关于地震电磁信号中隐失电磁波的研究

More and more field observations have recorded seismo-electromagnetic (seismo-EM) signals. Electrokinetic effect is one of the most possible generation mechanisms. In the last decade, some numerical simulation studies have been performed to understand some seismo-EM phenomena on the base of electrokinetic effect. However, the models adopted in previous studies consist of porous media only. In the real case, the media above the ground water level may be able to be approximately treated as solid. Besides, the media below a depth of several kilometers may comprise both solid and porous materials. Therefore, carrying out numerical simulation studies of the seismo-EM wave fields in a model consisting of both solid and porous materials may have great scientific significance for understanding and explanation of the observed seismo-EM signals. Adopting a two-layer model made up of a solid half-space and a porous half-space, the research group of the applicant recently conducted careful simulation study and theoretical analysis of the electromagnetic (EM) waves resulting from seismoelectric conversion at a solid-porous interface. Consequently, existence of the evanescent EM waves was identified. Given that the shallow earth media generally contains ground water and inhomogeneity, the observed seismo-EM signals presumably have already contained the evanescent EM waves. This project plans to carry out further studies on the evanescent EM waves. The decay characteristics, the propagation characteristics accompanying seismic waves, and the effect in the earthquake-inducing EM total fields will be investigated. Numerical simulations will be carried out on multi-layered half-space models consisting of solid and porous materials trying to explain some observed seismo-EM phenomena. Sensitivity study of the evanescent EM waves to the properties of the shallow underground fluid will be performed to understand its potential applications in detection or exploration.

越来越多的野外观测记录到了地震电磁信号，动电效应是最有可能的产生机制之一，近十年来基于动电效应对一些地震电磁现象开展了数值模拟研究，但采用的都是纯孔隙介质模型。而实际情况中，地下水位以上的介质也许能够近似看作固体；此外，几千米深度以下的介质可能既包含固体材料又包含孔隙材料。因此，针对由固体和孔隙材料共同组成的介质模型开展震电波场的数值模拟研究可能对理解和解释实际的地震电磁观测资料具有重要的科学意义。申请人课题组最近对固体–孔隙分界面上的震电转换作了细致的模拟研究和理论分析，发现了隐失电磁波的存在，它应该已经存在于实际观测到的地震电磁信号。本项目拟开展进一步研究，分析隐失电磁波的特征及其在地震引起的电磁总场中所起的作用，针对由固体和孔隙材料共同组成的多层半空间模型开展数值模拟以解释实际观测到的一些地震电磁现象，同时考察隐失电磁波对地下浅层流体性质的敏感程度以了解其在勘探方面的潜在应用前景。

近三十年来，关于地震电磁现象的观测报导越来越多，但由于相应的理论研究较少，目前对地震电磁现象还有很多认识不清的地方，这一现状制约了地震电磁观测资料的实际应用。动电效应作为可能的地震电磁信号产生机制之一，已经被应用于地震电磁学研究，而项目负责人在此前的研究中发现固体-孔隙分界面上的震电转换可以产生隐失电磁波，本项目以此为切入点，对震电隐失电磁波开展了更深入的研究。在考虑实际情况的基础上设计不同的模型，开发相应的程序并开展震电波场的数值模拟研究，同时也开展理论分析。推导出震电隐失电磁波振幅衰减因子的理论公式并验证其正确性，国外学者根据该理论公式预测震电频谱比值随着频率的升高而下降、并且发现天然地震的同震电磁数据确实有此特征，他们以此为基础开展浅层流体性质反演研究。我们考察了频率对震电隐失电磁波可探测性的影响，结果表明低频情况下震电隐失电磁波的振幅有可能远远超过界面辐射电磁波，国外学者的野外震电实验结果恰好印证了这一观点。通过理论分析以及合成数据提出震电信号也能反映Rayleigh波的频散特征，发现地震波和震电信号的频散谱具有很好的互补性，联合二者可以提取包含更宽频率范围和更多高阶模式的频散曲线，在此基础上提出一种新的面波勘探技术，有望进一步提高面波成像结果的准确性，该技术已获得一项国家发明专利的授权。获取2016年Kumamoto地震序列的同震电磁数据并针对其中一个余震进行个案研究，发现不同模型下震电隐失电磁波都对同震电磁信号有重要贡献，也发现在考虑动电效应的基础上引入一种人工散射效应可以合理地解释同震电磁信号的到时特征。利用曲线网格有限差分方法开发了一套带起伏地形二维孔隙介质弹性波模拟算法，未来该算法可以推广到三维情况并模拟动电效应产生的震电信号，从而为研究复杂孔隙介质中的地震电磁现象提供一套新的工具。另外，我们也对动生电磁感应效应产生的地震电磁信号形成一些理论认识。