A finite-element time-domain forward-modelling algorithm for transient electromagnetics excited by grounded-wire sources

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.