Studies On Acoustic Field Characteristics And Passive Localization Methods At Great Depth Of The Deep Ocean

As the ocean technology in deep water is paid much more attention than that in shallow water,the target detection techniques in deep water are also attracting much more attention.But the detection performance of the near-surface sonar is impeded by the vertical distribution characteristics of the sound speed profile.At present,the reliable acoustic path is able to fill the blind zone.Supported by the National Basic Research Program of China,this dissertation studies the spatial correlation of acoustic field with deep receiver in the direct zone in deep water.Based on the interference characteristics of the acoustic field in space/frequency domain,some new localization methods are proposed and verified using numerical simulations and experimental data.This dissertation provides theoretical and technical bases for further researches.The main contents and contributions are as follows:1.The spatial correlation of acoustic field with deep receiver in the direct zone in deep water is studied,and the theoretical solution of spatical correlation coefficient is derived using the ray theory.The acoustic field at great depth is mainly constructed by direct and surface-reflected arrivals,and the time delay between them determines the interference characteristics of the acoustic field.The spatial correlation coefficient is derived theoretically by using the ray theory.The spatial correlation has a fixed oscillatory period depending on center frequency and time delays between direct and surface-reflected arrivals.Through numerical simulations,the effects of source-receiver geometry on the horizontal-longitudinal correlation coefficient are studied and explained from the interference point of view.Finally,the experimental data collected in Western Pacific and South China Sea are analyzed,and the results reach good agreements with the numerical and theoretical ones.The conclusions are beneficial for the large-aperture array design and array signal processing.2.The oscillatory features of cross-correlated field in frequency domain are analyzed,and a moving target velocity measurement method using a single hydrophone is proposed.The cross-correlated broadband fields received by a single hydrophone deployed near the sea bottom show two kinds of interference patterns obviously.Based on the ray theory,the cross-correlaton field is derived theoretically and the interference characteristics are revealed clearly.The first kind of interference pattern is associated with radial source velocity,whose striations are called velocity-dependent striations.The second kind of interference pattern is associated with source depth,whose striations are called dependent-dependent striations.The radial source velocity and source depth can be estimated by Fourier transforming the oscillatory period of the striations.For the target moving track with a Closest Point of Approach(CPA),a joint estimation method of target motion parameters is proposed.For the radial moving target at far range,the radial source velocity approaches the source constant velocity.When the target range is unknown,one can discriminate the surface target from the underwater one by using the oscillatory period of the dependent-dependent striations.Experimental results confirm this single hydrophone localization method.3.The multipath characteristics in space and freqeucny domains are studied,and a target localization method using a vertical line array is proposed.The sparse characteristics of multipath arrivals make it possible to estimate source range by using the information of multipath arrival angles.The direct arrival angle is sensitive to target range,then a target range estimation method by matching the estimation value with the theoretical value of the direct arrival angle is proposed.Based on the Lloyd’s mirror effect,the interference characteristics of the beamforming output are sensitive to target depth,then a target depth estimation method by employing the relationship between the interference period and target position is proposed.Lastly,a dimensionless joint localization method is proposed.An obvious advantage is that it can enhance the interference features for the source depth estimation.Based on the depth variation features,a robust target depth classification method is also proposed.Besides,the detection zone and blind zone of the underwater target are defined by using ray method.The validity of the proposed methods is verified by experimental data.4.The bearing estimation error for the horizontal line array deployed near the sea bottom is studied,and at the detection region of the reliable acoustic path,a robust error calibration method is proposed.Due to multipath effect and frequency dispersion effect,a bearing estimation error will occur when converntional beamforming is used for the horizontal line array in the real ocean waveguide.Through numerical simulations,the effects of target range and targert direction on the bearing estimation error are studied in detail.The bearing estimation error reaches maxmum at the endfire direction of the horizontal line array and decreases to zero at the broadside direction.The estimated target bearing always lies between the true target bearing and the broadside direction.The physical mechanism is analyzed by using normal-mode theory and the ray method.Finally,an error calibration method is proposed at the detection region of the reliable acoustic path.Simulations show that the calibration results are not sensitive to the estimation error of the target range.Consequently,the proposed calibration method is robust.The conclusions are beneficial for the application of the horizontal line array in deep water in the future.