Three-Dimensional Inversion of GREATEM Data: Application to GREATEM Survey Data From Kujukuri Beach, Japan

Studies have shown that the grounded electrical-source airborne transient electromagnetics (GREATEM) method is a promising method for investigating resistivity structures in coastal areas, in addition to those in inaccessible areas such as volcanoes, mountains, and deep forest cover. To expand the application field of the GREATEM system, a three-dimensional (3-D) resistivitymodel that considers large lateral resistivity variations is required. In this paper, we present a frequency-domain 3-D electromagnetic inversion approach that can be applied to time-domain data from GREATEM. In the frequency-domain approach, TEM data are Fourier-transformed using a smooth-spectrum inversion method, and the recovered frequency response is then inverted. To deal with a large number of grids and a 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 computation time without significant loss of accuracy. First, we verified both our forward and inversion solutions. Then, we applied this approach to GREATEM survey data from Kujukuri Beach, central Japan. The inverted results of the field data were well fitted with previous study results in the Kujukuri area, suggesting applicability of this inversion approach for constructing 3-D resistivity models from GREATEM field survey data in the future.