Research On Inversion Of Seawater Parameter Based On Stimulated Brillouin Scattering

Temperature,salinity and sound velocity are important components of seawater parameters,which have a great impact on underwater communication and the performance of military equipment.At present,most of the seawater parameter detection uses on-board instrument to make contact measurement,which has many drawbacks,such as poor synchronization and long measurement time.Brillouin lidar is more suitable for the detection of seawater parameters because of its unique real-time,convenience and non-contact.Therefore,using Brillouin lidar to detect temperature,salinity and sound velocity of seawater is of great importance to the development of detection of seawater parameters.In this paper,the relationships between stimulated Brillouin frequency shift and linewidth with temperature,salinity and sound velocity of seawater are established,and the inversion models of temperature,salinity and sound velocity of seawater are obtained.The models are analyzed and evaluated from three aspects: error,sensitivity and uncertainty.Based on the data from the East China Sea,the inversion model proposed in this paper is used to invert the sound velocity of seawater.The influence of sea waves on the sound velocity of Brillouin lidar in detecting seawater is obtained through the analysis of sea surface model.The details are as follows:(1)Based on the basic theory of stimulated Brillouin scattering,the inversion models of temperature,salinity and sound velocity of seawater are established by least squares method with frequency shift and linewidth as independent variables and temperature,salinity and sound velocity of seawater as dependent variables.The effects of temperature and salinity on stimulated Brillouin frequency shift and linewidth are analyzed.It is found that with the increase of temperature,the frequency shift increases logarithmically and the linewidth decreases logarithmically.As salinity increases,the frequency shift increases with power function and the linewidth increases with power function.The maximum error of the temperature inversion model is 0.079 ℃ and the relative error is 0.212% by error analysis between inversion values and theoretical values.The maximum error of the salinity inversion model is 0.122‰,and the relative error is 0.156%;The maximum error of the seawater sound velocity inversion model is 0.124m/s and the relative error is 0.015%.The sensitivity analysis shows that the frequency shift increases,the temperature increases when the linewidth decreases,and the effect of the change of the linewidth on the temperature is greater than that of the change of the frequency shift on the temperature.When the frequency shift increases and the linewidth increases,the salinity increases,and the effect of the linewidth change on the salinity is less than that of the frequency shift change.When the frequency shift increases and the linewidth decreases,the sound velocity increases,and the effect of the change of the linewidth on the sound velocity is less than that of the change of the frequency shift.The maximum value of temperature uncertainty is 0.1945 ℃,the minimum value is 0.0039 ℃,and the average value is 0.0197 ℃.The maximum value of salinity uncertainty is 0.2665‰,the minimum value is 0.1262‰,and the average value is 0.1423‰.The maximum value of sound velocity uncertainty is 0.1999m/s,the minimum value is 0.1711m/s,and the average value is0.1734m/s.(2)Based on the World Ocean Atlas 2018(WOA18),the temperature,salinity and depth data of the East China Sea area were selected.The sound velocity of the East China Sea in different seasons and different longitude and latitude was calculated through the seawater sound velocity inversion model in this paper,and compared with the Richards’ model.Finally,the impact of the temperature halocline on the sound velocity of the sea was analyzed.(3)Based on PM spectrum,the simulation of sea waves under different wind velocity shows that the higher the wind velocity,the higher the power density of the spectrum,and the blurrier the underwater projection.The BRDF model of sea surface for laser scattering is adopted,and the BRDF of sea surface at different wind velocity is analyzed.Based on the above analysis and the detection principle of Brillouin lidar,the influence of sea surface wind velocity on the detection of seawater sound velocity is obtained.When the sea depth is 100 m and the wind velocity is 15m/s,the maximum error of sound velocity detection is 0.041m/s.