An explanation of abnormal TEM responses: coincident-loop negatives, and the loop effect.

Electromagnetic transients measured with coincident transmitter and receiver loops are usually positive at all delay times. The response occasionally switches to negative at late times; a possible cause of which is an induced electrical polarization. Unfortunately, it is not possible to explain the negative transients with a half-space or a single bedrock conductor unless the polarizability of the model is unrealistically large. A negative response can be obtained with a realistic polarizability if the coincident-loop response of a body is large and positive at early time and decreases very rapidly. The current associated with the positive response polarizes the body: the larger the positive response, the greater the polarization. However, the positive response must also decay away rapidly, so that the smaller response associated with the polarization will be observed as a negative transient at late time. A body with such a response is a conductive overburden. Because the large early-time currents induced in an overburden flow close to the transmitter loop, the most significant polarization will occur here. When this polarization is large, the sign of the vertical-field response will be reversed when the receiver is inside, or coincident with, the transmitter loop. If the polarization is small, the distortion of the response near the wire is similar to what is termed 'the loop effect'. These abnormal TEM responses can be modelled with a weakly polarizable overburden, suggesting that polarization effects are a plausible explanation.