Research On Modelling Method Of The Local Area Sound Velocity Field And Its Application In The Deep-sea

Seafloor resource exploration,marine environment observation,seafloor plate monitoring,and disaster monitoring require precise acoustic positioning technology.The variation of sound velocity is an essential factor affecting the precision of underwater sensor localization.Ocean sound velocity information can be accurately determined,but we only obtained point or line measurements rather than the whole observation data of the region.The existing parametric model of sound velocity field can provide the information on sound velocity field in a large area,but it has a large error and cannot meet the requirements of precise acoustic positioning in the deep sea.Given the above problems,combined with the temporal characteristics of the actual deep-sea sound velocity profile,this study carried out sound velocity field modelling and data analysis.The specific research contents are as follows:(1)The principles and methods of deep-sea sound profile stratification are sorted out,and the temporal model construction method of regional small time scale sound profile is proposed.According to the stratified model of deep-sea sound profile and the statistical characteristics of measured data at different depths,this study proposes a time-varying model construction method of regional small time scale sound profile based on layered Empirical Orthogonal Function(EOF).The accuracy of the layered EOF method is compared and verified with the measured data,the results show that the inversion sound velocity profile is better than the average sound velocity profile,and the root mean square error of fitting is improved by 0.050m/s.Then,by analyzing the time-varying characteristics of the layered EOF first mode coefficient and equivalent sound velocity of the measured data,two models for fitting the time-varying characteristics of the daily period of the sound velocity profile in the test area are proposed,and the fitting precision of the two models is analyzed and compared,and it is found that the fitting precision of all layers and different types of data is improved after adding the linear term.The middle layer improved more than the other layers.Finally,the factors influencing the spatial and temporal variation characteristics of the sound velocity profile were discussed,and its correlation with temperature,tide,ocean currents and the time-varying characteristics of the sound velocity profile was analyzed.It was found that the ocean current direction is approximately perpendicular to the horizontal gradient of sound velocity,and the variation of the first mode coefficient of EOF of the sound velocity profile in the experimental sea area was significantly related to the temperature,and the time-varying characteristics of the extracted sound velocity profile were consistent with the tidal cycle characteristics in the sea trial area.(2)The flow of calculating the horizontal gradient of sound velocity using measured data is summarized,and six calculation strategies of the horizontal gradient of measured sound velocity profile and calculation model of sound velocity disturbance related to vessel position based on measured horizontal gradient of sound velocity are proposed.Considering the time-varying characteristics of equivalent sound velocity in the test area and the correlation of parameters during the calculation of horizontal gradient,six algorithms are proposed to calculate the measured horizontal gradient of sound velocity profile,and the results are compared and analyzed.After obtaining the horizontal gradient of sound velocity,a model for calculating the sound velocity disturbance caused by the horizontal gradient of sound velocity and related to the ship's position is proposed in this study,and the accuracy of the model is compared and verified based on the constant gradient sound ray-tracing method.The results show that for all data sets,the propagation time calculated by the proposed method is closer to the reference true value of underwater positioning than that calculated by the alternative method of the average sound velocity profile.The horizontal gradient of sound velocity calculated by eliminating the time domain variation without adding linear term is better than that calculated directly.The horizontal gradient of sound velocity calculated by using the measured average sound velocity profile in the adjustment solution after eliminating the time domain variation without linear term is better than that calculated by using the measured average sound velocity profile to calculate the ship position sound velocity profile.Based on this model,the modified sound velocity profile information is obtained based on the measured data,and the comparison and verification of the measured sound velocity horizontal gradient with the estimation results of GNSS-A software is realized.Using SVP observation data,the accuracy of horizontal gradient of sound velocity calculated after eliminating the time domain variation of the sound velocity field is the best,and the calculated time domain variation characteristics of horizontal gradient of sound velocity are closest to the inversion value.Then,to evaluate the influence of the sound velocity disturbance caused by the horizontal gradient of sound velocity in the process of underwater acoustic positioning,the sound velocity disturbance caused by the horizontal gradient of sound velocity on the circular navigation line is calculated.Finally,to analyze the measured data and to quantify the ranging error caused by the acoustic velocity error,the slowness model is used to simulate the spatial distribution of acoustic positioning propagation time delay error caused by the horizontal gradient of sound velocity.(3)The sound velocity profile of standard depth interval is obtained through data preprocessing,and the applicable empirical formula for calculating sound velocity based on temperature,salinity and depth is selected.The sound velocity data set of standard depth intervals are obtained by using the data preprocessing method for different sound velocity data.The calculation accuracy of different empirical formulas of sound velocity in the test area is analyzed and compared with the measured data,and the applicable empirical formulas of sound velocity in data processing and analysis are determined.