Response characteristics of three-dimensional axial anisotropic media for transient electromagnetic method in underground whole-space

The study of the electrical anisotropy in the transient electromagnetic field could provide theoretical guid- ance for the data processing and interpretation of the underground full-space transient electromagnetic method under the condition of anisotropy,and it has important guiding significance to improve the detection accuracy of transient electromagnetic method. In order to study the transient electromagnetic field response characteristics of axial anisotrop- ic media in underground whole-space,an improved staggered mesh finite difference method was used to realize a 3D forward modeling of the transient electromagnetic field of axial anisotropic media. The conductivity tensor was added to Maxwell’s equations. Yee’s staggered grid was improved to facilitate the differential discretization of axial anisotropic media’ s transient electromagnetic fields. The whole-space finite difference forward algorithm for axial anisotropic media was developed. The correctness of the algorithm was verified by comparing the anisotropic finite difference nu- merical solution and the uniform whole-space analytical solution in a uniform whole-space model. Finally,combined with actual engineering detection,the influence characteristics of different axial anisotropic media on transient electro- magnetic field were analyzed by forward modeling,and the reasons for the deviation of the detection results were ex- plained. The results show that in the advanced detection mode,the change of anisotropy coefficient of HTI-X medium has no obvious influence on the transient electromagnetic field,the difference of induced electromotive force curves un- der different anisotropic coefficients is very small. For HTI-Y media and VTI media,the change of anisotropy coeffi- cient has a significant impact on the transient electromagnetic field,as the anisotropy coefficient of the medium increa- ses,the transient electromagnetic field response of the medium decreases continuously. With the increase of the angle between the symmetric axis of anisotropic media and the direction of detection,the transient electromagnetic field re- sponse generated by anisotropic media increases continuously. When the symmetric axis of anisotropic medium is con- sistent with the detection direction (HTI-X medium),the induced electromotive force generated by anisotropic medium is the weakest. When the symmetric axis is perpendicular to the detection direction (VTI medium),the induced elec- tromotive force generated by anisotropic medium is the strongest. Different types of axial anisotropic media have obvi- ous anisotropic transient electromagnetic field response characteristics.