Research On The Spatial Characteristics Of Ocean Ambient Noise Based On Ray Theory

Ocean ambient noise is a kind of background field in underwater sound channel.The gain of sensor array is improved by taking advantage of the difference between noise and signal fields in the matter of space-time statistical property.Consequently,the capacity of sonar devicein term of resisting interference is enhanced.Based on the ray theory,the models of spatialdirectivity and correlation of noise fields generated by surface noise in horizontal stratifiedocean waveguide were established in this article.On the basis of the above,the issues of noise field directivity in range-dependent waveguidewere further analyzed.By processing sea trial data,the vertical directivity and correlation of noise fields measured by array were obtained.First,the expression of vertical directivity of noise fieldsin horizontal stratified mediumwas derived based on ray theory.The impact on noise field vertical directivity from different ocean bottom parameters and noise source frequency was studied,as well as the change of directivity in various depthsof field was researched.The results show that the peak value of vertical directivity reduces with increasing of noise frequency;the variation of the vertical directivity of the noise field at different depths at the heterogeneous sound velocity is related to the sound velocity profile;the amplitude of the vertical directivity of the noise field is significantly magnified by the increasing ofocean bottomcompression wave velocity.Next,the spatial directivity function of ocean ambient noise pressure field was derived based on ray theory under " 2 " approximate condition in wedge-shape ocean bottom.The influence of the sound velocity profile on the spatial directivity of the noise field was studied,and the change of the directivity of the noise field with the various slopeswas alsoresearched.The studiesindicate that the noise with small grazing angle is very weak when acousticwave propagates upward slope,the noise intensityof small grazing angleis strong while sound goes downward;when the slope of the wedge-shape bottom increases,the directivity of the noise field is sharper with the variation of azimuth angle;when the sound velocity profile changes,the mode of sound transmission is altered by slope bottom topography within the certain scope of grazing angle.Then,the expressions of vertical correlation of the noise fieldin horizontal stratified mediumwere derived based on ray theory and wave theory respectively.The influence ofdifferentnoise sourcefrequency on the vertical correlation of the noise field was studied,and the change of vertical correlation characteristics invarious depthsof fieldwasresearched.The results of the two models were compared and verified.The results show that the results of the two models are identical and verified mutually;the correlation of noise field changes weakly with the changing of frequency within high noise source frequencyband;The vertical correlation of the noise field at the heterogeneous sound velocity is related not only to the vertical interval but also to the absolute depth,fundamentally,the impact is from the sound speed profile.At last,the ocean experimental data were processed.The vertical correlation of noise fields measured by vertical array was calculatedcompared with ray model predicted results.The effectiveness of the model was verified.The difference between the directivity of noise fields measured by vertical array calculated by using experimental data and noise field vertical directivity based on ray theory was well comprehended.The influence of differentnoise sourcefrequency on the vertical correlation and directivityof the noise field measured by vertical array was analyzed.The data processing consequences show that the changes with frequency of correlation function within low-frequency band is no more sensitive than that within high-frequency band;the peak value of directivity of the noise field measured by array changes with the variation of frequency within low-frequency band.