Induced polarization

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Induced polarization (IP) is a geophysical imaging technique used to identify the electrical chargeability of subsurface materials, such as ore. [1] [2]

Contents

The polarization effect was originally discovered by Conrad Schlumberger when measuring the resistivity of rock. [3]

The survey method is similar to electrical resistivity tomography (ERT), in that an electric current is transmitted into the subsurface through two electrodes, and voltage is monitored through two other electrodes.

Induced polarization is a geophysical method used extensively in mineral exploration and mine operations. Resistivity and IP methods are often applied on the ground surface using multiple four-electrode sites. In an IP survey, in addition to resistivity measurement, capacitive properties of the subsurface materials are determined as well. As a result, IP surveys provide additional information about the spatial variation in lithology and grain-surface chemistry.

The IP survey can be made in time-domain and frequency-domain mode:

In the time-domain induced polarization method, the voltage response is observed as a function of time after the injected current is switched off or on. [4]

In the frequency-domain induced polarization mode, an alternating current is injected into the ground with variable frequencies. Voltage phase-shifts are measured to evaluate the impedance spectrum at different injection frequencies, which is commonly referred to as spectral IP.

The IP method is one of the most widely used techniques in mineral exploration and mining industry and it has other applications in hydrogeophysical surveys, environmental investigations and geotechnical engineering projects. [5]

Measurement methods

Time domain

Typical transmitted current waveform and potential response for time domain resistivity and induced polarization measurements. Theoretical current waveform and potential response for time domain resistivity and induced polarization measurements..png
Typical transmitted current waveform and potential response for time domain resistivity and induced polarization measurements.

Time-domain IP methods measure considers the resulting voltage following a change in the injected current. The time domain IP potential response can be evaluated by considering the mean value on the resulting voltage, known as integral chargeability [2] or by evaluating the spectral information and considering the shape of the potential response, for example describing the response with a Cole-Cole model. [6]

Frequency domain

Frequency-domain IP methods uses alternating currents (AC) to induce electric charges in the subsurface, and the apparent resistivity is measured at different AC frequencies.

See also

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Soil resistivity is a measure of how much the soil resists or conducts electric current. It is a critical factor in design of systems that rely on passing current through the Earth's surface. It is a very important parameter for finding the best location of a transmitter working on low frequiencies as such radio stations usually use ground as counterpole. An understanding of the soil resistivity and how it varies with depth in the soil is necessary to design the grounding system in an electrical substation, or for lightning conductors. It is needed for design of grounding (earthing) electrodes for substations and High-voltage direct current transmission systems. It was formerly important in earth-return telegraphy. It can also be a useful measure in agriculture as a proxy measurement for moisture content.

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References

  1. Bleil, David Franklin (July 1953). "Induced Polarisation: A Method of Geophysical Prospecting". Geophysics . 18 (3): 636–661. doi:10.1190/1.1437917 . Retrieved 2018-05-15.
  2. 1 2 Zonge, Ken; Wynn, Jeff; Urquhart, Scott (2005). "Chapter 9. Resistivity, Induced Polarization, and Complex Resistivity". In Butler, Dwain K. (ed.). Near-Surface Geophysics - Investigations in Geophysics. Society of Exploration Geophysicists (SEG). pp. 265–300. doi:10.1190/1.9781560801719.ch9. ISBN   978-1-56080-130-6.
  3. Allaud, Louis A.; Martin, Maurice H. (1977-10-01). Schlumberger - The History of a Technique . Translated by Schwob, Marcel. New York, USA: John Wiley & Sons, Inc. ISBN   978-047101667-0.
  4. Olsson, Per-Ivar; Dahlin, Torleif; Fiandaca, Gianluca; Auken, Esben (2015). "Measuring time-domain spectral induced polarization in the on-time: decreasing acquisition time and increasing signal-to-noise ratio" (PDF). Journal of Applied Geophysics. 123: 316–321. doi:10.1016/j.jappgeo.2015.08.009. hdl: 2434/724249 . ISSN   0926-9851.
  5. "Induced Polarization (IP) What Is It?". Surface Search Inc. 2018. Archived from the original on 2018-05-17. Retrieved 2018-05-06.
  6. Pelton, William H.; Ward, Stanley H.; Hallof, Philip G.; Sill, William R.; Nelson, Philip H. (1978-04-01). "Mineral discrimination and removal of inductive coupling with multifrequency IP". Geophysics . 43 (3): 588–609. doi:10.1190/1.1440839.

Further reading