海底天然气渗漏的地球物理特征及识别方法

Geophysical features and identification of natural gas seepage in marine environment

海底天然气渗漏是在海洋环境中广泛分布的自然现象.海底天然气渗漏可以指示沉积层中的烃类聚集带,渗漏出的大量气体(主要是甲烷)可能影响全球的气候变化,此外,与海底渗漏相关的浅层气改变了海底沉积物的土工性质,可能对海底工程构成威胁.因此海底渗漏研究意义重大.海底天然气渗漏不仅影响海底沉积物的物理性质,而且还极大地改变海底地形地貌,它能在海底形成麻坑、泥火山、冷泉碳酸盐岩以及化学自养生物群落等现象.在海底渗漏发生的地方,地形地貌特征可以在海洋测深和逆向散射数据上得到反映;沉积物的声学特征可以在地震剖面上得到反映,如形成声混浊、声空白、亮点、多次波、速度下拉等;有些渗漏在海面形成油渍膜,油渍膜可以在合成孔径雷达图像上得到反映.根据海底渗漏的上述地球物理特征,可以识别出可能渗漏区域,海底渗漏的证实需要用到海底观测和取样分析资料.

Natural gas seepage is a widely distributing phenomenon in marine environment. Gas seepage may indicate the accumulation of hydrocarbon in sediments, and the large mount of venting gas （mainly methane） may impact the global climate change. Additionally, the seepage-related shallow gas accumulation, which can significantly alter the geotechnical properties and behavior of seafloor sediments, may threaten marine engineering. So it is important to study the natural gas seepage in marine environment. Marine gas seepage not only influences the physical properties of the seafloor sediments, but also significantly alters the geomorphology of the seafloor, such as the occurrence of pockmarks, mud volcanoes, authigenic carbonate and cold seep communities on seafloor. The latter changes can be mapped with bathymetry and backscatter data, and the former can be reflected on seismic profiles, such as the occurrence of acoustic turbidity, acoustic wipeout, bright spot, multiple, and velocity pull^down. Some seepage can form oil slicks on the sea surface, which can be imaged by SAR. Based on these gas seep-related geophysical features, the preliminary area of gas seeping may be identified. The verification of marine gas seeping needs ground truth data, such as the seafloor observation and core sampling.