Study Of Environmental Impacts On Ambient Noise Properties In The Deep Ocean

Properties of ocean ambient noise as background of many underwater acoustic technology applications are of significant interest to researchers and of great significance in aspects of military and defense and ocean resource exploitation.Previous experimental data have uncovered that there exists a decrease of the ambient noise level below the critical depth in the deep ocean.Based on this,application scenario of vertical line array(VLA)deployed below the critical depth for submerged targets detection has aroused great interests.The performance of VLA is dependent on ambient noise properties,such as noise level,noise spatial coherence and noise directionality,which are influenced by the ocean environmental conditions.While the general features of the ambient noise was understood,and individual data can be fitted by adjusting many of the unknown parameters,the general reasons for the difference in the observed data are not known.Since the experimental data are inherently tied to the ocean environmental conditions,in the thesis we conduct a systematic analysis of the depth dependence of surface-generated ambient noise at low frequencies using simulated data,and study the influence of various environmental conditions including local/distant wind and shipping traffic,noise source depth,sound speed profile change and bathymetry effects such as seamount blocking and bottom interactions in shallower-bottom areas.From the simulation results we conclude that,the decreasing rate of ambient noise level below the critical depth depends mainly on the amount of locally generated(wind)noise relative to the distant shipping noise(e.g.,a low decreasing rate suggests relatively strong local wind);while the depth dependence of ambient noise in the water column above the critical depth depends mainly on the bathymetry conditions.For the latter,as the depth increases from upper water column to around the critical depth,the noise level as a function of depth can be changed by the effects of seamount blocking(while with flat bottom the noise level would increase gradually with depth).The noise level could decrease gradually,or increase gradually,or remain almost constant with depth as the distance between the seamount and receivers and/or the size/height of the seamount changes.The intensity of distant shipping noise originating from shallow-water areas remains stable in the water column above the depth of the shallow ocean bottom,and decreases sharply below that depth till the deep ocean bottom.The rest of the environmental factors investigated is found to have no critical effects on the general tendency of the noise with depth.The results are then used to fit and interpret the existing experimental data and suggest critical tests to verify the theory/conclusions.The fitted environmental conditions are further used to predict/simulate the vertical coherence and directionality of the ambient noise for each experiment.The predicted noise properties are further explained from the perspective of normal modes using modal analysis.The study of the thesis is of important significance for the acquaintance of noise properties for a given area such as noise coherence and noise gain,and the deployment and design of VLA in order to improve its performance in practice of underwater target detection applications.