关键词: 时间域航空电磁, 激电效应, 三维正演, Debye模型

Abstract:

The time-domain airborne electromagnetic (AEM) method is an efficient geophysical exploration technique. Yet, due to the presence of the induced polarization (IP) effect, electromagnetic (EM) data from late-time channels often exhibit a phenomenon known as sign reversal. The traditional imaging and inversion methods without considering the IP effect cannot recover the real electrical structure of the underground, so it is necessary to develop the EM forward and reverse modeling technology with IP effect. This study introduces a three-dimensional (3D) forward algorithm for AEM with IP effect in the time-domain built upon the unstructured finite-element method. To capture the IP effect within the medium, we use the Debye model, transforming it into a derivative form in the time-domain through frequency-time conversion. For temporal discretization, an unconditionally stable second-order backward Euler's scheme is applied. Maxwell’s equations are discretized in space using a vector finite element method based on an unstructured tetrahedral mesh. The algorithm's accuracy is verified through a comparison with the 1D semi-analytical solution of the uniform half-space model. We compute EM responses for both a half-space and 3D geological formations under various polarization conditions. We observe that the polarization characteristics of surrounding rocks and anomalous bodies can be effectively identified according to the attenuation characteristics of the AEM response curves. In addition, by studying the characteristics of the influence of IP parameters on the time-domain AEM system, we show that the larger the resistivity of the geological body or the larger the chargeability, the stronger the IP effect.

Key words: time-domain airborne electromagnetic, induced polarization , effect, 3D forward modeling, Debye model