频率域可控源电磁法并行化三维正反演算法研究

发布时间：2018-02-28 16:39

本文关键词： 可控源电磁法三维正演三维反演反演理论 MPI 并行计算　出处：《中国地质大学》2015年博士论文　论文类型：学位论文

【摘要】：频率域可控源电磁法(controlled-source electromagnetic methods, CSEM)通过观测人工场源在地层、空气和海水等介质中激发的电磁场来获取介质的电性分布,长期以来一直是金属矿勘探中最重要的地球物理方法,并在最近十多年逐渐成为海洋油气勘探中一种不可或缺的地球物理技术。为了通过CSEM观测数据尽可能真实地还原出介质的电性结构以提升勘探效果,实施三维勘探并对数据进行三维模拟是必需的。反演是电磁资料的处理解释中极其关键的一步,目前可控源电磁法已经从最初的一维地电结构假设步入到二维甚至三维反演阶段。本文的主要目标是发展出一套高效、准确且既适用于陆地勘探又适用于海洋勘探的频率域CSEM三维正演与反演模拟工具。正演模拟是反演模拟的核心。电磁法的三维数值模拟是一个对数值算法和计算机硬件要求都非常高的问题。对常用的微分类方法如有限单元法和有限差分法而言,求解最后所得的大型线性系统方程是至关重要的一步,直接影响到正演算法的实用性。如何高效、稳定且准确地解线性方程长期以来一直是被探讨的问题。本文实现了基于线性系统直接求解技术的频率域可控源电磁(CSEM)三维正演。使用交错网格有限体积法(FV)来离散化关于二次电场的Helmholtz方程；使用直接解法取代传统的迭代解法来求解离散线性系统,即对系统矩阵进行完全LU分解,具体通过调用大规模并行矩阵直接求解器(MUMPS)来实现。基于理论模型做了一系列数值实验,证明了直接解法的高精度和稳定性,并考察了其内存需求、计算时间和并行可伸缩性等主要计算性能,检验了所开发的算法快速模拟多场源CSEM问题的能力以及对常规海洋和陆地CSEM模拟的有效性。在使用电偶极发射源的可控源电磁法(CSEM)勘探中,发射源的方位、长度、形状等对观测数据有重要的影响,然而现有的大部分三维数值模拟方法没有全面地将这些因素考虑进来,很多都只能应对非常简单的场源形态,例如单一方位的点电偶极子,这有可能显著降低模拟结果的准确性。本文的三维正演算法能够模拟形态相对复杂的场源,包括任意方位的有限长直导线和弯曲导线发射源。由于了使用一次场／二次场方法,只要在计算一次场时考虑复杂的场源形态便可以实现同样场源的三维正演。通过与一维理论模型的解析解对比验证了三维程序的准确性,并针对三维理论模型进行了一系列正演测试,初步考察了场源形态对三维正演结果的影响。阐述了当前电磁反演中主流的线搜索类方法的优化计算思路,对这一类反演方法中的最核心的数值计算问题——灵敏度矩阵的计算问题作了详细的讨论。基于面向对象的“模块化电磁反演系统”ModEM框架,实现了并行化的频率域CSEM的三维反演。通过理论模型的反演试验,验证了三维反演程序的有效性。
[Abstract]:The frequency domain controlled source electromagnetic method (CSEMEM) has been the most important geophysical method in metal ore exploration for a long time, by observing the electromagnetic fields excited by artificial field sources in strata, air and sea water, etc. In the last decade or so, it has gradually become an indispensable geophysical technique in offshore oil and gas exploration. In order to restore the electrical structure of the medium as truthfully as possible through the CSEM observation data, It is necessary to carry out 3D exploration and 3D simulation of data. Inversion is an extremely critical step in the processing and interpretation of electromagnetic data. At present, the controllable source electromagnetic method has moved from the initial assumption of one dimensional geoelectric structure to the stage of two-dimensional or even three-dimensional inversion. The main goal of this paper is to develop a set of high efficiency. The frequency domain CSEM 3-D forward modeling and inversion simulation tool, which is accurate and suitable for both land exploration and ocean exploration, is the core of inversion simulation. The electromagnetic three-dimensional numerical simulation is a logarithmic algorithm and calculation. For the commonly used micro-classification methods such as finite element method and finite difference method, Solving the final equations of large linear systems is a very important step, which directly affects the practicability of forward algorithm. The stable and accurate solution of linear equations has been discussed for a long time. In this paper, three dimensional forward modeling of frequency domain controllable source electromagnetic (CSEMM) based on direct solution technique of linear systems is realized. Discretization of the Helmholtz equation for the quadratic electric field; The direct solution is used instead of the traditional iterative method to solve the discrete linear system, that is, the complete LU decomposition of the system matrix. A series of numerical experiments based on the theoretical model are carried out to prove the high accuracy and stability of the direct solution, and the memory requirement of the direct solution is investigated. Main computing performance, such as computing time and parallel scalability, The ability of the developed algorithm to quickly simulate the multi-field source CSEM problem and the effectiveness of the conventional oceanic and terrestrial CSEM simulation are tested. The azimuth and length of the emission source in the electromagnetic-controlled source exploration of the electric dipole emitter are tested. However, most of the existing 3D numerical simulation methods do not fully take these factors into account, and many can only deal with very simple field source shapes, such as single azimuth spot electric dipoles. This may significantly reduce the accuracy of the simulation results. The 3-D forward algorithm in this paper can simulate relatively complex field sources, including finite length straight conductors and curved wire emitters with arbitrary azimuth. The three-dimensional forward modeling of the same field source can be realized by considering the complex field source shape when calculating a single field. The accuracy of the three-dimensional program is verified by comparing the analytical solution with the one-dimensional theoretical model. A series of forward modeling tests are carried out for the 3D theoretical model, and the influence of field source morphology on the 3D forward modeling results is preliminarily investigated. The optimal calculation ideas of the current mainstream line search methods in electromagnetic inversion are expounded. In this paper, the calculation of sensitivity matrix, which is the most important numerical computation problem in this kind of inversion method, is discussed in detail. Based on the object-oriented ModEM framework of "modularized electromagnetic inversion system", The parallel 3D inversion of CSEM in frequency domain is realized, and the validity of the 3D inversion program is verified by the inversion experiment of the theoretical model.
【学位授予单位】：中国地质大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：P631.325

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