Traditional forward modeling methods based on structured grids for Controlled-source Audio-frequency Magnetotelluric method (CSAMT) have been well developed. However, these methods have proved not to be effective for the complex topographic earth model as they have to approach the topography using stairs. In this paper, we present a finite-element method based on unstructured grids for CSAMT modeling. The finite-element method can effectively simulate the EM response for complex underground structures, while the unstructured grids can well approach the topographic earth surface. The accuracy check against other results shows that our method has a high accuracy. To analyses the characteristic of the topography effect on the CSAMT responses, we calculate the apparent resistivity for both 3D hill and valley models with and without abnormal bodies embedded.


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