Experimental observation and theoretical analysis of the low-frequency source interferogram and hologram in shallow water

Abstract

The interference pattern of the sound field of a broadband source in a shallow water waveguide is studied theoretically and experimentally in this paper. In the ocean waveguide, the sound source generates the interference pattern of the intensity distribution (interferogram) in the frequency–time domain. The mathematical theory of the interferogram structure is developed. The source interferogram consists of a set of quasi-parallel interference fringes in the frequency–time domain. It is shown that the slope of the interference fringes depends on the distance, velocity, and direction of motion of the sound source. The relationship between the slope angle of the interference fringes in the interferogram and the source parameters is derived in the paper. The two-dimensional Fourier transform (2D-FT) is used to analyze the interferogram. The result of the 2D-FT is called the Fourier hologram (hologram). It is shown that the hologram consists of a few focal spots in a relatively small area. The presence of these focal spots is the result of the interference of acoustic modes with different wavenumbers. The mathematical theory of the hologram structure is developed in this paper. The relationship between the coordinates of the focal spots on the hologram and the source parameters is considered. Consequently, the position of the focal spots can be used to estimate the source parameters (range, velocity, and direction of motion). The theoretical conclusions are verified in the context of computer modeling and the results of the acoustic experiment on the Pacific shelf (Yellow Sea, April, 2004) in the band 180–220 Hz.