Three-dimensional inversion of semi-airborne transient electromagnetic data based on finite element method

The semi-airborne transient electromagnetic method is characterized by a large detection depth, high signal-to-noise ratio, fast acquisition rate, low cost, and ability to be used for exploration in scenarios with complex geological conditions. As a result, the semi-airborne transient electromagnetic method has a great potential in the fields of mineral exploration, hydrological study and geohazard detection. However, most of the existing semi-airborne transient electromagnetic data interpretation methods still rely on one-dimensional forward modelling and inversion, which could lead to an incorrect geological interpretation. In this paper, we develop a three-dimensional inversion for semi-airborne transient electromagnetic data to recover a reliable subsurface conductivity distribution. The forward modelling is based on the effective vector finite element method with an unstructured tetrahedral mesh, which is capable of simulating complex geo-electric structures and the topography. Through the numerical experiments of several three-dimensional synthetic models, we study the influence of the number of excitation sources, flight mode and complex topography on the inversion results. The synthetic model studies reveal that: (1) by increasing the number of excitation sources, we can obtain a higher resolution conductivity image with less artefacts; (2) the detectability can be increased by flying along the terrain when the topography is complex; (3) the topography should be considered in the forward modelling and inversion to obtain a reasonable conductivity model using the semi-airborne transient electromagnetic method; and (4) the three-dimensional inversion still works when the inhomogeneity is present nearby the transmitter.