考虑多相耦合效应的海底含气沉积物中地震波传播特性及影响规律研究

The gassy marine sediments are widely distributed throughout China's offshore, the traditional theory of visco-elastic medium or solid-liquid two-phase medium can not accurately describe this kind of seismic wave problem. In this project, the gassy marine sediment is viewed as a three-phase media, the dynamic equations for three-phase porous media is introduced in the study of seismic wave in the gassy seabed, the theory of marine sediment characteristics is combined, the coupling effect among solid, liquid and gas phases of the sediments is analyzed, the three-phase analytic model for the seismic wave (body waves and surface waves) propagation in the seabed is derived and established, and then the propagation characteristics of seismic wave in gassy submarine sediments are discussed. This three-phase model could degenerate to two-phase or single-phase medium and then be compared with the calculation results of the corresponding classical models to verify its rationality and advantage. The influence of this important geological condition on seismic wave propagating procedure is studied and the effect laws of submarine gas content, interface effect, thickness of sediment, sediment dynamic parameters, seismic wave frequency on the seismic wave propagation and the corresponding seabed earthquake dynamic characteristics are specially analyzed. Based on this work, the effective new method and theoretical basis could be provided for the seismic analysis and ground motion input of China's offshore engineering infrastructures.

我国近海分布着较为广泛的海底含气沉积物，而传统的粘弹性介质理论或固-液双相介质理论无法准确地描述此类场地的地震波传播问题。本项目将海底含气沉积物视为三相多孔介质，尝试将三相介质动力方程引入海底含气场地的地震波传播研究中，结合海底沉积物的力学特性理论，分析沉积物中固-液-气三相介质之间耦合效应，推导并建立海底地震波（包括：体波和面波）传播分析的三相理论模型，进而探究海底含气沉积物中地震波的传播特性。将模型退化到两相或单相状态，与经典模型的计算结果进行对比，验证该模型的合理性和优越性。研究海底浅层气这一重要地质条件对地震波传播过程的影响，重点分析海底含气量、分层界面效应、沉积层厚度、沉积物动力参数、地震波频率等因素对地震传播及其引发海底地震动特性的影响规律。成果有望为我国近海工程中重要设施基础的抗震分析及地震动输入提供有效的新方法和理论依据。

本研究主要基于多孔介质理论，开展了非饱和土场地、含气土海床场地及海床结构物在复杂动力环境下的动力特性。主要的研究目标是期望通过开展相关一系列的理论研究，建立海床在地震或波浪作用下动力响应的分析理论模型，并在基础上进一步构建近海结构物的动力分析数值模型。为确定海底沉积物在地震波作用下的动力响应，首先建立反映沉积物中固、液、气三相质量守恒、动量平衡、能量守恒（熵不等式）等自然界普遍规律的基本方程，结合海底沉积物力学特性（如颗粒之间胶结作用差、骨架模量低）的最新理论，基于热力学基本理论建立封闭的动力控制方程组，再结合非饱和多孔介质的有效应力原理、修正的渗流Darcy定律等相关特性理论的最新研究成果，对建立的波动模型进行必要的修正和完善，使模型参数的物理意义更明确，并将模型参数与实验室中可测的试验参数建立理论或半经验公式。基于修正后的理论模型，可以在特定的初始条件和边界条件下解决本项目所涉及的地震波相关问题。地震波斜入射条件下海床中孔隙压力可视为两部分组成：瞬态振荡孔压（弹性分析）以及为累积残余孔压（塑性分析）。其中，瞬态振荡孔压采用弹性多孔介质理论直接进行求解；累积残余孔压则结合所建立的自由场计算分析模型进行预测；在此基础上得到海床中孔隙水压的时空分布规律。借鉴现有波浪荷载作用下海床液化的判别理论，对地震过程中海床中超孔隙水压力的科学含义进行了解释，进而揭示饱和砂质海床地震液化的物理机制，建议合理的判定准则，最后通过参数分析确定关键影响因素和规律。同时，对比和分析地震诱发海床液化与波浪诱发海床液化、海底地震液化与陆地地震液化在诱发机制和表现特点的异同。