Three-dimensional forward calculation of time domain electromagnetic response stimulated by a pseudo-Gaussian pulse source

Abstract

The calculation of three-dimensional (3D) electromagnetic responses using the controlled source time domain electromagnetic method suffers from low accuracy of the early response calculation and difficulty in the convergence of the later response for large-scale models. To address these problems, we propose a method of decomposing the time domain electromagnetic response into a primary field and a secondary field. This approach can easily address the above issues. In this paper, we use a time domain finite difference method based on a non-uniform step size and staggered grid and use an iterative algorithm with a non-uniform step size to calculate the late time domain electromagnetic response under high power pulse source excitation. Several example simulations were compared with analytical and spectral Lanczos decomposition solutions and demonstrate the correctness of the 3D forward modeling method. The response of a 3D anomaly excited by an electric dipole source pulse current was analyzed. The calculated time domain response revealed that the field excited by the high power pulse source diffused in the earth over time and exhibited a secondary diffusion process of the anomalous field generated by the 3D anomaly. The numerical simulation results showed that the time domain electromagnetic responses excited by the pulse source were stable and effective. The amplitude of the anomalous response was much greater than that of the step source, the signal-to-noise ratio was greater, and the detectability was better. The method may be used in inversion of the time domain electromagnetic response if the calculation speed can be further improved.