【作者】 蔡军涛；

【导师】 赵国泽；

【作者基本信息】 中国地震局地质研究所 ， 固体地球物理学， 2009， 博士

【摘要】 大地电磁研究和应用中,面临的实际地质结构多数是三维的,但是当前三维反演技术还不成熟,仍需要利用二维模型进行近似反演解释。在进行二维反演解释时,如何分析认识三维局部畸变的性质,最大限度地克服三维局部畸变的影响,获得尽可能可信的地质解释结果,如何认识三维模型的二维近似程度,如何利用观测的TE模式和TM模式的视电阻率和相位资料等问题,是受到普遍关注的科学问题和实际难题,也成为当前影响大地电磁向前发展的关键问题之一。本论文正是针对上面存在的问题,采用三维数值模拟方法模拟大地电磁局部畸变,在此基础上,研究局部畸变的影响特征,提出新的畸变校正方法和新的电性结构维性判定参数,并对三维数据的二维近似反演从数据差异性和反演结果合理性的角度做了详细的研究,最后通过对实测资料的应用检验本文的研究成果。论文主要研究内容和取得的成果概述如下:1.大地电磁局部畸变基础理论及研究状况归纳总结从大地电磁局部畸变的电磁场控制方程出发,推导了异常电场和异常磁场的积分表达式。论述了局部畸变模型成立的物理假设,并详细推导了阻抗张量畸变和倾子矢量畸变的计算表达式。介绍了局部畸变的传统分类中电流型畸变和感应型畸变的概念,它们对二维TE和TM模式视电阻率曲线的影响,最后简单分析了近地表三维局部畸变影响的一些特点。介绍了大地电磁构造维性分析中常用的阻抗张量基本旋转不变量、WAL旋转不变量以及基于这些不变量的构造维性判定方法、B-Q构造维性判定参数,指出应用这些不变量分析时应注意的问题。详细介绍了目前已有的常用畸变校正方法,对与本论文关系比较密切的算法如Swift、Bahr、GB方法做了详细的推导说明,并指出其优点和不足之处,为本文提出新的畸变校正方法提供了研究的出发点。另外,对电、磁场全畸变公式中的错误也作了修正。2.大地电磁局部畸变特征的三维模拟分析在第一部分理论研究的基础上,设计典型的局部畸变模型,采用改进的交错网格有限差分法进行了三维局部畸变的数值模拟,对计算结果进行了定性的分析,获得一些新的认识。研究发现TM模式视电阻率曲线在穿越低阻异常体界面时,曲线会先上移然后再下移,而在穿越高阻异常体界面时,曲线会先下移再上移。这与电性分界面处积累面电荷产生的二次电场有关。当异常体为高阻时,在其表面积累的面电荷产生的二次电场,在异常体上方其与一次电场方向相同,则观测总场是增大的。在异常体的两端边界附近,二次电场与一次电场的方向是相反的,因此导致观测的总场是减小的;当异常体为良导时,在异常体上方二次电场与一次电场的方向相反,则观测总场是减小的。在异常体的两端边界附近,二次电场与一次电场是同向的,观测总场是增大的。区域二维构造走向为南北方向时,在YX模式视电阻率平面图中三维低阻异常体在南北方向被拉伸,在东西边界处视电阻率分别存在一个增大的区域。而XY模式视电阻率平面图中三维低阻体异常体在东西方向被拉伸,在南北边界处存在视电阻率增大区域。在XY模式相位平面图中,三维低阻异常体在东西方向被拉长,在南北边界处存在相位值减小区。在YX模式相位平面图中,三维低阻异常体在南北方向被拉长,在东西边界处存在相位值减小区。该现象与二维TM模式曲线在穿越电性分界面时的规律是一致的,当测线沿东西方向展布时,此时的三维响应的YX模式对应二维情况下的TM模式。而当测线沿南北方向展布时,此时三维响应XY模式对应二维情况下的TM模式。低阻小异常体对二维区域响应的畸变比高阻小异常体要严重的多,高阻体对区域响应TM模式的畸变要稍大于TE模式,而低阻体则是对TE模式的畸变大于TM模式。同一极化模式,相位曲线相比视电阻率曲线而言,受畸变的影响要小。位于小异常体中心上方测点的三维畸变响应与对应二维区域响应的差异比较大,但可以等效于其他的二维模型响应,这种特殊畸变在资料解释时要特别注意。3.大地电磁局部畸变校正新方法研究介绍了国外研究者新近提出的得到广泛应用的相位张量,并在国内首次将该方法应用于理论数据和实测资料的解释。提出了一种新的单点单频畸变校正方法(SSSF)和单点多频校正方法(SSMF),推导了其理论计算公式,在此研究的基础上提出新的构造维性判定参数,编制了相应的局部畸变分析软件。首先采用合成理论数据验证了新方法的正确性,然后采用Swift、Bahr、GB、相位张量、WAL、SSSF和SSMF方法对三维局部畸变模拟的结果进行了定量的分析,最后将新方法应用于实测资料的解释。研究结果表明SSSF和SSMF法的计算结果与其他张量分解方法基本一致或更好,其提供的构造维性参数和畸变因子为大地电磁资料的分析提供了有力的补充。另外根据实际应用中存在的问题,修正了WAL方法维性判定准则。4.三维畸变数据的二维反演中极化模式的选择针对三维和二维模型响应数据的差异、如何认识二维模式的近似程度,如何利用观测的TE模式和TM模式的视电阻率和相位资料等问题,对三维模型数据的二维近似反演问题的研究,主要从三维模型和二维模型响应差异性和二维反演结果合理性两个方面进行了研究。研究结果表明:在三维模型条件下,利用二维模型进行反演时,TE模式对模型的二维的近似程度要求远远高于TM模式;当三维结构影响较明显时,仅利用TM模式数据进行二维反演比仅利用TE模式或利用TE+TM模式联合反演都更合理,反演结果中的虚假结构明显减少;对于TM模式,相位受三维畸变影响较小,视电阻率较大,所以反演中可适当加大相位的权;对于实测数据的二维反演,应优先考虑采用TM模式数据进行二维反演,其次是TM+TE模式,一般不要单独采用TE模式。本章对上述情况提出了物理分析的依据。5.论文研究成果在实测资料中的综合应用将论文研究成果综合应用于青藏高原东边缘带的美姑-绥江剖面大地电磁实测资料的解释中,对区域构造维性采用相位张量、SSSF方法和WAL方法进行综合判定,采用SSSF方法确定区域主轴方位角,局部畸变校正选用了论文新提出的SSMF方法。二维反演采用TM模式数据进行反演,在反演中还采用了多项新技术,其一是利用测点中心网格的自动生成技术构建带地形的二维反演初始模型,其二是正则化因子τ的分段取值。采用复杂模型大地电磁有效探测深度技术计算了二维反演结果的有效趋肤深度,同时参考相位张量不变量参数的结果,综合评价了反演结果的可靠性。取得了比较满意的反演结果,为研究区地电模型的建立和地质解释奠定了坚实的基础。更多还原

【Abstract】 In MT surveys, the actual structure of the Earth is often three-dimensional (3D). However the current state-of-the-art for MT data interpretation is two-dimensional (2D) inversion and 3D inversion will become available in the near future. When interpretation of 3D data with 2-D techniques, some problems are still exist, such as how to analyze the features of 3D galvanic distortion, how to overcome the influences of 3D local distortion as much as possible and get more reliable geological results, how to understand 2D approximate interpretation of 3D models, and how to make best use of apparent resistivity and phase of TE mode and TM mode. These are the focus of concern scientific problems and the key factors which delay the developing of MT. This thesis focuses on the problems in the above paragraph. MT responses of local distortion are computed using a 3D finite-difference on a staggered grid code. The effect of local distortion has been studied. New correction algorithm and new parameters for determination of geoelectric dimensionality are brought out. The study about 2D approximate inversion versus 3D data is done. The research is divided into two parts: one is the difference between 3D and 2D responses; another is validity of 2D inversion results. At last the research results of this thesis are applied to the interpretation of filed data. The conclusions of this thesis are summarized below.1. Basic theory and summary of research situation of galvanic distortion in MTBased on the governing equation for the EM field of local distortion, the integral expression of anomalous electric and magnetic field are deduced. The physics hypothesis for local distortion model is discussed and the expression of impedance distortion tensor and vertical magnetic distortion vector are derived from integral equation in detail. Galvanic distortion and induction distortion in classic classification of local distortion and their effect on apparent resistivity and phase curves of TE mode and TM mode are illustrated. Some features of local distortion caused by small-scale surface inhomogeneous are briefly discussed. Basic invariants of impedance tensor and WAL rotational invariants used in geoelectric dimensionality analysis are introduced. Some methods and B-Q parameters for determination of geoelectric dimensionality based on these invariants are presented, and some issues needing attention using these invariants are indicated. Several approaches close with this thesis such as Swift, Bahr, GB methods are reviewed in detail. The merits and shortcoming of these methods are point out. These work provide a research beginning for developing novel distortion correction algorithm in this thesis. The bug in formula describing distortion of electric and magnetic fields is fixed.2. Three-dimensional modeling and analysis of local distortion in MTBased on theory in the first part, several 3D local distortion models are designed and model responses are computed using a improved finite-difference on a staggered grid code. Qualitative analysis for model responses is done and some new insights are obtained. When apparent resistivity curve of TM mode across small conductive surface body, the curve will upward shift at first and then downward shift. While across resistive body, the curve will downward shift at first and then upward shift.. This phenomenon is associated with secondary electric field caused by the accumulative surface electric charges at the boundaries of the body. When small body is resistive or conductive, the polarity of the charges results in a secondary field additive to the primary field or opposing the primary fields. Upon vectorial addition of the primary field and secondary field, one can estimate the sense of distortion in the resulting total electric fields. In the resistive body, the total field is increased directly over the body, it is decreased off the ends, and an increase occurs along the sides of the body. With a conductive body, the total field is reduced directly over the body, it is enhanced off the ends of the body, and it is diminished along the sides.When the strike direction of 2D regional structure is NS direction, in the plane map of apparent resistivity of YX mode, 3D conductive body is elongated in NS direction, two zones with the values increasing are located respectively at east and west boundary. While in the plane map of apparent resistivity of XY mode, 3D conductive body is elongated in EW direction, two zones with the values increasing are located respectively at north and south boundary. For phase of XY mode, 3D conductive body is elongated in EW direction; two zones with the values decreasing are located respectively at north and south boundary. For phase of YX mode, 3D conductive body is elongated in NS direction; two zones with the values decreasing are located respectively at east and west boundary. This phenomenon is consistent with the case that curves of 2D TM mode across the electric boundaries. When the profile runs along EW direction, YX mode response in 3D corresponds to TM mode response in 2D. If the profile runs along NS direction, XY mode response in 3D corresponds to TM mode response in 2D.The distortion on 2D regional response caused by small conductive inhomogeneous are more severe than that of resistive inhomogeneous. The distortion on TM mode response is slightly larger than that on TE mode caused by resistive body. While for conductive body, the distortion on TE mode response is larger than that of TM mode. In the same mode the distortion on phase is less than that on apparent resistivity.At the position located directly over the small inhomogeneous, the difference between 3D distortion response and 2D corresponding regional response is much large. But this 3D distortion response can be equivalent to other 2D response. It is more important to pay more attention to this special distortion in MT interpretation.3. Research of new correction method for local distortion in MTPhase tensor approach put forward by overseas geophysicist is introduced and it is the first time applied to the interpretation of synthetic data and field data at home. New single site, single frequency correction algorithm (SSSF) and single site, multi-frequency algorithm (SSMF) are brought out. The formula for them are present in detail. Based on these research, new parameters for determination of geoelectric dimensionality are defined and a program to perform MT distortion analysis is developed. Firstly, synthetic data is used to check the validity of new methods. Secondly, quantitative analysis on 3D local distortion response is done using Swift, Bahr, GB, phase tensor, WAL, SSSF and SSMF approach. Finally, new methods are applied to field data. The research results manifest that the application of new methods are comparable to or even superior to other decomposition methods. The parameters for determination of geoelectric dimensionality and distortion factors provided by new methods are important complement to the analysis of MT data. According to problem occurred in application for WAL method, WAL invariants criteria is corrected.4. The choice of polarization mode in 2D inversion versus 3D MT distortion dataIn order to understand the difference between 3D response and 2D response, answer the questions how to understand 2D approximate interpretation of 3D models, and how to make best use of apparent resistivity and phase of TE mode and TM mode. the study about 2D approximate inversion versus 3D data is done. The research is divided into two parts: one is the difference between 3D response and 2D response, another is validity of 2D inversion results.The results manifest that in the 3D scenario the demand for approximate 2D structure in TE mode is much stricter than in TM mode. When 3D effect is relatively severe, by applying 2D inversion to 3D data, the results show that the inversion model using only TM mode data is more validity, and the phantom structures are less than using TE mode or TE+TM mode. In TM mode, the distortion on phase is less than apparent resistivity, so the weight of phase in inversion can be increased. For 2D inversion of field data, TM mode is first choice, next is TE+TM, TE mode is not proposed. The physics analysis basis is provided in this chapter.5. Application of research results in filed dataThe research results of this thesis are applied to the interpretation of Megu-Suijiang field data in eastern margin of Tibetan plateau. The dimensionality analysis is carried out by phase tensor, SSSF method and WAL invariants criteria. The strike direction of regional structure is determined by SSSF approach. SSMF method is used in local distortion correction. TM mode data is used in 2D inversion. Some new approaches are adopted in inversion, one is Site-Center Mesh (SCM) which used in constructing initial model, another is the regularization parametersτwhich determined through segmental variation. The validity of inversion model is checked by using effective skin depth of complex structure method and invariants of phase tensor. A good inversion results is obtained and it has laid a solid foundation for the construction of electric structure and geological interpretation.更多还原