Mapping of induced polarization using natural fields

The IP effect can be detected using natural fields in any finite body at low frequencies for which the body has a negligible induction number but exhibits a significant, complex, frequency dependent resistivity effect. The observed electromagnetic response of a conducting polarizable body is caused by induction and polarization currents in the body, as well as the distortion of induction currents in the surrounding medium. The polarization currents occur in a DC mode and the IP effect ill can be recognized when the DC response is clearly separable from the induction response. Since the DC response is not inherently frequency dependent, any observed change in response of the body for frequencies low enough to be in this DC limit must be associated with changes in the body conductivity with frequency. At such frequencies, the body distorts the currents in the medium but has no significant induction response on its own. Profiles of surface electric fields at these low frequencies are independent of frequency and show no associated phase shift unless the target body contains polarizable material. If the target is polarizable electric field ratio profiles, or the detection of phase anomalies with spatial extent comparable to the dimensions of the body, will reveal the IP response.;The detection of the IP response can be made more objective and quantitative if we apply a high pass spatial filter to extract the DC response and display, at each frequency, the residual lateral anomalies in electric field which are in the DC limit. This procedure eliminates any inductive response of larger scale conductive structures present in the data, but requires a continuous data profile that goes somewhat beyond the area of immediate interest.;Careful processing of data acquired in a standard continuous profiling MT survey has allowed the extraction of a clear IP response over a strong IP anomaly previously mapped with a conventional pole-dipole IP survey. The extracted IP response appears in both the apparent resistivity and normalized electric field profiles.