The Impacts Of Ocean Meso And Small Scale Processes On Underwater Acoustic Propagation

Sound,as the only known form of wave to transmit over a long distance in the ocean,is an important information carrier for underwater environmental detection and information transmission.As the core of the underwater warning system,underwater acoustic equipment is widely used in tasks such as underwater monitoring,target detection,recognition,localization and tracking,torpedo warning,underwater communication,and underwater acoustic warfare.The performance of the underwater acoustic equipment in real applications is affected by ocean environments,target characteristics,noise characteristics of the arrays,and technical regime of the equipment,among many others.There are complex multi-scale dynamic processes in the ocean such as large-scale circulations,mesoscale eddies,small-scale wind waves,and ocean internal waves.These processes may cause dynamic changes to the acoustic environments including temperature and salinity.As a result,underwater acoustic propagation characteristics are varied temporally and spatially,which has great effects on the performance of Sonar systems.Therefore,a deep research on acoustic propagation characteristics in the ocean will provide guidance to the design,evaluation,and operational effectiveness of Sonar systems.Three meso-and small-scale dynamic processes are studied including mesoscale eddies,internal solitary waves and wind waves,whose spatial structural features and corresponding effects on acoustic propagation are analyzed via numerical and experimental simulations.The results provide theoretical basis and technical supports to the evaluation and application of Sonar systems in dynamic ocean environments.The contributions of this paper mainly include:(1)An approach for the recognition and reconstruction of mesoscale eddies is proposed.The 3-D characteristic structure of mesoscale eddies is demonstrated systematically using data from remote sensing and floating buoyancy systems.Based on parabolic equation RAM-PE model and ray model,the acoustic propagation characteristics and the effect on multi-channel arrival times are analyzed when mesoscale eddies are at different positions.Results show that eddies have the greatest effects on the acoustic field near the ocean surface,resulting in 16 d B propagation loss at certain range and depth as well as 1 s arrival time fluctuation at approximately 100 km.Besides,the cause to the variations of the acoustic field due to eddies is explained qualitatively based on the ray theory.The effects on the performance of Sonars systems designed for detection and communication are also discussed based on the analysis of acoustic propagation characteristics.(2)Based on experiment data,the spatial characteristic structure of internal solitary waves in the northern of the South China Sea is systematically studied,and the difference between the internal solitary waves at the continental slope and shelf of the northern of the South China Sea is compared.We also analyzed how the characteristic structure of the internal solitary waves changes along the crest line of wave.A pattern is found about the effects of the internal solitary waves on temporal coherence using numerical methods.Based on acoustic propagation data received at a fixed position,the fluctuation rule of propagation loss due to the internal solitary waves is analyzed,and the changing rule of propagation loss when the internal solitary waves are present is statistically analyzed.The effects of the internal solitary waves on military activities are discussed.(3)Based on the model of 1-D PM spectrum fluctuating ocean surface and the model of bubble population due to wind waves,a calculation method for acoustic propagation is proposed that considers fluctuating ocean surface and bubble mixture layer simultaneously in the rough sea.Simulation results show that fluctuating ocean surface and bubble mixture layer have great effects on acoustic propagation within the isothermal layer near the ocean surface caused by wind waves,while they have less effects on acoustic propagation within the negative gradient layer in deep sea.Besides,the noise spectrum levels at different depths and frequency bands are analyzed using the observation data in the deep water in the South China Sea in 2018.It is found that the noise in high frequency band is significantly affected by environments,the time variation of noise at different depths is consistent,and the noise spectrum level distribution is close to normal distribution.