Recently, an airborne time-domain electromagnetic (ATEM) surveying with a grounded source, called GREATEM, has gained popularity because it provides a greater depth of investigation than moving-source ATEM surveys. However, unlike moving-source ATEM responses, GREATEM responses exhibit complicated behaviors that result from galvanic currents rather than vortex currents. Our model study showed that 1D inversion of GREATEM data is most likely to result in an erroneous resistivity model even for layered structures if there are some lateral changes in layer thickness and that 3D inversion is required to recover the subsurface resistivity. Our 3D modeling is based on a finite-difference technique in the frequency domain and inverse Fourier transformation to the time-domain response. The convolution with the system response is performed in the frequency domain. For 3D inversion, we introduced a scaling factor as an unknown parameter to take into account inaccuracies in the source moment and receiver gain. The inversion algorithm has been tested with a simple 3D subsurface model and 3D inversion result for a field data set taken over a known uranium deposit in Korea showed that the recovered conductive zones are consistent with the known structural direction and distribution of mineralization zones.


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