In this paper, we propose a new method for the inversion of large fixed-loop transient electromagnetic (TEM) data. The unstructured tetrahedral grids with flexibility to delineate the rugged topography are adopted to discretize the inverse model. The transmitting source is divided into short electric dipoles to simulate its practical shape, size and attitude. In the numerical simulation, we adopt the edge-based finite-element method (FEM) to discretize the governing equation in space, and the unconditionally stable second-order of Backward Euler (BE) scheme to discretize the time. The final forward modelling equation is solved for all time channels iteratively from an initial field. Since the full-channel forward modelling matrix is asymmetric, we use a backward recursive method to solve the adjoint forward problem to compute the Jacobian matrix. Considering the limited power of personal computational devices, we apply the limited-memory BFGS optimization method to accomplish our three-dimensional (3D) inversions. The inversion algorithm is tested firstly on a synthetic example. The inversion results show that the proposed inversion scheme can well invert TEM data with sharp topography. We further test our inversion code on a field survey data and obtained results that agree well with the geology and other geophysical information.


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