Experimental Study On Acoustic Properties Of Seabed Gas Charged Sand Sediments

Shallow gas is one of the most important engineering geological hazards in marine environment.It has a certain degree of distribution in the Bohai Sea,the Yellow Sea,the East and South China Sea,especially in the coastal area of the East China Sea.With the development of marine economy and the sustained growth of energy demand in China,it is inevitable that part of the engineering construction is carried out in the sea with shallow gas.A comprehensive study of the acoustic properties of seabed gassy sediments and the identification technology of gassy sediments have become demands of infrastructure construction,energy and resource exploitation and disaster prevention and mitigation engineering in marine and terrestrial environments.Acoustic detection is the main means of nondestructive detection of seabed sediments.In this paper,acoustic experimental study on the seabed gassy sand sediments of the Hangzhou Bay in the East China Sea is carried out,carrying out CT scanning experiments,obtaining bubble size distribution,selecting the appropriate excitation frequency according to the law of bubble resonance.The Anderson-Hampton model is compared with the experimental results of acoustic response,and an improved sound velocity model is proposed.The main research contents are as follows:First,upgrading and reforming experimental equipment.Bender-extender element testing system and dual pressure chamber system with high precision low range differential pressure sensor were added to GDS stress path triaxial apparatus.Acoustic experiment and preparation of gassy sand sediments is carried out combined with high-pressure vessel for preparing CO2 saturated solutions.Second,CT scanning system is designed for finding out the occurrence state and size distribution of gas bubble in sand sediments.The size distribution of gas bubble in gassy sand sediments is obtained using 225kV-3D-micro-CT by Processing scanned image with VGStudio MAX software.Characteristics of occurrence state and size distribution of gas bubble was described by log-logistic probability density function.At the same time,the size distribution of gas bubbles is predicted with the change of porosity and gas content in gassy sand sediments.Third,the range of bubble resonance frequency is obtained based on the bubble size distribution.The acoustic response of gassy sand sediments is studied under the bubble resonance frequency.The range of exciting frequency is determined in indoor acoustic testing.By selecting the appropriate frequency,the acoustic experiment is carried out to obtain the amplitude and sound velocity parameters of the gassy sand sediments.Finally,the applicability and limitation of the Anderson-Hampton model are mainly introduced.In addition,compared with the experimental data and explained the reasons for differences.Based on the relationship between gas content and compressive wave velocity,an improved sound velocity response model of gassy sand sediment is proposed.According to the parameters of the model,sensitivity analysis is carried out to test the applicability of the model.