IP effects in 3-D TEM data ? theory and case histories

The existence of negative responses in coincident-loop transient electromagnetic (TEM) data has been known for some time (e.g. Spies 1974; Lee 1975). Explanations of these responses have included instrumental errors, conductor geometries, current channelling and induced polarization (IP) effects. Theoretical studies by Bhattacharyya (1964), Morrison et at. (1969), and Weidelt (1982) established that the effects of a dispersive conductivity could cause sign reversals in coincident-loop data. These IP effects have since been computed for layered media (Raiche et al. 1985), 2-D horizontal cylinders (Lewis and Lee 1984), and spheres (Lee 1975). With the advent of the ability to numerically model 3-D conductors in conductive media (Newman et al. 1986) it has become desirable to extend the study of IP effects to this class of model.