The influence of the sound speed profile on signal waveform correlation for different normal mode in shallow water waveguide

The propagation of broadband acoustic signals in a shallow water waveguide has dispersion characteristics, and the received signal is in a form of the sum of several different normal modes. All the normal modes can be separated respectively by a warping transformation. The arrival time delays of two arbitrary normal modes can also be obtained by cross-correlation processing. The dispersion characteristics of normal modes can be applied to locating and ranging of the source. The dispersion characteristics of the normal modes are influenced by ocean environmental parameters, and sound speed profile (SSP) is one of the important factors which affect the waveguide. There has been significant research into the influence of SSP on the waveform of the normal mode. However, it is known that the ocean environment is complicated and has the characteristics of both temporal and spatial variation, so it is difficult to acquire SSP in situ, although the average speed of SSP can be acquired by experience. As it is difficult to acquire SSP in situ, the cross connection of normal mode waveforms are the subject of study in this paper under some typical SSPs. Simulation and calculation of the cross correlation between different normal modes is undertaken by taking SSP in situ, and average speed as the waveguide SSP. The arrival time delay is obtained and compared. The result is that, when the thermocline is not present, the correlation of pressure signals in time domain between the SSP in situ and the average SSP agrees well. Nevertheless, if there is a thermocline, the results are unsatisfactory. However if the average speed of the thermocline is combined with the average speed of other parts of this SSP, an acceptable result can be obtained. As a consequence, it can be seen that the influence of the variation of SSP has little influence on waveforms in the time domain of normal modes, and the SSP in situ of the waveguide can be replaced by the average SSP in order to analyze the characteristics of sound field. This is especially so in the analysis of the dispersion characteristics and simulation of received acoustic signals in the time domain, where the difficulty of acquiring the SSP can be further reduced.