A Plausible Mechanism for Generating Negative Coincident-Loop Transient Electromagnetic Responses

Abstract

Coincident-loop transient electromagnetic measurements are normally positive at all delay times, but occasionally they become negative at late delay times. The most likely cause of these sign reversals are polarizable conductivity structures (i.e. structures whose conductivity varies in the sense ∂σ/∂s > 0, where s is the Laplace transform variable representing the frequency). However, structures such as half-spaces, buried conductors or buried conductive layers are not capable of explaining the negatives unless the polarizabilities are implausibly large. The negatives can be explained with realistic polarizabilities if the structure is such that the following three 'favourable coupling conditions' are satisfied at the positions where negatives occur: (1) the transmitter couples well to the body, (2) at late delay times the electromagnetically induced current couples poorly to the receiver, and (3) the polarization couples well to the receiver. Polarizable structures which satisfy the favourable coupling conditions are: a buried conductor in a very resistive host, a thin conductive overburden, two interacting conductors, an overburden which comes to an edge and a thin dipping dyke. The field examples of negative transients presented in this thesis are all associated with these structures and the polarizabilities required to model the negatives are small, so the favourable coupling conditions provide a plausible general explanation of the negative transients.