Exploration Geophysics - Volume 9, Issue 4, 1978

The method advocated by Crone (1977) for determining the position of a "current axis" for a conducting ore body will give a positive result even if there is no ore body.

The depth, ℓ, to this axis in a uniform ground of conductivity σ and permeability p0 is given by:

In this expression t is time and consequently the result can be used to determine the conductivity profile of a nearly uniform ground.

The response of a coincident horizontal loop transient electromagnetic (TEM) system is expected to decrease as the loop is lifted off the earth. This effect is calculated for a uniformly conducting earth for a range of loop heights, ground conductivities, loop sizes, and delay times. Within the assumptions of the model, the calculations are exact and valid for any height.

It is shown that signal attenuation with respect to height does not preclude the use of a helicopter TEM system. It is also shown that for ground TEM surveys where the loop is held off the ground because of brush, the maximum attenuation that could be expected from a uniform half space is less than 20%.

The transient electric field excited by a step current in a loop of radius a on the surface of a ground of conductivity σ and permeability μ0 are concentrated in a toroidal zone. At early times the zone moves slightly inward from the loop and down but at later time the field maximum moves along a cone dipping at 30° away from the loop. At later times the maximum occurs beneath a point on the surface at radius R where R² ≈ 2.56 t/σμ0. At this maximum the time rate of change of the radial magnetic induction is zero.