GEOPHYSICAL ANALYSIS OF ABNORMAL SEISMIC (OCEANOGRAPHY) REFLECTION CHARACTERISTICS OF OCEANIC BOTTOM BOUNDARY LAYER

Abstract

Traditional seismic oceanography researches dominantly pay attention to the oceanographic phenomena 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 as 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 seafloor. 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-like 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 indicates 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 of 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 for 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 region “seafloor processes”.