考虑流固耦合效应的大容量气枪震源波场谱元模拟关键问题研究

Large-volume airgun sources can stimulate low-frequency seismic waves in an efficient way, thus it can be applied for deep earth exploration in regions with radius of several kilometers. The wave propagation, produced by airgun source, in the fluid-solid coupling media can be treated as near-field fluid-solid coupling wave problem. High efficient scheme of wave simulation should be devised for clear understanding of the generation of wave with different phases and for understanding the character of energy distribution of the wavefield. The scheme will play a important role in inversion of observed waveform in airgun experiment and in the design of source-receiver geometry in the experiment. In this study, we focus to solve two critical problems in design of such scheme, combined with spectral element method and the perfectly matched layer, that are (1) the derivation of a fluid-solid PML suitable for spectral element wave simulation, taking into account of the visco-elastic effect in solid, (2) the optimization of generalized Zener models applied for approximation of the constitutive relationship of visco-elastic solid. Then we combine the scheme with the airgun source’s model, described by the theory of air-gun bubble oscillation, to simulate the wave propagation stimulated by large-volume airgun sources. We will show for factors, such as the volume of water, the shape of the fluid-solid interface and the structure of the underground media, the way of their impact on the generation of wave with different phases and the distribution of wavefield energy. The scheme of near-field fluid-solid coupling wave problem, constructed in this study, should be also valuable for simulation of the submarine ground motion.

大容量气枪震源能有效激发低频地震波，适用于百公里范围的地球内部构造探测。其激发波动在水体-流固界面-地下介质这一复杂系统中的传播问题应归属近场流固耦合波动问题范畴。深入认识气枪激发地震波震相的发育机理及波场能量分布特征对气枪波形观测数据反演及气枪实验和观测方案的设计具有重要意义，需构建高效的波动数值模拟方法以模拟大容量气枪震源的波传播过程。本研究拟结合谱元，完美匹配层建立这一数值模拟方法，围绕方法构建中的若干关键问题展开深入研究：（1），计入固体介质的粘弹性阻尼，构建适用于谱元离散的流固耦合完美匹配层；（2），粘弹性介质时域本构模型的近似及优化。将该方法与基于气泡振荡理论建立的气枪震源模型结合，给出大容量气枪震源的波场模拟。深入分析气枪沉放深度、水体大小、流-固耦合界面，地下介质分布等因素对震相发育机理及波场能量分布特征的影响。本研究对海底地震动模拟亦具有重要参考价值。

大容量气枪震源能有效激发低频地震波，适用于百公里范围的地球内部构造探测。其激发波动在水体-流固界面-地下介质这一复杂系统中的传播问题应归属近场流固耦合波动问题范畴。深入认识气枪激发地震波震相的发育机理需构建考虑流固耦合的数值模拟方法。项目主要开展工作如下：1）实现了粘弹性固体介质时域本构的优化逼近方法。提供了考虑波传播距离，模拟带宽等因素的时域本构优化方案。在此基础上优化了以往粘弹性介质波动谱元模拟方案的计算效率及计算存储。2）构建了二阶理想流体-粘弹性固体近场耦合波动的高精度人工边界。针对以往基于强形式波动方程构建所得高精度人工边界条件-完美匹配层存在的层内运动方程与界面条件不匹配导致计算失稳的问题，提出了直接基于耦合波动方程和界面条件的弱形式波动方程亦建立完美匹配层的方法。应用考虑频移的复坐标延伸技术变换弱形式理想流体-粘弹性固体耦合波动方程建立了对应的完美匹配层。3）形成了近场理想流体-粘弹性固体耦合波动谱元模拟方案。在开源勒让德谱元模拟程序SPECFEM基础上，实现了二阶理想流体-粘弹性固体近场耦合波动的高精度人工边界完美匹配层，优化了粘弹性固体波动计算效率，给出了完整模拟方案。4）分析并模拟了气枪激发地震波场。结实现了基于开口系统的气枪子波模拟，综合考虑气体释放率、枪体体积等多种因素影响，构建新的单枪子波及气枪阵列子波模拟模型。建立了包含气枪震源，水体，流固界面以及地下介质构造的谱元分析模型。结合福建省地下介质模型，水库基底形状分析了气枪激发波场传播规律。结果表明：固定气枪位置，相比浅\窄水库，水库跨度、深度、水库向陆地延伸的坡度的增加会导致激发波场幅值的增加。对深大水库，激发波场幅值随增加气枪沉放深度的增加而增加。二维模型无法反映观测记录中呈现的S波震相强于P波震相的现象，这一特征只能基于三维模型模拟得到。