三维频率域可控源电磁法各向异性正反演研究

发布时间：2018-03-19 19:45

本文选题：可控源电磁法　切入点：各向异性　出处：《吉林大学》2017年硕士论文　论文类型：学位论文

【摘要】：可控源电磁法是一种在地球物理勘探中常用的人工源低频电磁勘探方法。通过电偶极源或长直导线源发射电磁波并接收电磁总场响应,进而获得地表或海洋环境下的电性信息。在过去,出于缓解反演多解性和节约计算成本的考虑,大多数国内外三维可控源电磁法正反演均通过假设地下介质的电各向同性来实现模拟计算。然而近年来的研究表示,在很多情况下,地下沉积层和晶粒排列方式会表现出很强的电各向异性。此时如果依旧采用传统的各向同性正反演方法,反演结果会非常容易受到观测响应中原本包含的各向异性信息的影响,并在原异常周围产生不准确甚至错误的各向同性异常体。针对这个问题,本文的研究主要集中在开发出高效、稳定的三维各向异性可控源正反演算法,并可同时适用于陆地勘探和海洋勘探。在正演方面,为了计算复杂三维电性介质的电磁响应,本文首先将不同频率的电磁波分解为发射源直接产生的一次场(背景场)与异常体感应产生的二次场,其中一次场可以利用一维全空间频率域层状介质偶极源格林函数进行计算;二次场则是在麦克斯韦方程组和边界条件的基础上推导出有限差分离散正演方程组后,再将主轴各向异性电导率通过电流密度项引入方程组中,最后采用交错网格有限差分法对推导出的离散二次电场的偏微分方程组进行求解。在有限差分各向异性正演的基础上,本文提出了一个三维频率域可控源主轴各向异性反演算法,并将其应用于理论模型算例中,得到了较好的反演效果。采用主轴各向异性的原因主要是主轴各向异性电阻率中存在较少的独立电导率分量,这可以缓解反演的多解性并更容易得到可靠的反演结果;另一方面,主轴各向异性模型也适合对典型地质情况分别进行三个主轴方向的理论分析。在模型试算中,本文重点测试了将不同各向异性异常体嵌入各向同性大地介质中的反演效果,以及两种不同收发装置对反演的影响,验证了算法的有效性和稳定性。
[Abstract]:The controllable source electromagnetic method is a commonly used artificial source low frequency electromagnetic exploration method in geophysical exploration. The electromagnetic wave is emitted by the electric dipole source or the long straight wire source and the total electromagnetic field response is received. And then get electrical information on the surface or in the marine environment. In the past, for the sake of mitigation, inversion, multiplicity, and cost savings, Most of the three dimensional controlled source electromagnetic forward inversion at home and abroad is simulated by assuming the electro isotropy of underground media. However, recent studies have shown that, in many cases, If the conventional isotropic forward and inverse method is still used, the inversion results will be easily affected by the anisotropic information originally contained in the observed response. In order to solve this problem, this paper focuses on the development of an efficient and stable 3-D anisotropic controllable source forward and inversion algorithm. And it can be applied to both land and ocean exploration. In the forward modeling, in order to calculate the electromagnetic response of complex three-dimensional electric medium, In this paper, the electromagnetic waves with different frequencies are first decomposed into the primary field (background field) generated directly by the emitter and the quadratic field induced by the anomalous body. The first order field can be calculated by using the Green's function of dipole source in layered media in one-dimensional full space frequency domain, and the quadratic field is derived from the finite difference discrete forward equations on the basis of Maxwell's equations and boundary conditions. Then the anisotropic conductivity of spindle is introduced into the equations by the current density term. Finally, the partial differential equations of discrete secondary electric field are solved by staggered grid finite difference method. On the basis of forward modeling of finite difference anisotropy, In this paper, a 3-D anisotropic inversion algorithm of controllable source spindle in frequency domain is proposed and applied to a theoretical model. The main reason of adopting spindle anisotropy is that there are less independent conductivity components in the resistivity of spindle anisotropy, which can alleviate the multi-solution of inversion and obtain reliable inversion results more easily. On the other hand, the principal axis anisotropy model is also suitable for the theoretical analysis of three principal axis directions for typical geological conditions. The inversion effect of embedding different anisotropic anomalous bodies into isotropic geodetic media and the influence of two different transceiver devices on the inversion are mainly tested in this paper. The validity and stability of the algorithm are verified.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P631.325

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