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

ABSTRACT 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.