Induced Polarization

Induced polarization (IP) is a second-order resistivity measurement that quantifies the charge storage capacity of earth materials.  IP data may be acquired in a variety of ways depending on instrumental design and environmental conditions.  IP data are traditionally acquired in either the time domain or frequency domain, each approach having its own benefits and short-comings.  IP data are typically acquired as an adjunct to a resistivity survey and require instrumentation specifically designed for IP acquisition.  As with resistivity, IP measurements are made using the same arrays, wires, and electrodes (usually).  The singular difference is the instrumentation required.

While originally developed for the prospection and characterization of mineral deposits, which represent well polarizable targets, in recent years the value of the IP method also has been recognized for near-surface studies in relatively low-polarizable, sedimentary environments.  The latter development has become possible due to considerable improvements over the last decade in instrumentation, macroscopic modelling and inversion techniques, and the understanding of the microscopic origin of IP.  Promising applications of the IP method are particularly seen in the rapidly emerging fields of hydrogeophysics and biogeophysics, including for instance the characterization of hydraulic properties or the monitoring of biogeochemical processes in the subsurface.

TDIP waveform.

Time domain IP (TDIP) consists of making an areal measurement of voltage decay after the cessation of transmitted current.  The area under a designated portion of the decay curve can be integrated and offered as an IP measurement.  Alternatively, IP data can be given as discrete values (of the decay curve or charging curve) at specified times (relative to current switching).  Decays are always measured for both positive and negative polarities to avoid DC offsets due to self potential and tellurics.  In order to make measurements comparable from one location to another or from one array to another, Newmont developed a “standard” for time domain IP measurement.  Other standards have been offered, but the principal goal in making an IP measurements is to determine anomalous responses from background responses.

FDIP waveform.

Frequency domain IP (FDIP) can be measured in two basic ways; a change in amplitude of the resistivity value at two frequencies, and, a phase shift of the received waveform relative to the transmitted waveform.  The amplitude differencing method is referred to as a “Percent Frequency Effect” or PFE and is generally made at two frequencies a decade apart in frequency; e.g. 0.1 to 1.0 Hz, although numerous variations of this approach have been used.  Peak-to-peak amplitudes avoid DC offsets due to self potential and telluric noise.  Although the image shows waveforms in the time domain, measurements are usually based on the amplitudes of the fundamental harmonics of the desired frequencies.

Phase waveform.

Phase measurements are usually made by comparing the phase of the fundamental harmonic of the transmitted waveform (usually but not necessarily a square wave) with the fundamental harmonic of the received waveform.  This approach allows the use of only one frequency but requires good phase stability of the transmitter.  Unlike amplitude measurements or time domain measurements, phase measurements are insensitive to DC offsets due to self potential and tellurics.

When both phase and amplitude measurements are made the resultant value is referred to as complex resistivity or complex impedance.  When multiple frequencies are used and both phase and amplitude are measured for all frequencies it is referred to as spectral IP (SIP) or spectral complex impedance.

The foregoing describe how IP data are instrumentally determined for a single measurement.  In order for an IP measurement to have relevance it must be compared with many other IP measurements as gathered, say, along a profile or in a borehole.  This requires a multitude of measurements, most commonly collinearly, so that sufficient background is obtained in order to establish the amplitude of anomalous response as well as its location and distribution.  Sufficient data coverage then allows for modeling and inversion of the data.

Results of resistivity and IP imaging at the kerosene-contaminated site. In addition to the inverted images of resistivity (middle) and phase (right), the lithologic stratification as determined from cored drillings as well as the occurrence of contamination as proved by chemical sample analyses is shown (left). (Taken from Slater et al, 2006)