井间电磁波场数值模拟与成像

1、井间电磁波场数值模拟与成像        【中文摘要】井间电磁波探测技术由于其低本钱、高效率的工作特点,以及高分辨率、高探测深度和对地下介质中孔隙流体的敏感性,使其一出现就立即受到了广泛地关注,并在众多专家学者的努力下取得了迅速地发展。目前,井间电磁波探测技术作为地质勘探的一种重要手段已经在油气田、核废物处理、地下水以及固体矿探测等领域成功应用,并逐渐成为地球物理学技术发展的重要前沿。井间电磁波探测研究的主要方向是正反演题目。正演的计算结果可对丈量仪器参数设置进行优化改进,更好地进步仪器的性能,同时,数值模拟的结果

2、决定着井间电磁波成像效果的优劣,是井间电磁波探测资料反演的基础和核心依据。相对于正演而言,目前反演方法的发展明显滞后,近年来仪器制造工艺和数据获取技术方面的巨大进步也对成像方法提出了更高的要求,也为本文提供了研究空间。本文在前人研究的基础上,针对井间电磁成像技术的最新进展,进行了探索性的尝试,旨在进步正反演的效果。在正演方面,主要研究了时域有限差分法(FDTD)的技术性题目,重点论述了吸收边界条件,并推导出了柱坐标系下FDTD的差分格式。而在反演方面,将小波自适应信号处理技术引进到井间电磁波全波形反演中,实现了小波自适应处理与井间电磁波成像的结合,并分析了井间电磁波全波形反演方法的优缺点和发展

3、趋势。本文可以分为以下五个部分:第一章,先容了井间电磁波探测技术的发展历史,具体讨论目前主要的井间电磁波正演数值模拟方法和反演成像方法及其适用范围,并分析了其优缺点和发展趋势。第二章,从电磁场基本方程组出发,具体讨论了井间电磁波场的FDTD数值模拟方法。考虑到井间电磁波探测的地层模型是横向地层,根据其观测方式,本文主要研究柱坐标系下,有耗介质中的FDTD方法及其在井间电磁波探测数值模拟中的应用。并编制了柱坐标系下的三维FDTD程序,通过设置合适的激励源、网格尺寸、空间和时间步上进行计算。为了吸收有损耗介质中的凋落波,数值模拟中采用通用完全匹配层(GPML)作为吸收边界来吸收向外传播的电磁波。第

4、三章,从反演基本理论出发,通过对Born迭代和Frèchet导数的先容,引出了全波形反演方法的基本理论,并给出了其公式的具体推导。在构造满足要求的目标泛函之后,本文采用了共轭梯度法进行最优化求解。针对井间电磁波全波形反演在实际情况中受噪声和源波形估计值的影响较大这一点,本文提出了采用基于二进小波分解的自适应LMS滤波方法来压制噪声,并采用基于小波变换的Kalman滤波自适应猜测反褶积方法来对源波形实现最优估计,以求改善反演效果。第四章,对本文所提出的方法进行了检验。在正演方面,本文对一组人工模型进行了计算,通过与解析解进行比较,检验了其正确性,并扼要分析了井间电磁波传播规律及其影响因

5、素。反演方面,在对井间电磁波电场垂直分量的灵敏度进行了具体讨论之后,本文对一组人工模型进行了全波形反演,并将结果同基于射线方法的层析成像结果进行了对比。第五章,总结了本论文在井间电磁波正反演方面获得的经验和熟悉,并指出了其不足之处,为以后的研究提供了理论依据。本文的创新之处主要有以下两点:其一,针对井间电磁波探测方式和地下介质的复杂情况,编制了柱坐标系下三维FDTD程序,并采用GPML作为吸收边界条件。经人工模型计算检验,证实了其正确性和有效性,并且以为该方法适合于作为反演的正演方法。其二,针对在实际情况中,井间电磁波全波形反演受噪声和源波形估计值影响较大的题目,本文首次将小波自适应滤波和自适

6、应猜测反褶积同全波形反演相结合,提出了自适应全波形反演的概念,并对其进行了具体论述。经人工模型计算检验,证实了该方法十分有效,能大大进步全波形反演的抗噪能力和适应能力。');【Abstract】 Crosshole electromagnetic wave detection technique becomes one of the popular techniques because its low cost, high efficiency, high accuracy, high detecting depth, and sensitive to pore fluid in un

7、derground medium. Nowadays, it is widely used in the area, such as oil and gas field, nuclear waste disposal, groundwater, and solid mine detection, and becomes the leading edge of geophysics.Forward and inversion problems are the two main research directions of Crosshole electromagnetic wave detect

8、ion. Forward can optimize the parameter settings of equipment, and can improve the ability of equipment. Meanwhile, the effect of crosshole electromagnetic tomography relies on the results of forward which is the basis of inversion. Compared to forward, the development of inversion seems slow. The p

9、rogress of technology brought more challenges to inversion. These provided the space of research to this dissertation.On the basis of existing literature, this dissertation analyzed the technical problems of FDTD, on forward aspect. In this part, the dissertation mainly focused on absorbing boundary

10、 conditions, and also deduced the difference scheme of FDTD in cylindrical coordinates. While on inversion, the dissertation brought the technique of wavelet adaptive signal processing to full waveform inversion, and discussed the advantages and disadvantages of it.The dissertation can be divided in

11、to five parts:In chapter one, the dissertation introduced the history of crosshole electromagnetic wave detection technique, the popular numerical simulation and inversion methods, and also discussed their advantages and disadvantages.In chapter two, from the fundamental equations of electromagnetic

12、, the dissertation analyzed the FDTD in loss medium, in cylindrical coordinates. Also, the dissertation provided a 3D-FDTD program, and adopted generalized perfectly matched layer (GPML) as absorbing boundary condition to absorb reflective wave.In chapter three, through the introduction of Born iter

13、ation and Frèchet derivative, the dissertation introduced the basic theories of full waveform inversion method, and also showed the derivation of formulas. After constructing misfit function, the dissertation used conjugate gradient optimized method to solve it. In addition, the dissertation pr

14、oposed an adaptive LMS filter and Kalman filtering deconvolution, based on dyadic wavelet, to de-noising and update source wavelet.In chapter four, the dissertation tested the methods brought above. The dissertation used a series of synthetic models to test FDTD code, compared to analytical results,

15、 and analyzed the diffusive rules and influential factors of crosshole electromagnetic wave. On inversion, the dissertation used synthetic data to test full waveform inversion method, comparing to CT.In chapter five, the dissertation drawled conclusions and the deficiencies of the whole research.The

16、 innovations of the dissertation are as follows:Firstly, wrote a 3D-FDTD program in cylindrical coordinates, and adopted generalized perfectly matched layer (GPML) as absorbing boundary condition. It is proved by synthetic modeling that it is correct and accurate, and is suitable for being the modeling method for inversion.Secondly, because full waveform inversion was suffered a lot from t