| 基于二次耦合势的广域电磁法有限体积三维正演计算 |
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| 引用本文： | 彭荣华,胡祥云,李建慧,胡祖志,杨辉.基于二次耦合势的广域电磁法有限体积三维正演计算[J].地球物理学报,2018,61(10):4160-4170. |
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| 作者姓名： | 彭荣华 胡祥云 李建慧 胡祖志 杨辉 |
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| 作者单位： | 1. 中国地质大学(武汉)地球物理与空间信息学院, 武汉 430074;2. 中国石油集团东方地球物理勘探有限责任公司, 河北涿州 072751;3. 中国石油勘探开发研究院, 北京 100083 |
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| 基金项目： | 国家重点研发计划（2016YFC0601104），国家自然科学基金（41274077，41704133）和湖北省自然科学基金（2015CFA019）联合资助. |
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| 摘 要： | 与可控源音频大地电磁（CSAMT）相比，广域电磁法通过采用全区视电阻率定义，突破了卡尼亚视电阻率所需的远区条件限制，极大拓展了可控源电磁观测区域和探测深度.考虑到电偶源激发场的三维特征以及地下复杂三维结构，为提高广域电磁数据解释精度，本文实现了基于二次耦合势的广域电磁法三维正演计算.该算法利用Helmholtz定理将麦克斯韦方程转化为库伦规范下的磁矢势和电标势耦合方程，有效改善了离散所得大型线性方程组的谱性质，并通过强加散度条件来消除电场伪解的影响.此外，采用散射场方法，其中一次场使用准解析法求解，二次场使用有限体积法求解，克服了局部激发场源奇异性问题.通过与一维层状模型下电偶源产生的电磁场准解析解对比，验证了本文算法的正确性.在此基础上，利用本文的正演算法对比分析了广域电磁法与CSAMT对典型三维目标体的探测能力，结果表明在相同的观测条件下，广域电磁法能够更准确地反映地下目标体信息，拥有更优的分辨能力.
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| 关 键 词： | 广域电磁法 可控源音频电磁法 有限体积法 三维正演 二次耦合势 |
| 收稿时间： | 2017-06-14 |
3-D finite-volume forward modeling of wide-field EM using scattered potentials |
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| PENG RongHua,HU XiangYun,LI JianHui,HU ZuZhi,YANG Hui.3-D finite-volume forward modeling of wide-field EM using scattered potentials[J].Chinese Journal of Geophysics,2018,61(10):4160-4170. |
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| Authors: | PENG RongHua HU XiangYun LI JianHui HU ZuZhi YANG Hui |
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| Institution: | 1. Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China;2. DBGP Inc, China National Petroleum Corporation, Zhuozhou Hebei 072751, China;3. Sinopec Petroleum Exploration and Production Research Institute, Beijing 100083, China |
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| Abstract: | Controlled-source audio-frequency magnetotelluric (CSAMT) method has developed into an established technique and been extensively used for a broad range of applications in geophysical exploration and near-surface investigations over the past decades. Conventionally, CSAMT measurements are carried out in the far-field zones to meet the plane wave approximation of CSAMT data, where the separation between transmitters and receivers should be sufficiently large. However, near-and intermediate-field CSAMT data are often collected due to improper employment of transmitter-receiver separation and limitations of logistics access in some prospecting areas such as mountainous regions. Under such circumstance, Cagniard apparent resistivity defined for magnetotelluric plane waves is no longer reasonable in these areas.In order to overcome the limitation of far-field measurements, wide field electromagnetic method (WFEM) introduces the wide field apparent resistivity which is defined from the analytical expressions of excited electromagnetic fields for homogeneous halfspace. The use of wide field apparent resistivity instead of Cagniard apparent resistivity greatly improves measurable regions and reduces distortion for data collected in near-and intermediate-field zones.
Considering both the three dimensional (3D) nature of the excited electromagnetic fields and complex subsurface structures, in this paper, we present an efficient finite volume 3D forward modeling algorithm for frequency-domain controlled-source electromagnetic simulations. To circumvent ill-conditioning of the system of linear equations resulting from discretization, Maxwell's equations are reformulated in terms of coupled scalar and vector potentials under Coulomb gauge condition, and an explicit divergence is enforced to avoid non-physical solutions introduced during solution process. In addition, the well-known scattering field formulation is employed to mitigate the numerical singularity in the vicinity of the impressed current source. The accuracy of the algorithm is validated against quasi-static solutions for 1D layered model. Besides, comparisons between CSAMT and WFEM measurements for typical 3D conductive structure under the same survey configuration demonstrate that the wide field apparent resistivity is less distorted in the non-plane wave zones and reveals better resolution capability than Cagniard apparent resistivity. |
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| Keywords: | Wide-field electromagnetic Controlled-source audio magnetotelluric Finite volume method 3D forward modeling Scattered potentials |
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