Studies On Sound Propagation And Source Localization Methods In Deep Water

After the Cold War,worldwide workers on underwater acoustics pay much more attenuation on the theoretical and experimental researches on the shallow water than these on the deep water.Domestic experts also focus on the coastal ocean,which is generally continental shelf.However,in recent years,as the situation in Western Pacific and South China Sea is becoming more and more tension,the requirement for the technique in deep water is greatly increasing.This thesis takes advantage of the experiments performed in deep water in last several years and analyzes the features of the sound propagation in deep water.Furthermore,based on these features,several robust methods for source localization are presented and verified using simulations or experimental data.The objects of study in this thesis are the surface duct,the bottom-reflected path and the reliable acoustic path.The contents of this thesis are as following: 1.Sound propagation in the surface duct is studied and the optimal depths for the source and the receiver to obtain the low transmission loss are presented.Understanding the effects of source-receiver geometry on the sound propagation in surface duct can improve the performance of the near-surface sonar in deep water.Lloyd-mirror and normal mode theories are used to analyse the features of surface-duct propagation in this thesis.Firstly,a method to calculate the minimum cutoff frequency(MCF)is derived.The presented method is source depth dependent and thus helpful for determining the working depth for sonar.Secondly,it is found that under certain environment there exists a layer of low transmission loss(TL)in the surface duct,whose thickness can be calculated by Lloyd-mirror method.The receiver should be placed in this layer to minimize the TL.Finally,the arrival angle on a vertical linear array(VLA)in the surface duct is analyzed based on normal mode theory,which provides a priori knowledge of the beam direction of passive sonar.2.To solve the problem of the blind zone in source detection,this thesis presents a solution using the energy leakage of the low-frequency sound outside the surface duct to illuminate the blind zone.The normal mode and the geometrical diffraction ray theory are used to explain the features of this propagation.Furthermore,the energy leakage contributes to the energy attenuation in the surface duct.Dominant parameters determining the value of the leakage attenuation are frequency and surface duct thickness.An expression for the leakage attenuation is derived with the aid of an extensive simulation study and verified using the data from two experiments.The expression shows differences in the attenuation determined by product of the frequency and the duct thickness raised to different powers,which indicates the strongly coupled effects of the two parameters on energy leakage.3.To solve the problem of the blind zone in source detection and localization,a method using a near-surface VLA and the bottom-reflected signals is presented.By casting the localization as separate estimations of the source range and source depth,the performance is much better than that of the Matched-Field-Processing(MFP)technique with the Bartlett processor.Source range estimation is based on the Weighted-Subspace-Fitting Matched Field(WSF-MF)method with modification to consider the array tilt.Source depth estimation is based on the time delay of multipath arrivals.4.To improve the signal-to-noise ratio for better sonar performace and to eliminate the blind zone within short range,this thesis provides a solution using the reliable acoustic path.The analysis of the physical properties of the reliable acoustic path(RAP)shows that the source localization method based on arrival angles is feasible in this environment.Consequently,the WSF-MF method based on a VLA is applied to do the localization.This method avoids the estimation of the arrivals angles,which is a key but unstable point for many conventional methods.5.To simplify the mechanical structure of the sonar system in the reliable acoustic path,this thesis presents two methods using a single hydrophone for the source localization.When one takes advantage of the RAP,the requirement for the mechanical structure is high to deploy a VLA near the ocean bottom.A better solution is just using single hydrophone or a planar array.Localizing a source of radial movement at moderate range using single hydrophone can be achieved by tracking the time delays between the direct and surface-reflected arrivals(D-SR time delays).The problem is defined as a joint estimation of the depth,initial range and speed of the source,which are the state parameters for the extended Kalman filter(EKF).The D-SR time delays extracted from the autocorrelation functions are the measurements for the EKF.Experimental results using pseudorandom signals show that accurate localization results are achieved by offline iteration of the EKF.An alternative for single hydrophone localization in the RAP is to exploit the interference structure of the sound field.The intensity spectrum recorded on a hydrophone for a continuous broadband signal from a target at moderate range,plotted as a function of range,exhibits light and dark striations,which is defined as the RAP striation.The physical mechanism of the striation is modeled using the ray-based description of the normal modes and a method is derived to depict the striation traces accurately.As the structure of the striations is sensitive to the source depth,two methods designed for certainty and uncertainty envrionments respectively are presented for source depth estimation.