The finite-difference time domain (FDTD) method is widely used in ground–airborne transient electromagnetic modelling. Generally, this method offers great limitations because the calculation of the air layer is avoided and the surface assumption is flat. This study proposes a new method for ground–airborne 3D electromagnetic forward modelling that includes air layers and effectively solves the airborne TEM forward calculation of the coastal ocean and rugged terrains. The optimum selection of the air-layer resistivity and the identification ability of the long-wire source are discussed. By adding the pulse source signal to the control equation and realizing a direct calculation that includes the source, the cumbersome process of calculating the complex initial field is avoided. After establishing the mountain model and coastal ocean model, the electromagnetic response is discussed. Subsequently, the ground–airborne TEM modelling is subjected to a 3D numerical simulation of complex geological conditions. The validity of this method is verified by analytical solutions through a uniform half space. The results show that the method can achieve 3D and high-precision numerical calculations when it includes complex terrain and seawater.

时域有限差分法（FDTD）被广泛用于地面机载瞬态电磁建模中。通常，此方法有很大的局限性，因为避免了空气层的计算并且表面假设是平坦的。这项研究提出了一种用于地面-机载3D电磁正向建模的新方法，该方法包括空气层，并有效解决了沿海海洋和崎terrain地形的机载TEM正向计算。讨论了空气层电阻率的最佳选择和长线源的识别能力。通过将脉冲源信号添加到控制方程式并实现包括该源的直接计算，可以避免计算复杂初始场的繁琐过程。建立山脉模型和沿海海洋模型之后，讨论电磁响应。随后，对地面机载TEM建模进行复杂地质条件的3D数值模拟。通过均匀半空间的解析解验证了该方法的有效性。结果表明，该方法在包含复杂地形和海水的情况下可以实现3D和高精度数值计算。