地震波引起华北地区地下流体变化运移及潮汐响应特征研究

This research is a cross subject of seismology and hydrogeology and it has been a subject of intense interest. The mechanism of water level variations induced by seismic waves is complex and uncertain, in addition, our latest studies indicate that shale (low porosity and permeability) will have influence on co-seismic fluid migration and tidal response. According to the above issues, this research will focus on the study of the characteristic of underground-fluid migration and tidal response induced by seismic waves in different lithologic porous rocks, based on the water level data of the groundwater observation stations in North China induced by large earthquakes happened in recent years. Integrated approaches will be employed, including in-situ observation of hydrogeology and seismology, lithologic property and physical property analyses of seepage stratum, and seismic numerical modeling. It will be implemented from three aspects: (1) Analyze the co-seismic change mechanism of physical properties of aquifers based on the computation and analysis of tidal effect of groundwater, and finally to get the relations among water level, volume strain, and aquifer parameters; (2) Analyze the lithology characteristics and the physical properties about the observation aquifers of each well, and explore the possibility of deducing the lithology of the underground medium (low porosity−low permeability shale) based on the characteristics of co-seismic groundwater migration and tidal response; (3) To do the numerical modeling of the propagation of seismic waves in porous rocks. To analyze the characteristic of underground-fluid migration in different lithologic rock medium (sandstone and shale). This study will have significant meanings, and will be essential for the thorough comprehension of the mechanism of earthquake induced groundwater migration, the characteristic of the co-seismic fluid responses in low porosity/ permeability shale, and earthquakes induced by the variations of pore-pressure.

地震地下流体研究是地震学与水文地质的交叉、热点课题。地震波引起水位变化的机理具有复杂性和不确定性，近期研究发现低孔-低渗页岩对同震流体运移及潮汐响应有较大影响。针对上述问题，本项目拟基于近年来发生的大地震引起我国华北地下流体观测台站的水位变化数据，采用实地台站观测、渗流层岩性物性分析与地震数值模拟相结合的方法，从三个方面研究不同岩性多孔岩石中地震波引起流体运移变化及潮汐响应特征：(1) 应用固体潮效应分析含水层物性同震变化机理，确定水位-应变-含水层参数的时间序列关系；(2) 分析各井孔观测含水层的岩性物性特征，探究利用同震流体运移及潮汐响应特征反推地下介质岩性（低孔-低渗页岩）性质的可能性；(3) 开展多孔介质地震波传播数值模拟，分析不同岩性介质中（砂岩与页岩）流体的运移特征。本课题对深入理解地震导致地下流体运移机理、低孔-低渗页岩同震流体响应特征、以及孔隙压变化诱发地震等具有重要意义。

本项目基于近年来发生的大地震引起我国华北地下流体观测台站的水位变化数据，采用实地台站观测、渗流层岩性物性分析与地震数值模拟及实验室三轴围压实验相结合的方法，从三个方面研究不同岩性多孔岩石中地震波引起流体运移变化及潮汐响应特征：(1) 基于流体的潮汐-气压响应分析相邻两井对大地震的不同水力响应模型，进而探究页岩的影响。研究发现页岩的复杂裂隙或各向异性可能会导致观测含水层处于半封闭状态，从而导致垂直向排水的发生，即呈现水平流＋垂直流的混合流动模型，所以宝坻井观测含水层与外界的孔隙压差异较小，导致同震渗透率上升较小甚至没有变化，这些因素是导致该井同震水位变化幅度总是非常微小的原因；(2) 采用基于Biot双相介质方程的不分裂卷积完全匹配层与高精度旋转交错网格有限差分方法精确模拟超声波在砂岩和页岩岩芯中传播,计算了散射衰减，对比了页岩和砂岩的物理特性；进行实验室页岩的三轴围压实验，证实页岩具有明显的各向异性且其非均质尺度在不同方向上存在差异。(3)对左家庄流体台站开展了长时间的流体潮汐-气压响应的计算研究，作为井观测含水层封闭性的探测手段，首次发现远场地震波的振动可以引起远距离范围含水层介质物理性质的永久性改变并导致含水层水平向渗透率的大幅度增加；此外，通过阜新井的流体潮汐响应分析、水位-体应变曲线对比研究探明了远场 (震中距>1000千米) 同震体应变-流体的耦合机理。本课题对深入理解地震导致地下流体运移机理、低孔-低渗、脆性页岩同震流体响应特征、以及孔隙压变化诱发地震等具有重要的理论和实际指导意义。