Study On De-noising Of Underwater Sound Signal And Cross-correlation Analysis Based Sound Speed Measurement

Underwater detecting and monitoring of ocean is an important means to protect the marine environment and ensure the rights and interests of the nation.The buoy is one of the important equipment for the purpose of detecting and monitoring of ocean.Therefore,the analysis of underwater sound signal of buoy has a significant impact on the theoretical and engineering applications.For the acoustic measurement system of the buoy,this thesis focuses on the research in two aspects.One is the analysis and the de-noising of the underwater sound signal measured by buoy,the other one is the measurement and estimation of sound speed in sea water by using underwater sound signal measured by buoy.The acoustic field in oceanic environment is very complicated.There are many different noises come from the surface of water and underwater.Therefore,when we use the acoustic measurement system on buoy to detect and monitor the marine environment,the noises will be added to the total signals collected by buoy.It could have a negative impact on the precision of the detecting and monitoring.This thesis studies on the analysis of noise components of the originally measured signals and the separation and elimination of the sound of ocean waves from the measured signals are implemented.The traditional method for sound speed measurement in sea water is to use CTD instruments on buoy to measure the salinity,temperature and depth of the target sea area.Then the sound speed in sea water is obtained through the empirical formula of sound speed.This method relies on many sensors and accurate empirical formula of sound speed.This thesis proposes an innovative measurement method of sound speed in vertical direction based on cross-correlation analysis theory for the signals collected by hydrophone array on buoy.The vertical sound speed in sea water is estimated by calculating the time delays of mechanical impulse noise contained in the measured signals.The main work of this thesis is summarized as follows:The first chapter introduces the background,purpose and significance of this research.The development of the buoy acoustic measurement,the technology of the sound speed measurement in sea water,and the engineering application of cross-correlation method are reviewed in this part.The second chapter presents the processing of underwater sound signal of buoy.The noise components of the measured signals are recognized by the playback function of ArtemiS 12.0.Next,the analysis of the characteristic frequencies is done for each of noise components.A digital filter is designed for eliminating the sound of ocean waves from the measured signal at the characteristic frequencies of each of noise components.The third chapter introduces the basic knowledge of the correlation analysis and physical system.Then the mathematical model of estimating sound speed in sea water is established based on the cross-correlation analysis method.In the fourth chapter,the theoretical model of sound speed measurement in sea water is verified by the test in the sea.In tests the buoy developed by a company is placed in a arranged sea area.At last,the sound speeds of sea water at different depths are successfully estimated by calculating the cross-correlation function of mechanical noise component contained in the measured signal.The last chapter summarizes the main research of this thesis.The conclusions and contributions are stated.Also some comments and outlooks are given for the future research about the processing of underwater sound signal and the sound speed measurement in sea water.