Mapping of induced polarization using natural fields

Abstract

The observed electromagnetic response of a finite body is caused by induction and polarization currents in the body and by the distortion of the induction currents in the surrounding medium. At a sufficiently low frequency, there is negligible induction and the measured response is that of the body distorting the background currents just as it would distort a direct current (dc). Because this 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 result from frequency-dependent conductivity. Profiles of lowfrequency 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 electric field profile over the body becomes frequency dependent and phase shifted with respect to a reference field.The technique was tested on data acquired in a standard continuous profiling magnetotelluric (MT) survey over a strong induced polarization (IP) anomaly previously mapped with a conventional pole-dipole IP survey. The extracted IP response appears in both the apparent resistivity and the normalized electric field profiles.

Feasibility of Natural Source Induced Polarization

“…Based on the knowledge of the frequency which reached the DC limit and using the EMAP technique, they designed the observation schemes of measuring the real and imaginary components of electric field, the PFE and the impedance phase for the lower frequencies (0.1 Hz to 0.001 Hz) for the TM mode (electric field E perpendicular to strike), in order to realize the IP survey using natural electromagnetic fields. It is worth to notice that they [8,9] themselves also pointed out that the method cannot work in a layered earth because in this case there is no DC limit -induction is always present. Actually, the geoelectric structure in the earth is generally a layered earth.…”

Audio-magnetotelluric survey to characterize the Sunnyside porphyry copper system in the Patagonia Mountains, Arizona

“…The MT method measures and records the Earth's natural time-varying electromagnetic signals as they pass through the subsurface. A relatively new technique, known as natural field induced polarization (Gasperikova and Morrison, 2001), extracts induced polarization parameters from MT data. To do so, however, the skin depth of the target must be known in order to distinguish the electromagnetic response of the mineral deposit from the response of the surrounding host medium.…”

New Methodology for Computation of the Earth?s Polarizability from Telluric Borehole Logging Measurements Taken at Different Frequencies

“…Achieving an adequate signal-to-noise ratio therefore requires excessive power or costly attempts to reduce contact resistance at the current electrodes. Even for exploration at shallow depths, the need to inject current reduces the usefulness of the technique in many situations in which it is difficult to achieve low contact resistances (GASPERIKOVA and MORRISON, 2001).…”