The Correlation Of Seafloor Sediment Acoustic Properties And Physical Parameters In The Southern South China Sea

Seafloor surface sediment is the interface of sea water and seafloor, and it is an important part of marine acoustic field environment, is also an indispensable content to the marine resources survey research。The relationship between seafloor surface sediment acoustic properties and physical parameters has important application value in the field of marine engineering construction, submarine resources, marine military development and security, et al. This paper offers a summary of a systematic study on the relationship between seafloor surface sediment acoustic properties and physical parameters in the the southern South China Sea.Seafloor sediments of different geographical areas in the southern South China Sea(continental shelf, continental slope, and Okinawa Trough) were gravity cored at 21 locations, and two samples were from the continental shelf, 13 samples were on the continental slope, and six samples were in the Okinawa Trough. Sound velocities of the samples were measured on the deck immediately upon retrieval, and the improved coaxial differential distance measurement method was adopted. Sediment physical parameters, including porosity, wet bulk density, and mean grain size were measured later in the laboratory. According to the actual measurement data, the relationship between sediment sound velocities and physical parameters were statistical analysised, and the single parameter equations of sound velocity- physical parameter were established by least squares regression. Empirical equations from previous studies were applied to predict velocities of sediment samples from the measured physical properties, and it was found that the sound velocities derived from the existing equations did not closely match the measured sound velocities. Possible explanations for the discrepancies between the measured data and those of previous studies were investigated and found that physical properties, sediment types, geographical area, etc. are important factors that influence sound velocity.Based on the core samples collected in different geological tectonic units in the southern South China Sea, the double parameter empirical equations between sediment sound speed and physical properties were developed. According to the double parameter models we established, the influences of physical parameters(porosity, mean grain size, wet bulk density, and clay content) on sediment sound speed were analyzed. The error norm method was introduced, and the sensitivities of each parameter in the double parameter equations are compared using the error norm method. In addition, the most influencing physical parameters with relative order were analyzed. The results show that porosity is the most important physical parameter that influences the sound speed, and the most influential physical parameters on sound speed are as follows: porosity > wet bulk density > clay content > mean grain size.The inversion theory between sediment sound velocity and physical parameters were researched in this paper. Based on foregoing researches, the Gassmann equation was rearranged and utilized to estimate porosity based on the relationship between porosity and compressional wave velocity. The compressional wave velocity was measured on the deck based on the core samples collected in the southern South China Sea, and the porosity was measured in the laboratory. Porosity calculated using Gassmann equation was compared with the porosity measured in laboratory. The results show that Gassmann equation can better predict the seafloor sediment porosity, and it is particularly accurate at shallow water areas. The sensitivity of Gassmann equation was studied by error nom method, and the results show that compressional wave velocity is the most influential parameters for porosity prediction.