The time-domain/transient electromagnetic (TEM) method employs an alternating magnetic field that is established by passing a bidirectional pulsed current through a transmitter loop. A large wire loop on or above the earth is one of the most widely used sources in TEM survey systems. To simulate TEM responses of the large loop system, the effect of the loop size must be considered in a numerical modeling scheme. We have developed an efficient three-dimensional (3D) TEM modeling algorithm using a staggered-grid finite-difference time-domain (FDTD) method. In this paper, we first derive FDTD TEM equations using fictitious displacement currents based on an explicit mid-point method. Then, we employed a conversion method to efficiently obtain stepoff TEM responses from step-on responses. Further, following the concept of the conversion method, the charging and discharging processes of current waveform in loop does not need to be identical, implying we can apply the simulation scheme for more general source waveforms. The developed algorithm has been verified through comparison of numerical results for a homogeneous halfspace medium. Finally, we analyze TEM responses generated from a large loop source for a 3D conductive body buried in a homogeneous half space.

时域/瞬变电磁（TEM）方法采用交变磁场，该交变磁场是通过使双向脉冲电流通过发射器环路而建立的。地球上或地球上的大铁丝线圈是TEM测量系统中使用最广泛的信号源之一。为了模拟大回路系统的TEM响应，必须在数值建模方案中考虑回路尺寸的影响。我们使用交错网格有限差分时域（FDTD）方法开发了一种有效的三维（3D）TEM建模算法。在本文中，我们首先基于显式中点方法，使用虚拟位移电流推导FDTD TEM方程。然后，我们采用了一种转换方法，以从踩踏响应中高效获取踩踏TEM响应。此外，遵循转换方法的概念，环路中电流波形的充电和放电过程不必相同，这意味着我们可以将仿真方案应用于更通用的源波形。通过比较均匀半空间介质的数值结果，已验证了该开发算法。最后，我们分析了埋在同质半空间中的3D导电体的大型环路源产生的TEM响应。