Bing Deng ^1* Junbing Chen ²

• ¹ East China Normal University, Shanghai, China
• ² Zhejiang Institute of Hydrogeology and Engineering Geology, NinBo, China

Based on high-resolution sub-bottom seismic profilers collected in the coast of Zhoushan Archipelago of the East China Sea, the distribution and characteristics of the shallow gas have been analyzed. The strata in the area were divided into three geologic units, including Holocene fine-grained neritic facies muddy strata ， late-Pleistocene coarse-grained fluvial or lacustrine facies sandy strata and bedrock. The presence of shallow gas were evidenced by various acoustics indicators, including acoustic blanking, enhanced reflections, gas chimneys, bright spots, and small mounds.The shallow gas accumulations are observed through the lower and upperHolocene strata, presumably related to degradation of organic matter buried in the Holocene deposits and sealed by soft muddy deposits. The presence of ultra shallow gas charged sediment sealed by fine grained muddy sediment in coastal area with frequent human disturbance suggested a high potential for sedimen instability. Moreover, The frequent upward gas migration signatures were found in the study area. These gas migration features mainly occur in nearshore areas where tidal disturbance and human activities are both active. We thereby deduced that the deduction of sediment above probably decreased the presure and therefore resulted in increasing upward migration. The presence of active shallow gas within thin soft muddy strata, along with the strong tidal turbulence and human activities indicates the seafloor of study area is vulnerable to sediment erosion and shallow gas is easy release to cause potential danger to the stability of marine engineering foundation. Since the Changjiang sediment supply has been decreasing in the past decades, it would inevitably lead to a corresponding morphological adjustment in study area, such as seafloor erosion. To what extend the coastal erosion should release the buried shallow gas and lead to unpredictable consequences is worthy of our sustained attention.