【摘要】 对南海东北部973项目采集的地震测线进行了处理,阐明了恒春海脊处天然气水合物似海底反射(BSR)的特征.不同类型剖面的分析表明,单道地震剖面可以揭示南海东北部地区BSR一定的分布,但地震处理对揭示BSR分布全貌起重要作用.南海东北部的BSR具有世界大陆边缘BSR典型的特征,切穿了沉积层理反射,与海底起伏大体平行,为一强振幅的负极性反射.该BSR特征明显,意味着南海东北部地区存在含天然气水合物沉积.利用甲烷水合物与多组分天然气水合物相平衡曲线,计算了稳定带底界的埋深,并与BSR深度进行了对比分析.无论是甲烷水合物稳定带底界还是多组分天然气水合物稳定带的底界,单一地温梯度计算的结果不可能与BSR深度在整个剖面上对应.可知本区横向上地温梯度变化较大.利用BSR资料估算了地温梯度并求得热流值.计算表明,地温梯度与热流值由西向东,随着离海沟距离的增大、离岛弧距离的减小而减小.BSR计算得到的热流值为28~64 mW/m2,与台湾西南实测热流值的结果基本可以对比.更多还原

【Abstract】 Processing of a recently acquired seismic line in the northeastern South China Sea by Project 973 has been conducted to study the character and the distribution of gas hydrate Bottom-Simulating Reflectors(BSRs) in the Hengchun ridge.Analysis of different-type seismic profiles shows that the distribution of BSRs can be revealed to some extents by single-channel profile in this area,but seismic data processing plays an important role to resolve the full distribution of BSRs in this area.BSR’s in the northeastern South China Sea have the typical characteristics of BSRs on worldwide continental margins: they cross sediment bed reflections,they are generally parallel to the seafloor and the associated reflections have strong amplitude and a negative polarity.The characteristics of BSRs in this area are obvious and the BSRs indicate the occurrence of gas hydrate-bearing sediments in the northeastern South China Sea.The depth of the base of the gas-hydrate stability zone was calculated using the phase stability boundary curve of methane hydrate and gas hydrate with mixture gas composition and compared with the observed BSR depth.If a single gradient geothermal curve is used for the calculation,the base of the stability zone for methane hydrate or gas hydrate with a gas mixture composition does not correspond to the depth of the BSRs observed along the whole seismic profile.The geothermal gradient therefore changes significantly along the profile.The geothermal gradient and heat flow were estimated from the BSR data and the calculations show that the geothermal gradient and heat flow decrease from west to east,with the increase of the distance from the trench and the decrease of the distance to the island arc.The calculated heat flow changes from 28 to 64 mW/m~2,which is basically consistent with the measured heat flow in southwestern offshore Taiwan.更多还原