1.School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China 2.Anhui Huizhou Geology Security Institute Co., Ltd, Hefei 231202, China
Fund Project:Project supported by the Open Fund of Key Laboratory of Exploration Technologies for Oil and Gas Resource (Yangtze University), Ministry of Education (Grant No. K2016-08) and the Technology Research and Development Program of Guizhou, China (Grant No. [2018]3003-2).
Received Date:21 August 2018
Accepted Date:03 December 2018
Available Online:01 February 2019
Published Online:05 February 2019
Abstract:We present a newly developed algorithm for three-dimensional (3D) magnetotelluric model based on the vector finite element method. In this paper, unstructured grids which are composed of irregular tetrahedrons are used in our finite element model, which can be refined locally and adaptively according to the complex geometry of computational domain or subsurface structure. For obtaining more accurate solutions, secondary field rather than total field is numerically computed, which makes the errors limited to relatively small secondary field. Traditional node-based finite element method does not satisfy the condition that the normal electrical field is discontinuous at the interface of electric separatrix and the electrical current density is divergence-free throughout the regions without source, which obviously violates Maxwell equations. In order to overcome these drawbacks of node-based finite element, vector finite element method is employed to solve the secondary field-based partial differential equation. Moreover, in this study, the heterogeneous permeability is taken into consideration in our algorithm, as a consequence, which can deal with heterogeneous permeability model. The accuracy of our approach is verified by comparing with previously published numerical simulations of a COMMEMI-3D model. The advantages of our approach are also illustrated by the numerical simulations of model with arbitrary topography and complicated anomalous body. In addition, the simulation results of the irregular anomaly and the complex model of arbitrary terrain are both ideal results. It proves that the effect of 3D terrain is more serious and complex than that of two-dimensional (2D) terrain, and dealing with 3D models or data with 2D algorithm may bring in large errors. In the area where the magnetic permeability is abnormal, the magnetic permeability has an important influence on the numerical simulation results, and the magnetic permeability must be treated as an independent parameter in the magnetotellurics survey. Keywords:electromagnetic diffusion/ vector finite element method/ numerical simulation/ magnetization/ magnetotellurics
图 4 基于总场和二次场的二次插值的模拟结果与解析解的对比T为周期) (a) 视电阻率${\rho _{\rm{s}}}$; (b) 相位$\phi $ Figure4. Comparisons of modelling results of quadratic interpolation and analytical solutions based on total fields and secondary fields: (a) Apparent resistivity ${\rho _{\rm{s}}}$; (b) phase $\phi $.
24.2.三维模型COMMEMI-3D1 -->
4.2.三维模型COMMEMI-3D1
对于三维地电模型, 本文采用COMMEMI-3D1 (见图5, 其中${\rho _{\rm{0}}}$, $\rho $分别代表背景和异常体的电阻率)模型验证算法的正确性, 生成的四面体网格82992个, 由图6可知(三角符号为平均值, 竖线为标准差), 对于0.1 Hz的剖面, XY模式和YX模式, $x$测线和$y$测线, 模拟的结果均在参考结果的标准差以内, 且较为靠近均值; 对于10 Hz的剖面(见图7), YX模式勉强落在标准差以内, 对于XY模式, x测线上?500—500 m和y测线上?1000—1000 m (即异常所在的区域), 模拟的结果明显比参考结果偏低. Mitsuhata和Uchida[25]基于势(电流密度的矢势和标势)的算法, 采用结构化六面体网格, 混合使用矢量有限元和节点有限元法, 模拟得到的XY模式10 Hz剖面的结果相对于参考结果偏低, 为了验证其结果, Mitsuhata和Uchida[25]还用交错网格有限差分算法进行了模拟, 结果也偏低, 本文结果与其结论相吻合. 综合以上讨论, 验证了本文算法的正确性. 图 5 COMMEMI-3D1模型, 其中(a)图中虚线为x和y测线 Figure5. COMMEMI-3D1 model. The dashed lines in (a) are the x and y survey lines.
图 6 (a), (b) x和(c), (d) y测线模拟结果和Zhdanov等[24]的结果对比(剖面为0.1 Hz) Figure6. Comparisons of modelling results and Zhdanov et al[24] of (a), (b) x survey line and and (c), (d) y survey line (profile is 0.1 Hz).
图 7 (a), (b) x和(c), (d) y测线模拟结果和Zhdanov等[24]的结果对比(剖面为10 Hz) Figure7. Comparisons of modelling results and Zhdanov et al[24] of (a), (b) x survey line and and (c), (d) y survey line (profile is 10 Hz).
图 13 三维和二维模拟结果对比 Figure13. Comparisions of three-dimensional and two-dimensional modelling results.
为对比三维地形对MT响应的影响, 仍沿用图10模型, 仅将其水平地表改成起伏地表(见图14), 山峰底部长宽均为3 km, 顶部长宽均为1 km, 高为0.5 km, 以原点为中心对称分布, 类同Ren等[26]的带地形模型, 测线沿着x轴分布, 生成有限元网格20355个. 对于图14带地形地电模型, 其背景模型为两层模型, 水平地表为空气和地下介质的分界面, 山峰看作空气层中的异常体来处理. 由图14可看出, 在异常体边界及地表附近场变化剧烈的区域对网格进行了加密, 另外将黄色计算域进行了适当的向外延伸, 使边界条件更为贴近实际. 图 14 山峰模型及其非结构化网格剖分(左图点为测线) Figure14. Peak model and discretization using unstructured grids (the points in left diagram are survey line).