Mapping of induced polarization using natural fields

The observed electromagnetic response of a finite body is caused by inducti on and polarization currents in the body and by the distortion of the induc tion currents in the surrounding medium. At a sufficiently low frequency th ere is negligible induction and the measured response is that of the body d istorting the background currents just as it would distort a direct current (dc). Because this de response is not inherently frequency dependent, any observed change in response of the body for frequencies low enough to be in this de limit must result from frequency-dependent conductivity. Profiles of low-frequency natural electric (telluric) fields have spatial anomalies over finite bodies of fixed conductivity that are independent of frequency and have no associated phase anomaly. If the body is polarizable, the elect ric field profile over the body becomes frequency dependent and phase shift ed. with respect to a reference field. The technique was tested on data acquired in a standard continuous profilin g magnetotelluric (MT) survey over a strong induced polarization (IP) anoma ly previously mapped with a conventional pole-dipole IP survey. The extract ed IP response appears in both the apparent resistivity and the normalized electric field profiles.