Study On Geoacoustic Model Of Typical Sediments On The East China Sea Continental Shel

Seafloor shallow sediments,as the interface between the seafloor and seawater,have acoustic characteristics that influence sound ray paths and sound field structures,which is crucial for accurate forecasting of ocean sound field,exploration of new types of seafloor resources,marine engineering construction,and marine disaster prevention and mitigation.The content of the thesis mainly includes three parts.Firstly,it summarizes and analyzes the acoustic and physical-mechanical characteristics of typical seafloor sediments in the East China Sea.It explores the differences in acoustic characteristics and physical-mechanical properties of different types of seafloor shallow sediments,revealing the distribution characteristics of acoustic and physical parameters in the vertical direction.Secondly,it studies the correlation between sound speed and physical-mechanical properties of seafloor sediments in the East China Sea.It establishes sound speed prediction models for seafloor sediments in the East China Sea,including single-parameter models,two-parameter models,and multi-parameter machine learning models.Thirdly,it investigates the dispersion characteristics of acoustic properties of typical sediments in the East China Sea.It analyzes the dependence of sound speed on frequency for different types of sediments and explores the propagation laws of multi-frequency sound waves and energy dissipation mechanisms.The results have further improved the acoustic model of seafloor sediments on the East China Sea continental shelf,providing effective methods and basic data for the study of acoustic characteristics of seafloor sediments in the region.This study is based on the"Research on the Spatial Distribution Patterns of Acoustic Characteristics of Seafloor Sediments in the East China Sea Shelf and the Experimental Study of the 2020 National Natural Science Foundation of China’s Shared Voyage Project(42049902)".During the voyage,conducted in the East China Sea shelf area,30 sampling sites were selected using a gravity corer in three types of sediment areas:clayey sand area(Zone A),fine sand area(Zone B),and clayey silt area(Zone C).Additionally,six sites were selected along the S profile,which is located entirely in the clayey silt area.Columnar samples were obtained from these sites,and laboratory measurements were conducted to determine the acoustic and physical-mechanical properties.Based on the measured data,the acoustic and physical properties of different sediment types and their differences were analyzed.The study revealed the variation patterns of acoustic and physical properties with burial depth in the three types of sediment areas.The results indicate significant differences in the acoustic and physical properties among the different sediment types.The sediment in Zone B have higher sand content and exhibit characteristics of high density,low water content,low porosity,and high sound speed.The sediment in Zone C have higher clay content and exhibit characteristics of low density,high water content,high porosity,and low sound speed.The acoustic and physical properties of The sediment in Zone A fall between those of Zones B and C.The acoustic and physical properties along the S profile are intermediate between the three zones.There is a close relationship between the sound speed and physical-mechanical properties of sediments in each zone.In the vertical direction,the sediment grain composition in Zone A exhibits complex changes,and the sound speed initially decreases and then increases with burial depth.The sediment grain composition in Zone B remains relatively constant with burial depth,while the sound speed gradually increases.In Zone C,the sound speed remains relatively constant with burial depth.Based on the measured data,the analysis of the correlation between sound speed and physical parameters was conducted.Single-parameter and two-parameter regression equations for the prediction of sound speed were established,and the study showed that there is a good correlation between sediment sound speed and physical parameters.The determination coefficient(R~2)of the two-parameter regression equation was generally higher than that of the single-parameter regression equation.Using the XGBoost machine learning method,a sound speed prediction model for seafloor sediments was developed based on five physical parameters:density,water content,porosity ratio,sand content,and average particle size.Sensitivity analysis was performed on each physical parameter,effectively addressing the issues of overfitting or underfitting that can occur with the single model and improving the accuracy of sound speed prediction for seafloor sediments.This study utilized sound speed data(25-250k Hz)and physical property information from samples of three typical sediment types:clayey silt,coarse silt,and silty sand,collected in the research area.The research focused on the sound speed dispersion characteristics of different sediment types.By optimizing the input parameters of four theoretical models(Biot-Stoll,EDFM,GS,and VGS),a sound speed dispersion model suitable for characterizing typical seafloor sediments in the East China Sea shelf was established.The study found that the sound speed of the three sediment types increases with frequency.For silty sand sediments,the Biot-Stoll,EDFM,GS,and VGS models can predict sound speed dispersion accurately.The GS and VGS models,which account for the dual nature of elasticity and fluidity,are suitable for clayey silt sediments.The Biot-Stoll and EDFM models,which focus on porosity,are more applicable to coarse silt sediments.