Transient electromagnetic smoke rings in a halfspace during active transmitter excitation

The smoke ring concept is a useful device for understanding how the electromagnetic fields induced in a 1-D earth propagate and diffuse through a medium. Aside from facilitating a physical understanding of field propagation, the smoke ring concept has been used to interpret behavior of vertical and radial magnetic fields at the surface and used to estimate depth of penetration for conductivity-depth transforms. Past studies have focused on the current distribution during the off-time. We calculate and illustrate the current in a halfspace from a half-sine excitation (which provides a continuous induction). In comparison, the current pattern from a continuously excited waveform is more densely distributed near-surface than the off-time current system, suggesting that measurements during a continuously excited on-time are more sensitive to shallow targets. For airborne applications, where the primary field coupling changes and is an important noise source, a primary field-stripping algorithm impacts the current distribution but does not deleteriously affect near-surface sensitivity.