Studies On Very Low Frequency Sound Propagation Characteristics Of Deep Ocean

Today,deep ocean acoustics has become a hot topic at home and abroad,especially for China,the theoretical and experimental basis is relatively weak.Understanding the acoustic propagation characteristics is the basis of underwater acoustic investigation,underwater acoustic communication and positioning and sonar system design.At present,human beings have not paid enough attention to the problem of deep ocean very low frequency(VLF,10 Hz to 100Hz)sound propagation,and it is of great value to be familiar with the characteristics of VLF sound propagation in ocean scientific research,ocean development and national defense construction.This paper focuses on the propagation characteristics of VLF sound waves in sound fixing and ranging channel(SOFAR),also has made a theoretical summary of the simulation phenomenon.The results have important theoretical and practical significance.The main contents of this paper are as follows:1.Firstly,the COMSOL results were used as the standard solution to verify the reliability of three kinds underwater acoustic numerical programs,KRAKEN,BELLHOP and RAM,in dealing with VLF sound propagation problems of SOFAR.It was found that the calculation results of KRAKEN and RAM were well matched with COMSOL when they dealt with range independent environment.Although the simulation conditions met the high frequency approximation of ray theory,the results of BELLHOP was far from COMSOL,so it is not suitable to deal with problems of deep ocean VLF sound propagation.When dealing with weak range dependent environment,RAM still have high reliability.2.Two kinds of SOFAR models were established,and the spatial distribution characteristics of transmission loss of VLF sound were simulated and analyzed.It was found that the VLF sound field of SOFAR existence frequency boundary.When source frequency was less than this frequency,the sound field was mode like.Conversely,when source frequency was higher than this frequency,the sound field was ray like.At the same time,the range of the convergence zones were frequency dependent when source frequency was less than the frequency boundary.3.Based on the study of the spatial characteristics of transmission loss,the interference structure of deep ocean VLF sound field and the distribution of waveguide invariants were analyzed.The conclusion is as follows: when the ocean environment was determined and the sound source and receiver parameters were determined,the waveguide invariants generally show "invariance",but there still has some cases in which the waveguide invariants change obviously.For example,when the source frequency was lower than the frequency boundary,the value of waveguide invariants of converging zones would change with the frequency,and the existence of surface channels would also change the value distribution of waveguide invariants.Fundamentally,these two situations were caused by the change of the multi-path structure of eigen rays between source and horizontal line array,this resulted in the change of phase velocity and group velocity,finally,the value of waveguide invariants were changed.