海底边界层异常(地震海洋学)反射地震特征的地球物理分析

Geophysical analysis of abnormal seismic(oceanography)reflection characteristics of oceanic bottom boundary layer

通过对南海北部与西部大量反射地震剖面海水层部分进行再处理,与以往地震海洋学主要关注海水层内部的反射结构不同,本文重点对海底附近水体的各种复杂反射地震特征进行分类、分析与总结.与传统对海底边界层的定义不同,我们将海底边界附近的水体称之为海底边界层.本文利用传统地震相分析方法,分析海底边界层各种复杂反射地震结构的几何形态、内部反射结构、连续性、振幅以及视频率特征,结合过去相关的地震海洋学研究成果、海底边界层理论与其它各种海底附近作用/过程,不仅对中尺度涡旋、内孤立波和背风波在地震剖面上的反射地震特征进行了归类与分析,并推断最新发现的一些反射地震特征可能揭示的各种海洋作用/过程,例如不同的地震相特征可能反映了海底湍流边界层,海底沉积物再悬浮,天然气渗漏羽状流和麻坑内部异常上升流相关海底界面作用过程.结果分析表明,地震海洋学方法不仅能够对海洋内波、涡旋等物理海洋现象进行研究,同时也能够对海底附近各种复杂海洋作用/过程进行成像,从而拓展了地震海洋学的研究领域,一定程度上也能为过去不能有效对海底边界面发生的各种冷泉热液活动、生物和沉积等作用过程进行现场观测提供新的探测方法和研究视角.

Traditional seismic oceanography researches dominantly pay attention to the oceanographicphenomena within the water column,such as internal wave,eddy,thermohaline fine structure,water mass boundaries,internal tide,thermohaline staircase,lee wave and so on,and could provide extra information quantitatively and qualitatively,compared with physical oceanography method.So far,very few researches try to study the water column near the seafloor,which is a significant boundary layer where water-sediment dynamic interaction,cold seepage,hydrothermal vents,biochemical activities and energy dissipation of many oceanic processes may occur.To be different from previous seismic oceanography researches,this paper mainly focuses on seismic reflections of seawater columns near the seafloor,by reprocessing a large amount of seismic sections acquired in the west and north of the South China Sea.In this paper,conventional seismic facies analysis method is used to analyze,classify and summarize the external geometry,internal configuration,continuity,amplitude and apparent frequency of some complex seismic reflections of seawater column near the oceanic bottom.Combined with the past research results of seismic oceanography,theory of bottom boundary layer and other various processes near the seafloor,this article not only classifies the reflection characteristics of meso-scale eddies,internal solitary wave and Lee wave,but also speculates that some possible processes could result in a few of the newly discovered abnormal seismic reflections.For example,sheet seismic facies unit may reflect turbulent bottom boundary layer;hair reflection configuration can be caused by the sediment resuspension,resulted from the interaction of bottom current and high frequency undulating seafloor,such as sand dunes;plume seismic facies unit indicate the characteristic of seep plume;and broom seismic facies unit could be associated with the upwelling currents and sediment resuspension in pockmarks.Results indicate that seismic oceanography can image not only processes within the ocean water column,such as internal wave and eddy,but also some complex processes near the seafloor,which greatly expands the research field of seismic oceanography and provides a new method and research perspective on the field observation of processes near the seafloor.Here,bottom boundary layer refers to the water column near the seafloor,and we call all the processes of this area＂seafloor processes＂.