Three-dimensional inversion of controlled-source audio-frequency magnetotelluric data based on unstructured finite-element method*
Chen Xiang-Zhong 1, Liu Yun-He 1, Yin Chang-Chun 1, Qiu Chang-Kai 2, Zhang Jie 1, Ren Xiu-Yan 1, and Zhang Bo 1
1.College of Geo-exploration Science and Technology, Jilin University, Changchun, Jilin, China
2.Development and Research Center, China Geological Survey, Beijing, China.
Abstract：
We propose a new 3D inversion scheme to invert the near- and transition-zone data of CSAMT with topography accurately. In this new method, the earth was discretized into unstructured tetrahedra to fi t the ragged topography and the vector finite-element method was adopted to obtain precise responses and good sensitivity. To simulate the attitude and shape of the transmitter, we divided a long-grounded transmitter into dipoles and integrated these dipoles to obtain good responses in the near- and transition-fi eld zones. Next, we designed an L2 norm-based objective functional and applied a standard quasi-Newton method as the optimization method to solve the inverse problem and guarantee steady convergence. We tested our 3D inversion method first on synthetic data and then on a field dataset acquired from select sites near Changbai Mountain, China. In both tests, the new inversion algorithm achieved excellent fitting between the predicted and observed data, even in near- and transition-fi eld zones, and the inversion results agreed well with the true model. These findings reveal that the proposed algorithm is eff ective for 3D inversion of CSAMT data.
. Three-dimensional inversion of controlled-source audio-frequency magnetotelluric data based on unstructured finite-element method*[J]. APPLIED GEOPHYSICS, 2020, 17(3): 349-360.
2020, Vol. 17 
