Study On The Vertical Spatial Characteristics And Nonlinear Dynamical Characteristics Of Ocean Ambient Noise In Deep Ocean

Ocean ambient noise is a background noise that always exists in underwater environment,and a deep understanding of its temporal and spatial characteristics is a necessary condition for improving sonar performance and developing passive information sensing methods.In the context of the development of China’s Marine industry towards the deep sea,this dissertation,supported by the Key Program of National Natural Foundation of China(No.11534009),deeply studies the vertical spatial characteristics(including vertical directionality and vertical correlation)and the nonlinear dynamic time characteristics of deep-ocean ambient noise.In this dissertation,the numerical modeling theory of noise vertical spatial characteristics in deep ocean is developed,the law and mechanism of the influence of common environmental disturbances on noise vertical spatial characteristics in deep ocean are explored,and the nonlinear dynamic characteristics of ocean ambient noise are deeply discussed through the comparative analysis of deep-ocean noise data collected in absence/ presnece typhoon.The conclusions of this dissertation are important basises for improving the target detection and passive inversion performance of vertical arrays in deep ocean,and developing nonlinear noise suppression and target detection methods.The specific contents and innovation points of this dissertation are as follows:1.The expressions of the noise vertical spatial characteristics in deep ocean derived,and the basic characteristics of noise vertical spatial characteristics in typical deep ocean environment are analyzedA noise field model based on sound propagation theory is established,and the Green’ function representation(general representation)of noise vertical spatial characteristics are derived.By revising and extending the normal mode representations of noise vertical spatial characteristics in shallow ocean,the normal mode representations of noise vertical spatial characteristics in deep ocean is derived.Numerical simulations are carried out in typical deep ocean environment to verify the validity of the given expressions,explain why the normal mode representations of noise vertical spatial characteristics have different forms in shallow and deep ocean,and analyze the laws and physical mechanisms of the changes of noise vertical characteristics versus depth in typical deep ocean environment.2.The laws and physical mechanisms of the influence of surface duct on noise vertical spatial characteristics in deep ocean are revealedUsing the normal mode representations of noise vertical characteristics,numerical simulations are carried out in the cases where there is no surface duct and there is a surface duct with varying parameters(including the thickness of the duct and the sound velocity gradient within the duct),to reveal the influence of surface duct on noise vertical spatial characteristic at the depth below the duct in deep ocean.According to the propagation characteristics,the normal modes induced by noise sources are classified into four categories: invalid modes,trapped modes,refracted modes and reflected modes.The influence of surface duct on the numbers and intensities of these four categories of normal modes is analyzed based on the WKB approximation of the mode function,and which is varified by numerical simulations.Based on the normal mode representation of noise vertaical characteristics,the effect of changes of these four categories of normal modes on noise vertical spatial characteristics are discussed,thereby the physical mechanisms of surface duct changing noise vertical spatial characteristics are revealed.3.The laws and physical mechanisms of the influence of eddies on noise vertical spatial characteristics in deep ocean are revealedUsing the general representations of noise vertical characteristics,numerical simulations are carried out in the cases where there is no eddy and there is a cold or warm eddy with different intensities,to reveal the influence of eddies on noise vertical spatial characteristic at the eddy center with different depths in deep ocean.In the numerical simulations,a simple and effective Gaussian eddy model is used to describe the sound speed disturbance caused by eddies.Considering that the sound speed distribution is range-dependent in presence of eddies,the ray and parabolic equation models are used to calculate the Green’s function from the near and far field noise sources to the receiving point respectively,so as to give consideration to the calculation accuracy and efficiency.The effect of eddies on the pitch angle and energy of noise arriving at the receiving point are analyzed using a ray inverse tracing method,hence the physical mechanisms of eddies changing noise vertical spatial characteristics are revealed.Numerical simulations also show that the above analyses and conclusions can be approximately extended to the cases where the receiving point deviates from the horizontal center of the eddy,but they are close to each other.4.The internal priciple of the temporal variation of ocean ambient noise is revealed from the perspective of nonlinear dynamicsIn the study,deep-ocean noise data recorded in absence and presnece of typhoon were used,both of which were obtained from a noise observation experiment carried out by the research group in the South China Sea.In pre-analysis,the time domain waveform,power spectral density and probability distribution of the noise data are analyzed,and the intensities of ship noise and wind-generated noise,which are two dominant components of ocean ambient noise,in the two types of noise data are compared.In nonlinear dynamical analysis,phase space is reconstructed by means of the averag mutual information and false nearest neighbor mehtods,and whether chaotic behavior exists in the the two types of noise data is analyzed using the largest Lyapunov exponent,permutation entropy and permutation test methods.The common characteristic of the above chaos discriminantion method is that they are less affected by stochastic noise and can indicate the strength of stochastic noise in the data.According to the results of pre-analysis and nonlinear dynamic analysis,it is pointed out that ocean ambient noise contains both chaotic and stochastic processes,and shipping noise and wind generated noise are chaotic and stochastic respectively.