Study On The Sediment Characteristics Of The Seabed Pockmark In The North Yellow Sea And The Model Test Of Pockmark Forming

Pockmark is a kind of special seabed geomorphology,widely developed in the area around the world of the continental shelf,continental slope.In recent years,due to its environmental significance,pockmark has been becoming one of the hotspots of research in the field of marine geology.Many pockmarks developed on the shallow water shelf seabed at northwest of the North Yellow Sea.Some reserchers have been studied on the pockmarks in this area,however there are some scientific problems still need to be further studied.Two gravity cores sediments sampled from inside and outside of the pockmark in the North Yellow Sea.Multi-parameter core scanning,physical properties,mechanical properties and SEM were carried out to analyse two gravity cores sediments,then the difference between them and the reason were discussed.In this study,a model test on the formation process of pockmark was carried out at the same time,and several factors was analyzed in order to provide supporting for mechanism analysis?Based on the model test studying and previous studies,A genetic model were discussed for the formation of seabed pockmarks.The following conclusions and understanding can be drawn from this study.As for the core scanning parameters,on average,the shear wave velocity of the sediment inside of the pockmark is higher than it outside of the pockmark;the acoustic impedance value of the sediment inside of the pockmark is higher than it outside of the pockmark;the scanning porosity value of the sediment inside of the pockmark is lower than it outside of the pockmark;the scanning density value of the sediment inside of the pockmark is higher than it outside of the pockmark.Those represent that compaction of sediment inside of pockmark is higer than it outside of the pockmark.As for the character of grain size of sediments,the lower part sediment of inside of the pockmark is relatively coarse while upper is fine.However,graine size keeps stable for the sediment outside of the pockmark.As for the physical properties,on average,wet and dry density,specific gravity and plastic index of sediment inside of the pockmark are higher than those outside of the pockmark,while the moisture content,void ratio,liquid limit value and liquid index are lower.As for the mechanical property,on average,the resistance to compression,permeability and shear strength of the sediment inside of the pockmark are higher than those outside of the pockmark.The carbonate content of sediment inside of the pockmark is obviously higher than it outside of the pockmark.As for the microstructure,there are many directional pore channel in sediment inside of the pockmark,and the pore connectivity of sediment inside of the pockmark is better than that outside of the pockmark.There are higher carbonate contention in the sediments inside of the pockmark than outside.Some evidences of gas seepaging were analysed to explain differences of sediments characteristics both inside and outside of the pockmark.A model experimental device for the formation of pockmark was desigened and constructed,and preliminary model tests were carried out.By the pockmark formation model test,the water depth,soil layer thickness,the output gas pressure were found related to the process of gas breakthrough overlying fine sand to form the pockmark.The time required for the gas to break through the fine sand is an exponential increasing relationship with the thickness of the fine sand,and the relationship with the gas pressure is a negative exponential reduction,while it is a linear rising relationship with water depth.The development law of pockmark formation is the tendency of the diameter and depth to be stable after the rapid increase of jet time for fine sand.The seepage and shear failure of the gas breakthrough for fine sand and silty clay were discussed.The pressure threshold formula for gas breakthrough is constructed.The test results verify the accuracy of the theoretical formula.Finally,the theoretical formula is used to calculate the gas pressure threshold value of the two kinds of overburden soil layer.A genetic model were discussed to explain the formation of pockmarks including four steps: firstly,shallow gas accumulate;secondly,gas break into the overlying soil and form the gas transport channel;thirdly,gas escape from the seafloor after broken through the overlying soil;fourthly,the gas takes away the surface sediments and forms a pockmark.