| The change of ocean water medium with distance and undulation of seabed topography will cause normal mode coupling in the acoustic field.The influence of this phenomenon on long-distance sound propagation cannot be ignored;interference between normal modes will cause space-time-frequency change of the low frequency and long distance acoustic field.The interference between normal modes will cause space-time-frequency change of the acoustic field,which will cause amplitude fluctuation of received signals and change of frequency-distance received signal interference pattern in the low-frequency long-distance sound propagation,leading to the decreasing strength for received signal and lower signal-to-noise ratio,which will affect detection performance of the sonar equipment.Therefore,modeling and analysis of the coupling effect and interference characteristics of the normal mode in low-frequency and long-distance acoustic propagation is one of the important contents for hydroacoustics.This thesis focuses on acoustic coupling and interference characteristics in complex ocean environment.First,a coupled normal mode acoustic field prediction model under the condition of a range-dependent elastic seabed is established.Under the standard wedge-shaped sea area environment with elastic seabed,the model is compared and analyzed with elastic parabolic equation acoustic field calculation program RAMS and finite element commercial software COMSOL.The results verify correctness of acoustic field calculation results of the coupled normal mode model,and analyze influence of the ocean environment and acoustic source parameter changes on the acoustic field coupling and interference characteristics;secondly,single normal mode broadband coupling interference characteristics under the influence of solitary internal waves are studied.Also,based on single normal mode broadband coupling interference pattern under the influence of solitary internal waves,a method for estimating distance between the solitary internal wave and the acoustic source is proposed,and the simulation realizes accurate estimation of distance between solitary internal wave and acoustic source;finally,modeling studies interference characteristics of broadband acoustic field in distance-frequency domain,and proposes a new method for passive ranging of underwater broadband pulse acoustic sources based on periodicity of horizontal wavenumber difference of the broadband acoustic field normal mode;also,frequency spectrum of the single normal mode broadband received signal under influence of internal waves is periodic,and a method for estimating distance from solitary internal wave to acoustic source is proposed.The specific research content is as follows:1.Study on acoustic field modeling of complex eigenvalue coupled normal mode under the condition of range-dependent elastic seabedThe acoustic field prediction model of complex eigenvalue coupled normal mode under the condition of range-dependent elastic seabed is established.First,the elastic coupling matrix expression under the complex eigenvalue condition is derived.Based on above derivation,a sound field prediction model of complex eigenvalue coupled normal wave under the condition of distance-dependent elastic seabed is given.then,in the environment of elastic seabed standard wedge-shaped sea area,a comparison is made among the calculation results of sound pressure field in water for the model and RAMS and COMSOL,to verify accuracy of acoustic field calculation of the model,which is better than that of RAMS;finally,a series of ocean environment and acoustic source parameters such as different seabed slopes,seabed topography,seabed transverse wave sound velocity,and acoustic source depth are compared to analyze their effect on acoustic field characteristics under elastic wedge seabed conditions.Analysis shows that influence of leaky modes in acoustic field calculation of standard wedge-shaped sea area on elastic seabed is not negligible.The coupling matrix of leaky normal modes is complex,which results in normal mode phase change and complex acoustic field interference pattern change.The normal mode coupling has a strong influence on interface wave amplitude.Increase of seabed transverse wave sound velocity will cause increase of normal mode attenuation coefficient.When the normal mode is close to cut-off frequency,the trapped normal mode has a strong coupling effect on the leaky normal mode,and the mechanism of the increase of coupling strength when the normal mode is close to the cutoff frequency is revealed.Also,there is a linear relationship between coupling strength of the normal mode and the seabed slope.2.Research and application of single normal mode broadband coupling interference characteristics under the influence of solitary internal wavesPrevious studies on influence of solitary internal waves on broadband acoustic field interference characteristics mainly focused on broadband acoustic field interference characteristics under combined action of all normal modes.The method in this thesis focuses on single normal mode broadband coupling interference characteristics under influence of solitary internal waves,and the wideband pulse signal received by VLA elements during soliton internal wave propagation along direction of the source to receiver array under the static condition of a wideband pulse acoustic source and a vertical receiving array separated by 50 km is simulated.Simulation results show that there are multiple pulses in single normal mode broadband received signal caused by the solitary internal wave,and the resulting single normal mode broadband coupling interference pattern;imitating the aoucstic source measurement method based on waveguide invariant,based on quantitative relationship between normal mode group velocity,phase velocity,signal frequency and interference fringe slope,a method for estimating the distance between solitary internal waves and the acoustic source is proposed.Simulation results show that this method can accurately estimate the distance during movement of the solitary internal wave from the source to receiver.Moreover,the precision and real-time performance of measurement for the distance between the soliton and source are improved,which opens a new perspective for internal wave ranging based on the periodicity of normal mode interference in frequency.3.Application research of broadband acoustic field interference frequency periodicity in solitary internal wave ranging and underwater acoustic source rangingBased on frequency periodicity of wide-band acoustic field interference,and quantitative relationship between frequency period of single normal mode broadband coupling interference pattern caused by the solitary internal wave and distance between solitary internal wave and acoustic source,a new method for estimating distance between solitary internal wave and acoustic source is proposed.According to frequency period of horizontal wavenumber difference of the normal mode in broadband acoustic field interference pattern and distance from receiver to the source,a new method of passive distance measurement for broadband pulse acoustic sources is proposed.Frequency period of horizontal wavenumber difference of normal modes decreases with increase of distance,which leads to decrease of internal wave distance or acoustic source distance estimation error.Therefore,simulation results show that the method of estimating distance between the soliton internal wave and acoustic source based on frequency periodicity can accurately estimate the distance when the distance is farther than the minimum distance for the normal mode interference to cover one interference period in frequency,Improving the real-time performance for ranging of soliton internal waves;the passive ranging method based on frequency periodic broadband pulse acoustic source can also accurately estimate the broadband pulse sound distance when the distance between the receiver and source is farther than the minimum distance for the normal mode interference to cover one interference period in frequency,improving the accuracy and real-time performance of passive ranging of broadband acoustic sources.Simulation analysis and marine test data processing show that this method can reach a long-distance underwater broadband pulse acoustic source positioning distance of up to 30 km,and the ranging error is less than 1% under the condition of high signal-to-noise ratio.The method is feasible.At the same time,the new method proposed in this thesis opens a new perspective of passive ranging of broadband sound sources,and solves the passive localization problem of acoustic sources based on periodicity of acoustic field interference in frequency.This method has been used in this thesis to simulate the calculation of the distance from the soliton internal wave to the acoustic source,as well as to measure the distance from the receiver to the broadband acoustic source in simulation experiments.This method is expected to be expanded in other fields to solve a series of ranging-related problems. |
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