Complex Geoelectrical Structure Responses In AMT Using Finite Element Method

As the geological structure is very complex, not only for terrain but also for the structure underground, often syncline, anticline, fault and other complex combination of geological bodies, which make AMT forward modeling very difficult. Audio-frequency magnetotelluric (AMT) based on the magnetotelluric (MT) theory is researched on the response to the audio section of electromagnetic signals. It is necessary to achieve the simulation of AMT data under complex structure to practice.The rectangular elements with bilinear interpolation finite element method (FEM) was used to compiled the two dimensional AMT forward modeling program in this paper. Meanwhile, the effects of grid size and its boundaries to the accuracy of MT forward modeling using FEM were discussed. Besides that, complex geological structure responses with terrain and without terrain were detailed simulated in this paper, so were anticline and syncline models.The results show that, for one-dimensional media, left and right boundaries are not necessary to place at infinity, for two dimensional media, the effects of boundaries could be weakened seriously only laid at 5 times the size of object distance. Both lower boundary and ratio of horizontal to vertical have little effect on accuracy; vertical grid is closely related to the smallest skin depth and it can not larger than 1/4 skin depth to make sure maximum relative less than 1% for bilinear interpolation. There consecutive elements below the surface should not cross the interface. The horizontal resolution of TM mode is higher than TE mode and the shape of isoline about apparent resistivity section is not a true reflection of structure. Topography can not be ignored. The maximum value is always on the ridge and minimum on the valley for TE mode, however, TM mode on the contrary. The apparent resistivity profiles and their pseudo-sections on anticline and syncline of three layers are regular. Without terrain correction, referred to the characteristics of apparent resistivity profiles and their pseudo-sections, it can make a qualitative analysis and be guided for practice.