基于合成孔径声学深拖调查的海底浅表层流体活动研究——以SAMS DT6000深拖在琼东南海域调查为例

声学深拖作为一个声学设备搭载平台，主要功能是获取高分辨率的声学数据，精细刻画海底地形地貌特征以及浅层剖面结构，对于研究海底浅表层流体活动系统的类型、形成机制和演化模式有着重要作用.本文介绍的合成孔径声学深拖（Synthetic Aperture Sonar Deep-tow）搭载了合成孔径声呐、浅地层剖面仪以及多波束系统等声学设备，相比于传统的侧扫声呐，合成孔径声呐采用小物理孔径基阵通过信号处理虚拟合成大孔径基阵来获得方位向高分辨率，大大提高了测绘速率，同时结合高分辨率的浅地层剖面和多波束背散射数据，可实现海底浅表层特征的三维立体显示.为查明调查区海底浅层流体活动的声学特征，分析天然气水合物相关的流体渗漏活动性与浅层构造之间的关系，我们利用声学深拖对研究区进行了全覆盖的扫测，获得了高分辨率的合成孔径声呐图像、浅地层剖面资料以及多波束背散射数据，平面上识别出多个呈条带状的海底丘状体，火焰状的流体渗漏，新月形的麻坑构造等流体活动地质构造；浅层剖面上可见气体聚集的声学空白段落，凸起的活跃喷口，以及反射杂乱的柱状浑浊带.通过识别流体活动的特征，我们总结了浅层流体活动演化模式具有周期性：游离气体通过高渗透运移通道上升至海底，首先扩散聚集造成局部沉积物体积膨胀形成丘状体；然后受其各种外界因素影响丘状体崩塌而引起气体渗漏；最后流体逸散剥蚀海底松散沉积物而形成麻坑构造；随着流体排出，喷口重新闭合，流体在地层中再次聚集，聚集的气体又将沉积地层上拱，在麻坑底部又可能生成含气丘状体.海底浅表层蕴藏着丰富的地质信息，这对于研究海底复杂的流体活动有着重要意义.

Research and application of synthetic aperture sonar deep-tow in the seafloor fluid flow system: SAMS DT6000 deep-tow in the northern of South China Sea as an example

Acoustic Deep-tow is a platform equipped with different acoustic transducers, and is used to obtain high resolution acoustic data, to describe seafloor topography and shallow profile structure in detail.It plays an important role in studying the type, formation mechanism and evolution mode of seafloor fluid flow system.In this paper, Synthetic Aperture Sonar(SAS) Deep-tow we introduced is integrated with synthetic aperture sonar, chirp sub-bottom profile and multibeam system that just provides backscatter data for the blind area. The high resolution sonar image, and high quality sub-bottom profile are generated by data process may explain the seabed microtopography, and analyze the seafloor fluid flow system, it helps to establish a three dimensional exploration of the fluid flow system of the seafloor. In contrast to the traditional acoustic deep-tow, SAS deep-tow has a higher resolution and a better efficiency in that it uses a small physical aperture to obtain higher along-track resolution by signal processing, and combined with the high resolution of the seabed sub-bottom profile and multibeam backscatter data can realize the three-dimensional display of the seabed shallow surface characteristics. In order to clarify the shallow acoustic features related to seabed fluid flow system, and analyze the relationship between the seepage and the structure, the mosaic image of SAS and the sub-bottom profile are employed, stripe shape of sea mounds, flame shape of gas plumes and new-moon shape of pockmark are identified by the sonar image, in the sub-bottom profile gas blanking, active fluid vent and fluid turbidity are recognized. According to the characteristics of fluid activity recognized, we conclude that the evolution pattern of shallow fluid activity has periodicity: the free gas rises to the seabed through the high permeability migration channel, due to the diffusion and accumulation of gas, causing the volume expansion of local sediments to form the sea mound; Then the gas leakage is caused by the collapse of the sea mound under the influence of various external factors. Finally, the loose sediment near gas leakage are carried away and then form the pockmark structure. As the fluid is discharged, the vent closes and the fluid accumulates again in the formation. The accumulated gas lifts the sedimentary formation up and form a gas mound at the bottom of the pockmark. In the end, seabed provides the more detail information about fluid flow system, it is accessible to pay attention to the shallow seafloor sonar image and sub-bottom profile.