南海北部陆坡白云深水区海底斜坡失稳的发育特征及主控因素研究

Submarine slope failure areas are widely distributed on the north slope of the South China Sea, this study focuses on Baiyun deep-water area. Based on the high accuracy multi-beam data and 2D/3D seismic reflection data, we will build submarine digital terrain model and carry out geological and seismic comprehensive interpretation. Then the topographical features of the seabed and seismic identification features of the strata will be analyzed. By the fine dipiction of the development characteristics of the slope failure area, we can confirm the slope failure time, internal structure and the spatial extension. And on this basis, the sequence stratigraphic framework will be established by recognition and tracking of the key sequence boundaries of seismic data. According to the geological influence factors of submarine slope failure, we will build the preliminary geologic mechanics model. With finite element method, the reappearance of the slope deformation process will be displayed and the effect of the slope stability with different control factors (such as the water depth, slope gradient, gas hydrate zone thickness and its dissociation quantity) will be revealed and the stability coefficient of the submarine slope will be calculated by the Bishop's Method. Combined with the analyses of tectonic and sedimentary evolution of the study area, we will make comprehensive analysis of the dominant factors of the slope failure. The results of this study can provide theoretical basis for subsequent geo-hazard risk assessment and avoidance of geological disasters.

海底斜坡失稳在南海北部陆坡广泛分布，本项目以南海北部陆坡白云深水区为研究区域，首先利用近年来在研究区获取的高精度海底多波束测深资料和二维、三维高分辨率地震反射资料，构建高分辨率海底数字地形模型（DTM）结合地质-地震资料综合解释，分析研究区的地形地貌特征和地震相特征。通过对斜坡失稳发育特征的精细刻画，阐明其形成时间、内部结构和空间展布特征。在此基础上，重点对地震资料关键反射界面进行标定和追踪，建立研究区层序地层格架，并根据海底斜坡失稳的地质影响因素构建合理的初始地质力学模型。利用有限差分数值模拟方法再现斜坡失稳的演化过程，揭示水深、海底坡度、水合物带厚度和水合物分解量变化对海底斜坡稳定性的影响，并采用Bishop方法计算海底斜坡的稳定系数。结合研究区海底斜坡的构造及沉积演化背景，综合分析海底斜坡失稳的主控因素。为下一步开展地质灾害风险评价和规避灾害事故提供理论依据。

项目利用多波束、多道地震、浅地层剖面及沉积物样品数据，结合测井和岩芯数据，构建了研究区三维海底数字地形模型（DTM）和浅地层精细层序地层格架,研究南海北部深水区海底斜坡失稳的发育特征和控制因素。在南海北部和西部陆缘识别了4套规模较大结构典型的海底斜坡失稳分布区。其中，白云深水区发育了一套面积逾10,000km2的大型海底斜坡失稳。.为了进一步探讨海底斜坡失稳与天然气水合物分解的相关性，首先对水合物的分解机理及其对海底沉积地层的强度产生的影响进行了分析。利用研究区底层剖面和沉积物物性参数建立了研究区海底地质力学模型，采用Bishop法对海底斜坡是否稳定作出评价。研究结果表明，白云凹陷北侧的海底峡谷给南侧大型海底斜坡失稳的发育提供了充足的物源保障，区内水合物的分解对海底斜坡失稳的发育具有诱发作用。在敏感性影响因素的定量研究方面，通过数值模拟的手段，首先对水合物分解量控制斜坡失稳进行了模拟，结果显示：随着水合物分解量的增加，斜坡的稳定性逐渐减弱，当水合物的分解达到一定数量时，陆坡沉积物便会完全失稳。此外，针对水合物分解量为10%，30%和100%的情况下，进行了海水水深，陆坡坡角以及水合物带厚度三个因素对海底失稳的敏感性分析。结果显示，海水水深的增加能有效的抑制海底斜坡失稳的发育，斜坡坡角和水合物带厚度的增加都是失稳发育的启动因素。通过项目研究，我们认为研究区内水合物的分解对海底斜坡失稳的发育具有诱发作用。