中文摘要：

传统时间域航空电磁全波形正演模拟主要采用间接法（褶积算法）和直接法（时域有限差分方法等），然而褶积算法需要获得精确的电流二阶导数，这给发射电流数据采集工作带来极大挑战；时域有限差分方法受到网格和时间步长的严格限制，缺乏灵活性．为解决这些问题，本文采用时域有限元方法，通过直接改变每个时间道上的瞬时电流强度模拟任意发射波形的电磁响应．由于无需计算电流二阶导数，大大提高了正演结果的精度．利用基于非结构四面体网格的矢量有限元方法和后推欧拉技术对时间域电场扩散方程进行空间和时间离散，实现三维航空电磁时间域全波形的直接正演模拟．由此不仅可以模拟复杂的地电结构，而且基于后推欧拉法的无条件稳定性，可以更加灵活地选取时间步长，提高计算效率．通过与1D数值模拟结果进行对比验证了该方法的准确性．本文对三维柱状体模型上HELITEM MULTIPULSE和VTEM系统实际发射波形电磁响应进行模拟，并与褶积算法的结果进行比较，验证了本文算法模拟实际发射波形电磁响应的优越性．对复杂三维地质体模型上不同发射波形电磁响应进行模拟，验证了时间域有限元算法可有效处理复杂地下地质结构．

英文摘要：

Traditional airborne EM full-wave forward modeling may be problematic when simulating complex transmitting waveforms and underground structures. The convolution algorithm requires high-precision of second order of derivative of transmitting current, which brings big challenges to AEM modeling, while the FDTD method is restrained by the mesh size and time step. To solve these problems, we adopt the edge finite-element method based on unstructured grids in combination with backward Euler scheme to perform 3D time-domain airborne EM （ATEM） modeling. This research will lay a foundation for the processing and interpretation of 3D full-wave time-domain airborne EM data. We start from time-domain electric field diffusion equation and discretize it with edge finiteelement method based on unstructured grids. The vector basis functions can automatically satisfy the divergence-free property of electrical fields. The tetrahedral grids have big advantages in simulating complex geological structures. We further adopt Galerkinrs method to obtain the finite-element governing equation. The backward Euler scheme is then used to discretize the governing equation to establish an unconditionally stable implicit equations system. We simulate the full-wave responses of arbitrary transmitting waveform by directly changing the instantaneous current for each time channel. To check the accuracy of our algorithm, we first construct a homogeneous half-space model for a center-loop AEM system. We calculate the responses for a half-sine and trapezoid transmitting wave that shows a good agreement with 1D solutions. Secondly, we simulate the EM responses for the transmitting wave used by HELITEM MULTIPULSE system over a 3D geological model with both convolution algorithm and FETD method. The modeling results of dBz/dt from the convolutional algorithm shows drastic oscillations for both on- and off-time. That＇s due to the low precision of second derivative of transmitting current. However, the results obtained from FETD are stable and smo