Classification of sign changes in borehole TEM decays

Abstract

Sign changes at a particular receiver location are commonly observed in downhole TEM data as a function of delay time. There are many causes but generally they can be classified as being due to either a change in the sense of coupling between the downhole probe and the analagous secondary field, or a reversal of the analagous secondary field itself. The most common coupling-related reversals are due to analagous current migration in finite plate-like, overburden and host conductors. On the other hand, analagous secondary field reversals are more frequently seen in multiple conductor cases. In particular the impulse response exhibits a sign reversal for the case of a conductor directly under a transmitter placed on an overburden. In this case the step response does not. For the case of the same conductor located at some horizontal distance away from the transmitter, the step response may exhibit a sign reversal while the impulse response may exhibit two. The overall response is further complicated by the fact that the superimposed overburden response changes sign as well. Sign reversals may also arise from non-uniform primary field coupling, current channeling and induced polarization effects and these can add further uncertainty to the interpretation process.