Transient electromagnetic smoke rings in a half-space during active transmitter excitation

Abstract

The smoke-ring concept is a useful device for understanding how the electromagnetic fields induced in a 1D 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 have calculated the current in a half-space from a half-sine excitation (which provides a continuous induction). In comparison, the current pattern from a continuously excited waveform is more densely distributed in the 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, in which the primary field coupling changes and is an important noise source, a primary field-stripping algorithm affects the current distribution but does not deleteriously affect near-surface sensitivity.