Different geophysical exploration methods have significant differences in terms of exploration depth, especially in frequency domain electromagnetic (EM) exploration. According to the definition of skin depth, this difference will increase with the effective detection frequency of the method. As a result, when performing three-dimensional inversion on single type of EM data, it is not possible to effectively distinguish the subsurface geoelectric structure at the full scale. Therefore, it is necessary to perform joint inversion on different type of EM data. In this paper we combine the magnetotelluric method (MT) with the controlled-source audio-magnetotelluric method (CSAMT) to study the frequency-domain three-dimensional (3D) joint inversions, and we use the unstructured finite-element method to do the forward modeling for them, so that the numerical simulation accuracies of different electromagnetic methods can be satisfied. By combining the two sets of data, we can obtain the sensitivity of the electrical structure at different depths, and depict the full-scale subsurface geoelectric structures. In actual mineral exploration, the 3D joint inversion is more useful for identifying subsurface veins in the shallow part and blind mines in the deep part. It can delineate the morphological distribution of ore bodies more completely and provide reliable EM interpretations to guide the mining of minerals.

In this paper, the authors propose a method of three-dimensional (3D) magnetotelluric (MT) forward modeling algorithm based on the meshfree and finite element coupling method. The model is discretized by regular nodes in the central area, and the radial point interpolation method (RPIM) based on the global weakness is utilized to construct the meshfree shape function. The Governing equations in each background gird are solved by Gaussian integration. In the extended area where the points are sparsely distributed, to avoid the instability of the meshfree method, finite element method (FEM) with regular grids is used to solve the governing equation. Finally, the meshfree and finite element governing equations are coupled by the continuity of the field at the interfaces, and the direct solution technique is used to realize the 3D MT forward modeling. Numerical experiments of several typical electrical models are used to verify the effectiveness of the method.