We apply the SE method in the frequency range (900 Hz to 56 kHz) to airborne EM modeling. Starting from the vector Maxwell's equations, we use the SE method to establish spatial-discrete forms of 3-D vector Helmholtz equations and adopt GLL integration to calculate the matrix elements. The magnetic field is calculated using Faraday's law. To check the accuracy of the SE algorithm, we compare our results with semi-analytical solutions for a homogeneous half-space and a layered earth model. We further analyze the influence of grids, expansion of outside boundary of model domain, and the order of interpolation polynomial on EM modeling accuracy and reveal that increasing the SE polynomial order can especially improve the accuracy for complex models. Finally, we demonstrate the feasibility of our SE algorithm for modeling an airborne EM system by numerical 3-D experiments.

]]>A processing algorithm was developed to analyze the time series of the measured magnetic fields. Transfer functions for the vertical magnetic field were derived using both a scalar and a bivariate analysis approach. After rotating the transfer functions into the strike direction of the 2D subsurface anomaly, a two dimensional inversion method was applied for the interpretation of the conductivity structure to a depth of 15 m.

In addition, radiomagnetotelluric (RMT) measurements were carried out on the same UAS-VLF profiles. The RMT data were interpreted by 2D inversion for comparison and are used here to derive the background resistivity, which cannot be achieved by VLF measurements. As a result, good correlation was found between the UAS-VLF and RMT conductivity models.

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