与天然气水合物有关的加拿大温哥华岛外海Slipstream海底滑塌构造的力学机制研究

Submarine landslides, associated with the occurrence of natural gas hydrates, have been identified worldwide on continental slope. Being a potential risk for marine environment and engineering projects, it has been being a hot topic of hydrate research. However, quantitative study on the mechanical mechanism of gas hydrate associated submarine landslide is still rare, since it is hard to obtain the key rock-soil mechanical parameters (Young’s modulus, Poisson’s ratio, shear strength, etc.) of hydrate bearing sediment layer, due to not only the special property of hydrate stability (immediately dissociation once leave the stable temperature and/or pressure condition) but also the complexity of existing modes of gas hydrate within the sediment’s pores or fractures..This research proposal will try to tackle this fundamental issue at the Slipstream Slide, which is located at the frontal ridge of northern Cascadia margin, offshore Vancouver Island, Canada. The glide plane of the Slipstream Slide has been proved closely relating to the high concentration layer of gas hydrate at very shallow depth of ~100 m below seafloor. Using the sediment samples collected at the nearby drill hole U1326 of IODP 311, we will systematically measure the physical properties of ultrasonic P- and S-wave velocities and rock-soil mechanics at different gas hydrate saturation with a test equipment in laboratory, and analyze the primary rules of variations of the elastic parameters for gas hydrate bearing sediment as referring to the saturation and existing characteristics of gas hydrate within pore space. Based on the laboratory measuring results and effective medium theory for marine unconsolidated sediment, an appropriate rock physics model could be constructed for the gas hydrate layers of the Slipstream Slide, which in turn will be used to transform the subseafloor P- and S-wave velocity structures from OBS and surface towed seismic data sets collected during SeaJade (Seafloor Earthquake Array – Japan Canada Cascadia Experiment) project to the 3D geological model of the Slipstream Slide. Finally, the finite element method will be applied to the geological model to calculate the stress status around the glide plane at critical stable state and compare them with the shear strength of different gas hydrate saturation and model boundary condition of external slide triggering forces, such as earthquake. Thus, the slide controlling factors, such as gas hydrate layer’s mechanical properties and earthquake magnitude could be quantitatively studied for their contributions. .Since the seafloor stability is an important consideration during marine gas hydrate production, the research results of this project can provide scientific basis of sediment stability assessment for the experimental production project in South China Sea.

与天然气水合物相关的海底滑塌在全球陆坡区被广泛发现。由于其对海洋环境和工程有潜在威胁，一直是水合物研究的热点之一。由于水合物一旦离开其稳定温压条件即分解的特殊性和孔隙赋存形式的复杂性，获得杨氏模量、泊松比、抗剪强度等岩土力学参数是定量研究水合物诱发海底滑塌力学机制的关键。位于北Cascadia增生楔前缘的Slipstream滑塌构造与水合物富集层高度相关。本课题将对研究区内的IODP 1326钻孔水合物层沉积物进行声学和岩土力学参数随水合物饱和度变化的测量，以实验数据为核心建立岩石物理模型，从而可以将滑塌区沉积物的OBS地震速度结构转换为地质模型。最后，用三维有限元方法计算滑塌面临界失稳时的应力条件，从而定量分析水合物层力学性能及地震等不同诱因对滑塌的控制作用和贡献。由于海底地层稳定性是水合物开采重要考量之一，本研究的成果能够为我国南海天然气水合物开发过程中的地层稳定性评估提供科学依据。

由于对气候、环境和海洋工程的潜在威胁，水合物与海底滑塌的关系一直是国际水合物研究的热点问题之一。本项目以与天然气水合物有关的加拿大温哥华岛外海Slipstream海底滑塌构造为研究对象，利用研究区丰富的水合物勘探资料，结合实验室含水合物松散沉积物物性测试和岩石物理建模结果，运用三维有限元实体模型定量分析不同力学条件下的坡体稳定性，以确定水合物控制该滑塌构造的可能机制和过程。.本项目的水合物实验结果表明，沉积物纵横波速度和电阻率与甲烷水合物饱和度的关系呈现明显的阶段性。饱和度<6%~8%时，水合物主要附着于沉积物颗粒表面，对沉积物骨架的增强作用有限，从而声波速度变化较小；此阶段的“离子浓缩”效应盖过“孔隙堵塞”效应，使得沉积物电阻率小幅度降低。之后，逐步有水合物以“桥接”模式胶结沉积物颗粒，沉积物骨架逐渐变“硬”，纵横波速度较快抬升；另一方面，“孔隙堵塞”效应逐渐盖过“离子浓缩”效应，沉积物电阻率开始明显抬升。.根据实验结果，以颗粒介质接触理论、Gassmann方程和Archie定律为理论基础，形成了一套适用于含甲烷水合物沉积物的声电联合岩石物理建模方法。利用该方法先将Slipstream滑坡体的OBS速度结构换算为相应的水合物饱和度，再结合已知的土力学参数随水合物饱和度的变化关系，得到了三维数值模型分析所需的滑塌区海底各层沉积物的力学参数。.构建的Slipstream滑坡三维实体模型较为精细，滑坡面和滑坡前地貌得到重建，内部的三维结构（包括浅部含水合物高速层、BSR界面和断层）被精确定位。对模型进行弹塑性应力应变分析，结果显示坡体在自重条件下稳定，0.2g的地震加速度和60%的水合物分解对坡体稳定性的影响也有限，然而断层处流体汇聚导致的沉积物液化则可以轻易造成塑性失效，是滑坡产生的最可能诱因，而这种流体汇聚可以由该区域的大地震所导致。.通过Slipstream滑坡体的研究，本项目建立了一套具有实用价值的分析水合物地层稳定性的技术流程，可以为我国南海水合物开发的安全评估提供参考。.

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