参量阵浅地层剖面仪在海底羽状流探测中的应用--以ATLAS P70在马克兰海域调查为例

海底渗漏的羽状流是沉积层赋存天然气水合物的重要证据之一,基于非线性水声学原理的参量阵浅地层剖面仪作为海洋探测的重要设备,对于获得羽状流在水体中的物性特征和渗漏点的浅地层信息有着重大意义.本文根据ATLAS P70浅地层剖面仪在马克兰海域调查中得到的浅地层剖面数据,结合多道地震数据、多波束数据以及地质样品等资料,刻画了研究区内羽状流形态特征,分析了羽状流区海底地层流体运移的通道以及近海底微地形地貌特征.通过研究发现在羽状流区伴随泥火山喷发,自生碳酸盐岩发育,剥蚀海底松散沉积物形成大小不一的麻坑,滋生生物群落等特征.反映在浅剖初始高频(20 kHz)数据界面上羽状流表现为柱状浑浊反射异常,形态呈火焰状,高度由80 m到1500 m不等;对应在次级低频(4 kHz)信号界面可以清晰显示流体渗漏的浅地层结构特征,从中不仅可以识别出流体的运移通道,如泥火山和管状通道等,而且揭示了流体逸散的残留地貌,如麻坑构造和海底滑坡等.本文依托参量阵浅地层剖面数据,对巴基斯坦马克兰海域羽状流有了较全面的认识,为天然气水合物的研究垫定了基础.

Application of parametric sub-bottom profile in gas plumes detection:ATLAS P70 in Makran area as an example

Methane-bearing gas plumes escaping from the seafloor as a common geological phenomenon is one of the important geophysical evidences of gas hydrate.Generally,in the effect of the temperature and press of stratum in the ocean,especially under the background of changing geological structure,gas hydrate is very susceptible to gasify,and then leak out by the effective migration path in form of gas plume.As a parametric acoustic detection device,the echosounder ATLAS P70 was used for detecting bubbles in the water column that causes strong backscatter in the echographs(gas plume).ATLAS P70 is one of the most important technique in ocean exploration which could not only provide visualized datum for sea bottom geological survey,but also identify the characteristics of the plume in the water where gas seeps,which are of great significance for the research on the fluid geology.According to the sub-bottom profile P70 acquired,multichannel seismic data,multibeam data by EM302 and carbonate encrust by TV-Grab in Makran area,lots of gas plumes has been confirmed,along with the fluid escape structures,the migration path and the profile reflection characteristics have been described,attributes and features associated with submarine plume in the water column near the seafloor were analyzed.A seismic section across the site shows a BSR suggesting the presence of gas hydrate,as well as the spot of gas chimney and diaper above the BSR indicating free gas.The gas seeps are usually associated with mud volcano eruption,pockmark formation,the methane derived authigenic carbonate and a mount of microorganism and molluscs accumulation.The gas or fluids which seep from the seafloor to seawater are columnar turbidity reflex,the range of height is from 80 metres to1500 metres,reflected in the P70 profile on primary high frequency(20 kHz)data,corresponding to the secondary low frequency(4 kHz)we can obtain the submarine structure where gas seep,which reveal the migration path of fluid,such as mud volcano and gas pipe,which reveal the residual structure during the seepage,such as pockmark and seabed slide.Mud volcano stands for the overpressure fluid activity,the gas pipe is related to the dissolution of carbonate,Gas blanking was observed in sub-bottom profile(SLF=4 kHz)through the sites,which indicates the dissolving gas from gas hydrate in storage of stratum.Finally,parametric sub-bottom profile provides the more detail information about gas plume,it is accessible to recognize and explore the gas hydrate in the future.