Interference Patterns In Shallow Water And Broadband Source Ranging

At ranges far enough (greater than several water depths) in shallow water waveguide, the acoustic intensity can be written in terms of normal modes,and every mode has its own eigenfunction and horizontal wave number. It is the mode coherence that gives rise to the interference patterns on the range-frequency,range-depth and depth-frequency coordinate.When the parameters of waveguide are time dependent, the variance of the interference patterns should also be considered.The waveguide invariant theory is used to depict the interference striations on the range-frequency plane.In this thesis,the waveguide invariant theory is revisited and the frequency interference spectrum is found to'shift'and'zoom'along with the variance of range.The frequency-shift processing has been used in several papers described in the first chapter, while the'zoom' character has not been maturely used in underwater acoustics,as long as the author knows it.This character is used for broadband source ranging in this thesis.Firstly,a simple relationship between the range of the source and the local slope of the interference striations is derived and used for source ranging by a horizontal line array and a guide source. Besides the popular method for deriving the slope of the interference striations based on the image processing, an alternative method based on the frequency-shift is provided. Secondly, the interference striations are considered along the frequency coordinate and the range coordinate respectively.The relationship mentioned above can be simplified further into that between the quasi-period on the interference spectrum in frequency domain and the range of source,by which only a single receiver and a guide source are needed for source ranging.It should be noted that every pair of normal modes produce a quasi-period in the frequency interference spectrum(Ω-IS),and if there are more than two modes,the spectrum will be a superposition of different quasi-periods which will make the direct identifying of the quasi-period difficult.To solve this problem, the MUSIC algorithm, a popular and effective method for spectral analysis, is resorted to for identifying and separating the interference components in theΩ-IS made up by pairs of normal modes. Furthermore, such kind of processing may be used in ocean tomography.The work in this thesis can be summarized as follows:(1)The different kinds of interference patterns are provided by simulation and are discussed regarding to waveguide invariant theory.(2)A ranging method using a horizontal line array and a guide source is proposed,requiring no modeling and no knowledge about ocean environment.To make things easier, a method for source ranging in shallow water using a single receiver and a broadband guide source is proposed.(3)Aaron M. Thode has provided a method using a full-span vertical line array and a guide source on the basis of waveguide invariant theory,and another clearer explanation is provided in this thesis.(4) The methods are validated by simulation and (or) verified with experimental data.