Characteristics of Sea Floor Shallow Gas in the Eastern Part of Zhoushan Dongji Island

Sea floor shallow gas detection has been conducted by using acoustic profiler in the eastern area of Zhoushan Dongji Island.It is found that there are obvious features of the shallow gas in this zone:The first,there are cloudy reflection phenomena in the sea water;the second,it is revealed by side scan sonar image that there are three offshore concaves in the sea bed;the third,there are acoustic reflection anomalies in acoustic reflection profile of shallow strata,mainly for acoustic curtains and acoustic blanket.The result of CPT shows that there is dissolved methane in this zone.It is concluded through the analysis that cloudy reflection phenomena were formed by the shallow gas.Offshore concaves were the result of the uneven subsidence of submarine sediments,which were caused by the shallow gas.