideal environmental condition and the uncertainty of each parameters inversed agrees with the sensitivity of corresponding parameters to the vertical coherence when there has low -frequency disturbance. This...