BASIC CHARACTERISTICS OF SUBMARINE SHALLOW GAS IN LINGDINGYANG FIRTH
伶仃洋海底浅层气的基本特征*

hallow gas mostly distributes in the areas of estuary and continental shelf and is a kind of potential geo-hazards because it can influence the physical-mechanical property of the sediments.It is also detected in the Pearl River estuary,a well developed area with a wealth of engineering underwater constructions,such as pipe lines,bridges,and cables,which might be threatened by shallow gas.Guangzhou Marine Geological Survey(GMGS) carried out a comprehensive project of marine geo-environment & geo-hazards investigation in the Pearl River estuary between 2003 and 2005,and shallow gas of Lingdingyang Firth was investigated by Sub-Bottom Profiler system(SBP),Single-Channel Seismic system(SCS),Side Scan Sonar system(SSS),Underwater Undulation Towed Vehicle system(U-TOW),and Drilling Core.Based on these data and results of soil sample test,we suggest that the blurry reflected areas by inversing represent the areas with shallow gas.Shallow gas in Lingdingyang Firth locates between 8.2m and 12.3m under seafloor and includes CO_2 and CH_(4).The SBP and SCS data,lithology,as well as soil sample testing have shown that the geotechnical properties of the gas strata are different from other areas.The natural moisture content,natural void ratio,and compression coefficient of the gas strata are larger than other areas,but the natural density is smaller,which means that the strata are swelled by shallow gas with their framework destroyed.The total area of shallow gas is quite large,about 900km~2,distributing in several places.As the density of shallow gas is low and no escaping has been detected which was inferred by no pockmarks indicated by SSS and no CH_(4) detected by U-TOW,exploitation is not likely to present.However,soil mass deformation which might be caused by future gas escapes should be considered for marine engineering.