TEM Coincident Loop Negatives and the Loop Effect

If the conductivity of the ground increases with frequency (i.e. the ground is polarisable) sign reversals can occur in coincident-loop transient-electromagnetic (CLTEM) measurements. The CLTEM response of a polarisable ground is comprised of two parts: the positive fundamental inductive response, and a weaker, negative, polarisation decay. The time dependence of the polarisation decay can be calculated using a convolution approximation.

CLTEM sign reversals (negatives) occur when the polarisation decay becomes greater than the fundamental inductive response. For most conductivity structures, negatives will not be observed unless the IP chargeability of the ground is exceptionally large. However, in special circumstances the fundamental inductive response is particularly small, and thus negatives can be produced by conducivity structures with geologically feasible chargeabilities. Examples of such circumstances are: in localised zones between inductively interacting conductors, at the edge of overburdens, and over relatively resistive grounds which have a thin polarisable surficial layer.

The polarisation current in a horizontal layer causes the vertical field response to be enhanced outside the transmitter loop and depressed inside the loop. When the response due to the polarisation current is measurable, but not large enough to cause negatives, this produces a situation known as ‘the loop effect’.