| 核磁共振与瞬变电磁三维联合解释方法 |
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| 引用本文： | 李貅, 刘文韬, 智庆全, 赵威. 核磁共振与瞬变电磁三维联合解释方法[J]. 地球物理学报, 2015, 58(8): 2730-2744, doi: 10.6038/cjg20150810 |
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| 作者姓名： | 李貅 刘文韬 智庆全 赵威 |
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| 作者单位： | 1. 长安大学地质工程与测绘学院, 西安 710054; 2. 中国地质科学院地球物理地球化学勘查研究所, 河北廊坊 065000 |
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| 基金项目： | 国家重点基础研究发展计划项目(973项目)(2013CB036002)、国家重大科研设备研制项目(ZDYZ2012-1-05-04)和国家自然科学基金重点项目(51139004)联合资助. |
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| 摘 要： | 传统核磁共振地下含水量解释多采用基于均匀半空间或层状导电模型的一维反演, 分层给出地下含水信息.然而, 这些方法忽略了地下复杂电阻率分布信息对结果的影响, 也不能很好地反映局部三维含水构造.本文从三维电介质中核磁共振响应的正演理论出发, 提出首先利用瞬变电磁数据进行基于等效导电平面法的快速电阻率成像, 然后将成像结果作为核磁共振三维反演的电性模型, 进行联合解释.激发磁场的分布采用有限元法直接求解, 通过引入伪δ源实现电流源的加载, 并强加散度条件排除了三维磁场模拟中"弱解"的影响.针对核磁共振灵敏度矩阵的病态性和数据中存在的干扰信号, 提出考虑罚项的非线性拟合目标函数, 利用线性化方法进行核磁共振反演.模型数据表明该方法能较准确反映地下三维含水构造, 实测算例进一步证明了方法的有效性.本研究将促使核磁共振方法在岩溶、裂隙水、孤立水体等复杂水文地质条件及隧道、矿井灾害水源探测等方面得到有效应用.
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| 关 键 词： | 核磁共振 瞬变电磁 等效导电平面 三维联合解释 |
| 收稿时间： | 2014-12-14 |
| 修稿时间： | 2015-07-08 |
Three-dimensional joint interpretation of nuclear magnetic resonance and transient electromagnetic data |
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| LI Xiu, LIU Wen-Tao, ZHI Qing-Quan, ZHAO Wei. Three-dimensional joint interpretation of nuclear magnetic resonance and transient electromagnetic data[J]. Chinese Journal of Geophysics (in Chinese), 2015, 58(8): 2730-2744, doi: 10.6038/cjg20150810 |
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| Authors: | LI Xiu LIU Wen-Tao ZHI Qing-Quan ZHAO Wei |
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| Affiliation: | 1. School of Geology Engineering and Geomatics, Chang'an University, Xi'an 710054, China; 2. Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geosciences, Hebei Langfang 065000, China |
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| Abstract: | The traditional interpretation methods of nuclear magnetic resonance(NMR)usually use 1D inversion based on homogeneous half space or layered conductive earth model and the information of water amount in each layer will be derived. However, these methods usually ignore the influence of the inhomogeneous underground conductivity to the interpretation results. Meanwhile, these results are easily restricted to local water-bearing structures. For the purpose of avoiding the problems aforementioned, a novel joint interpretation method combining transient electromagnetic(TEM)and NMR data is proposed in this paper, which provides an effective and practical interpretation method for the detection of complex hydrogeological conditions.#br#This paper proposes a novel joint interpretation method combining transient electromagnetic(TEM)and NMR data based on the forward modeling theory of NMR method in 3D conductive media. Firstly, a fast resistivity image is derived from floating plate interpretation results of TEM data. Afterwards, the resistivity imaging data will be used as the electrical model in 3D NMR inversion. The distribution of transmitting magnetic field is calculated by FE method directly. The transmitting current source is loaded by introducing a pseudo-delta source, and the influence of "weak solution" in three dimensional simulation of magnetic field is eliminated by a force introduction of divergence condition. In order to improve the illness of sensitivity matrix and eliminate the noise, a non-linear fitting objective function considering the penalty terms is proposed and the linearized inversion method is used in the inversion process.#br#The results of homogenous half-space model show that solutions of excitation magnetic field are almost the same as FEM solutions and digital filtering solutions and the influence of "weak solution" is eliminated. Results of the distribution of the NMR kernel function in different conductive models show that:(1)The background conductive of a model affects the distribution of the kernel function, especially when it is a low conductive one, and the lower of the conductive, the smaller range of the influence of excitation at the same pulse moment will be;(2)Local low resistive bodies with reasonably scale also have a significant effect on the distribution of the kernel function, lower resistivity and shallower buried depth causing greater influence, vice versa; Several conclusions can be drawn from the inversion results:(1)smaller variation of the geo-electrical model from the real case will lead to more accurate inversion results;(2)The inversion result can deviate from reality when using a homogenous half-space as the electrical model directly while the low conductive body have a certain scale, and the accuracy of the inversion result will see a significant improvement with the introduction of the floating plate interpretation method based on TEM data;(3)Though the accuracy of the floating plate interpretation decreases with increased depth, the impact of low conductive bodies on the distribution of the kernel function also decreases with that. Therefore, the fast resistivity image result based on TEM data, in spite of being an approximate result, can still offer a reasonable electrical model for NMR inversion.#br#This paper proposes a novel joint interpretation method combining transient electromagnetic(TEM)and NMR data based on the forward modeling theory of NMR method in 3D conductive media. Results of the 3D distribution of kernel function show that low resistive bodies have a significant effect on the distribution of the kernel function, and the media conductivity should be considered during the NMR inversion. The floating plate interpretation based on TEM data can provide a reasonable geo-electrical model, and the accuracy of the NMR inversion will be improved significantly after introducing joint interpretation method combining transient electromagnetic(TEM)and NMR data. |
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| Keywords: | Nuclear magnetic resonance Transient electromagnetic Equivalent conductive plane 3D joint interpretation |
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