时间域航空电磁各向异性大地三维自适应有限元正演研究

Three-dimensional adaptive finite-element method for time-domain airborne EM over an anisotropic earth

各向异性介质模型电性结构复杂,如何进行合理的网格剖分成为获得高精度正演结果的关键,为此本文开展时间域航空电磁各向异性大地三维自适应有限元正演算法研究.通过结合非结构时间域有限元算法和自适应网格优化技术,实现各向异性介质条件下三维时间域航空电磁自适应正演.考虑到时间域航空电磁响应随时间的衰减特性,为了综合评价不同时刻的后验误差,本文将时间作为加权因子,调整各个时刻后验误差的相对权重,进而实现对浅部和深部网格的同步优化.通过与一维解析结果进行对比验证了本文算法的可靠性.数值实验结果显示电导率各向异性对自适应网格影响严重,其最大主轴电导率的数值及其分布特征直接决定了网格加密效果.此外,各向异性对时间域航空电磁三分量响应的分布形态和异常幅值也会产生严重影响,利用全域视电阻率极性图,可以很好地识别各向异性主轴方向.

How to generate a reasonable mesh for the anisotropic medium model with complex electrical structure is the key to obtaining high-precision modeling results.In this paper,we study the adaptive finite-element(FE)method for 3 Dtime-domain airborne EM modeling over an anisotropic earth.By combining the time-domain unstructured FE algorithm and the adaptive mesh refinement technology,we successfully realize the 3 Dtime-domain airborne EM adaptive forward modeling for anisotropic media.To deal with the big difference of EM responses at different time channels,we bring in a temporal weighting to achieve a synchronous refinement of shallow and deep meshes.The reliability of our algorithm is verified by comparing our results with 1 Dsemi-analytic solution.The numerical experiments show that the anisotropic conductivity has a big influence on the mesh refinement.The value of the maximum principal-axis conductivity and its distribution directly determines the mesh refinement.In addition,anisotropic conductivity can also seriously affect the distribution pattern and anomalous amplitude of the three-component EM responses.By using the polarity maps for full-domain apparent resistivity,the direction of the anisotropic principal axis can be well identified.