With the development of computers and forward modeling techniques, three-dimensional (3D) transient electromagnetic inversions are being developed. However, there are few 3D transient electromagnetic inversions based on explicit forward modeling methods. Therefore, in this study, we propose a 3D transient electromagnetic inversion method based on explicit finite-difference forward modeling. For forward modeling, we first calculate the whole-space initial field excited by magnetic dipole sources at an initial time after the current is switched off. Then, we step Maxwell’s equations in time using the modified DuFort-Frankel method and staggered grids. For inversion, we employ the least squares optimization method and test it using synthetic and field data. Using synthetic data, we invert the model of a conductivity brick in a half-space. The results reveal that the conductivity brick is well recovered. Inversion results using the field data are consistent with known geological conditions. The synthetic and field examples demonstrate that the inversion method based on explicit finite-diff erence forward modeling is reliable.

随着计算机和正演建模技术的发展，三维（3D）瞬变电磁反演正在发展。然而，基于显式正演建模方法的3D瞬变电磁反演还很少。因此，在本研究中，我们提出了一种基于显式有限差分正演建模的3D瞬变电磁反演方法。对于正演建模，我们首先计算电流关闭后初始时间磁偶极子源激发的全空间初始场。然后，我们使用改进的 DuFort-Frankel 方法和交错网格对麦克斯韦方程进行时间步进。对于反演，我们采用最小二乘优化方法并使用合成数据和现场数据进行测试。使用合成数据，我们反转半空间中的电导砖模型。结果表明，导电砖恢复良好。使用现场数据的反演结果与已知的地质条件一致。综合实例和现场实例表明，基于显式有限差分正演模型的反演方法是可靠的。