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In shallow water exist the stable and significant interference characteristics of low frequency sound propagation, which contain the information of the sound source state and waveguide peculiarity. A simplified mapping method for describing the scalar and vector sound field interference structure radiated by a moving target, and an indicatory mechanism of the target state implicated in the energy distribution of the mapping domain are investigated in this paper. The mapping characteristics of two-dimensional Fourier transform of the vector sound field time (space) frequency interference spectrum are analyzed theoretically. Relations among waveguide invariant, range-rate, heading angle, and energy ridge slope of the mapping domain for time-frequency interference spectrum produced by a uniformly moving target are derived. Indication of target attacking or moving away, and the degree of threatening through symbols or the absolute value change of mapping domain’s ridge slope are demonstrated. Then numerical simulation and sea trial research are carried out. Experimental results with theoretical analysis and simulation results are in good agreement with each other. Research results show that the scalar and vector field time (space) frequency interference structure can be simplified by the two-dimensional Fourier transform. The mapping domain ridges, range-rate, heading angle and waveguide invariant show an analytic relationship among them. Variation embodied in the form of scalar and vector field interference structure obtained after mapping are more consistent with each other. The ridge of mapping domain can indicate the moving state of target concisely.

The microscopic structure characteristic of low frequency sound propagation in shallow water is that a stable space-frequency interference structure represented by waveguide invariant exists. Both the scalar field and the vector field determine all the features of the acoustic field. The vector field characteristics of space-frequency interference structure in shallow water acoustic field are investigated. The theoretical analyses of the mechanism of interference structure in the sound pressure spectrum, the kinetic energy density spectrum, and the acoustic intensity flow spectrum in shallow water vector acoustic field are conducted, the numerical simulation studies of the interference of the Pekeris waveguide in energy and energy flow density characteristics are performed, and the sea trials for broadband sound source radiation vector sound field interference properties and characterization are also carried out. The experimental results accord with the theoretical analyses and simulation results very well. The research results show that stable space-frequency interference structure in acoustic vector field exists in many forms for modal coherent in short-distance and long-distance. In addition to various forms of energy and energy flux density spectra, in coherence-coefficient spectrum there appear the interference characteristics. These forms of space-frequency interference structure can be described by waveguide invariant theory effectively.