A FETD forward modelling algorithm for transient electromagnetics excited by grounded‐wire sources

ABSTRACT A three‐dimensional finite‐element time‐domain forward‐modelling algorithm is developed to simulate transient electromagnetics excited by grounded‐wire sources. The main advantage of this finite‐element time‐domain algorithm is that full transmitting‐current waveforms and complex‐shaped sources resulting from topography can be directly dealt with in this algorithm. The models used to test this algorithm include a homogeneous half‐space model, a stratified‐medium model, the model of a complex conductor at a vertical contact and the Ovoid Zone massive sulfide deposit at Voisey's Bay, Canada. The homogeneous half‐space model is used to determine the truncation boundary for a computational domain, and to compare with the electromagnetic responses excited by step‐off, step‐on and direct current waveforms. For the stratified‐medium model, results demonstrate that full transmitting waveforms have strong effects on the observed electromagnetic responses. The model of a complex conductor at a vertical contact is designed for the grounded electrical source airborne transient electromagnetic method and is also used to examine the effectiveness of the broadside and inline configurations for such a vertical, thin plate embedded in the subsurface. The area of the Ovoid Zone massive sulfide deposit possesses non‐negligible topography, the effects of which on the shapes of the grounded‐wire sources must be taken into account when implementing the finite‐element time‐domain solver. The results show that both the broadside and inline electromagnetic responses are strongly affected by the massive conductive ore body.