3-D Inversion of the Grounded Electrical-source Airborne Transient Electromagnetic (GREATE) Survey Data

Studies have shown that Grounded Electrical-Source Airborne Transient ElectroMagnetics (GREATEM) is a promising method for modelling three-dimensional (3-D) resistivity structures in coastal areas, in addition to inaccessible areas such as volcanoes, mountains and deep forest cover. To expand the application of the GREATEM system, a 3-D resistivity model that considers large lateral resistivity variations is required. In this study, we present a frequency-domain 3-D electromagnetic inversion approach that can be applied to time domain data from GREATEM. To deal with a huge number of grids and wide range of frequencies in airborne datasets, a method for approximating sensitivities is introduced for efficient 3-D inversion. Approximate sensitivities are derived by replacing adjoint secondary electric fields with those computed in the previous iteration. These sensitivities can reduce the computation time without significant loss of accuracy. We verified our inversion approach using electromagnetic response generated from two synthetic models that consist of one conductor and two conductors buried in a host resistive half-space. The inverted results were able to recover conductive zones of potential interest within the resistive region. These results are in good agreement with the synthetic model, confirming the accuracy of our inversion approach.