Numerical And Experimental Research On The Temporal Coherence Of Underwater Acoustic Signals Influenced By Internal Waves

The coherence time of acoustic signals can be decreased by internal waves in the ocean. Physical medium determined coherence time, which is a critical parameter to the detection and identification of sonar targets, matched field locaton and underwater communication, represents the delay time when the coherent signals become incoherent. In this paper, the temporal coherence influenced by internal waves is analyzed via numerical simulation and acoustic data process. And the coherence time of the matched field and range-depth dependence in the conditions of typical sound transmissions are both obtained.Based on parabolic-equation (PE) method, the temporal coherence function along with the spatial distribution of coherence time is calculated by Monte-Carlo simulations. An unfrozen model for environment simulation is developed in this paper on the basis of the frozen acoustic environment model used in previous study, The results of temporal coherence show that the frozen model is feasible within a certain frequency and receiving range. And the frozen model becomes less reliable with the increasing frequency and receiving distance. During the telecommunication, the matched field process is too difficult to be carried out by the sonar system. Numerical and experimental investigations demonstrate that the range-depth dependence of the coherence time is almost analogous with the interference pattern of the sound field. And the coherence time in the convergence zone is longer than that in the shadow zone, which is further testified by the exsistence of stochastic noises in the ocean. Moreover, the coherence time is no longer power law dependent with the frequency, range and sound speed standard deviation because of interference structure fluctuations of sound field.Besides the Monte-Carlo simulation research, a statistic method based on the coupled mode theory is proposed to study the temporal coherence of acoustic signals. In the condition of longe distance sound propagation in deep water, the statistical study also shows that a similar power law frequency, range and sound speed standard deviation dependence of the matched field coherence time is predicted by the adiabatic approximation, Monte-Carlo simulation and one-way coupled theory. The coherence time is much more overestimated by the adiabatic approximation which can not describe the range-depth distribution correctly. However, the adiabatic approximation is almost feasible in shallow water. The coherence time obtained through the two dimentional statistical model developed in this paper is longer than the three dimentional model developed previously. Within the range of the parameters discussed in this paper, the numerical results can converge with only five internal wave modes and dozens of normal modes considered. But more normal modes should be included to obtain a precise dimentional distribution of the coherence time.The results of acoustic data from ASIAEX2001 demonstrate that the coherence time is much more decreased when strong solitons appear upon the propagaton paths from sources to receivers, when the coherence time of 300Hz signals can be less than 10 seconds and that of 500Hz signals even falls below 10 seconds. The coherence time of signals with a higher frequency is shorter than that with a lower frequency. Besides, the coherence time also shows an obvious depth dependence. The results of received 400Hz signals at R transmitted from two different sources S1 and S2 are different. The coherence time of S2-R is longer than that of S 1-R, which may be caused by the faster phase velocity of solitons from S1 to R. The difference of source depth and seafloor between the two propagation paths can also contribute to the conclusion. The waveform and strength changes during the propagation of solitons are neglected in the frozen environmnet used by PE simulations, resulting in the longer coherence time of simulations compared with datas. Therefore, in order to develop a much more realistic unfrozen model, enough temperature sensor strings should be used to observe the evolution of solitons from sources to receivers.