3D Step-by-step inversion strategy for audio magnetotellurics data based on unstructured mesh

A three-dimensional (3D) step-by-step inversion strategy for audio magnetotellurics (AMT) is investigated in this study. The objective function is minimized by iteratively solving the Gauss-Newton normal equation, and the inversion region is discretized with unstructured tetrahedral elements. The inversion proceeds step-by-step from a coarse mesh to a fine mesh. In the inversion iteration process, a mesh is adaptively optimized according to the spatial gradient information about the model resistivity to fine delineate the boundaries of abnormal bodies. In the early stage of inversion execution, a coarse mesh is used for inversion, and the inversion stability is improved by reducing the number of inversion elements. In addition, mesh refinement is performed in the iterative inversion process. The inversion results obtained from the previous mesh are used as the reference and initial models for the next mesh iterative inversion. The step-by-step inversion strategy can ensure that the inversion is performed in the correct direction, improving the inversion stability and results gradually. Synthetic results show that the step-by-step inversion strategy with a Gauss-Newton method for 3D AMT inversion is stable and reliable, which lays a foundation for further practical 3D AMT data inversion.