The Interference Structure Of Shallow Water Low-frequency Acoustic Field And Its Application

Underwater acoustic technology has made great progress recently, but it also faces new challenges. With the development of quiet submarine, whose radiated noise has been greatly reduced, how to detect the quiet submarines in long range becomes difficult. Towed line array sonar is one of the important sonar developing directions, because it has been playing an important role in anti-submarine warfare and torpedo alarm. However, until now there is no any technology making this sonar possessing the functions of passive ranging and velocity estimation. The important feature of the ocean waveguide is that the stable interference structure can be observed in the low-frequency sound field. The purpose of this paper is to explore new target detection, identification and passive ranging techniques based on the physical nature of the sound field.Sound field interference structure is the function of the range, the depth and the radiated noise frequency of the target, so it should be simplified by adopting space-time transformation to enhance useful information for application. The interference structures in shallow water low-frequency sound field have been researched and many attentions are paid to the line spectrum interference structures in the very low frequency sound field and the continuous spectrum interference structures in the low-frequency sound field. For the former, the vector field of normal modes in shallow water stratified medium waveguide is concerned, the expression of complex acoustic intensity, especially the analysis of their characteristics are proposed; the reactive component and active component of the vertical (interactive) or horizontal interactive complex sound intensity are analyzed quantitatively; Further more, the phase angle of complex acoustic intensity is discussed and its distribution regularities are analyzed numerically. For the latter, the interference striations on the continuous spectrum interference structures, also called LOFARgram or coherent function records, in the low-frequency sound field are described based on the theoretical framework of waveguide invariants; Other than coherent function, the power response function, dual-frequency function and spread function from the space-time filtering theory of coherent sound field are adopted to promote the characterization means for continuous spectrum interference structures; The coincidence of the results from the simulation and sea trial, which are analyzed combined waveguide invariants, suggests the correctness of the theory; Then continuous spectrum interference structures in other scenarios are introduced briefly.Researches on the line spectrum interference structure in very low frequency sound field show that:the sign of the sound pressure cross-spectrum (or cross-spectrum between sound pressure and horizontal velocity) active component is independent of range by placing the receiving transducers in appropriate depths when only two normal modes trapped in the waveguide, so the target can be classified by comparing its depth with the critical depth on which the sign of the sound pressure cross-spectrum active component changed oppositely, in other words, the sound pressure cross-spectrum active component can be used to tell whether the sound source is near the surface (surface ship) or underwater (submarine); while the active component sign of the sound pressure cross-spectrum changes periodically with the horizontal range, and the period is defined as horizontal coherence distance which can be obtained by acoustic field modeling or predicting using effective depth approximate model, then the target radial velocity can be estimated by measuring the time during which the moving target passes one horizontal coherence distance scale. Approximate theoretic analysis of sound pressure cross-spectrum active component characteristics which deepens the understanding to the characteristic of shallow water low-frequency acoustic field is proposed in the paper combined with the concept of effective depth, and the prediction formula of appropriate deploying depth of transducers is presented which can be used to guide the actual application of the algorithm. The theoretic analysis indicates that the sum of the bi-hydrophone depths should be equal to the waveguide effective depth, then the sign distribution of the sound pressure cross-spectrum active component is independence on the horizontal range, and bi-hydrophone deploying depth is uniquely determined by setting a expected critical depth which is adjustable. Further stability analysis shows that the depth classification algorithm and radial velocity estimation algorithm are applicable to the different shallow water environment.Researches on the continuous spectrum interference structure in low frequency sound field show that:the interference striations on the LOFARgram obtained in the shallow water low-frequency acoustic field are a family of quasi hyperbolas, and the waveguide invariant, the range of the closest point of approach and the corresponding time are linked by the interference striation equation; Among the four system functions of the space-time filter theory, there is only one independent, every function can be adopted to describe the characteristics of the continuous spectrum interference structure, however, each system function owns special characteristics, which can give prominence to certain features of the interference structure. Hough transform based parameter estimation algorithms are proposed combining the image processing to LOFARgram and bearing-time records, and the correctness and feasibility of the parameter estimation algorithms under two scenarios that is whether there is closest point of approach have been validated by the simulation researches and sea trial data analysis, which also show its high accuracy of parameter estimation. For the scenario including the closest point of approach, four dual-array (element) based passive ranging algorithms are proposed when the estimated parameters are as the prerequisite, and the positioning accuracy is analyzed for the four algorithms. And the improved positioning algorithm is studied to overcome the poor stability of the second algorithm. For the scenario without the closest point of approach, two dual-array (element) based passive ranging algorithms are proposed, and then the corresponding simulation researches, accuracy analysis and stability analysis are finished. Furthermore, the improved positioning algorithms for both passive ranging algorithms are proposed to improve the positioning accuracy when the SNR is low.The parameter estimation algorithms and passive ranging algorithms proposed in this paper have important application prospect in many aspects, such as marine integrated monitoring, coastal forecasting and alarming system, airborne sonobuoy, marine research, acoustic measurement, torpedo alarm and so on. Further researches by sea trial will be needed for their practical engineering applications.