Find the electrical charge of the subsurface for mineral exploration, environmental assessment and engineering geophysics.
The Induced Polarisation (IP) method extends the resistivity methods by making an additional measurement of the ability of the ground to store electrical charge. Originally developed for mineral exploration, it is now finding applications in the fields of environmental and engineering geophysics. The method measures the rates with which electrical charges build up in the ground due to applied voltages, and with which they re-equilibrate after such voltages are removed. Common sources of the effect are charge polarisation on individual grains, charge buildup within clay layers, and electrochemical interactions at grain surfaces.
As the IP effect results from currents passing through the ground, IP surveys measure resistivity in addition to some index of polarisation. The polarisation index that is reported is different for different IP equipment; it may be chargeability in mV-sec/V, per cent frequency effect, or phase shift in milliradians (mR) between transmitted and received signals. The equipment used in IP surveys is similar to that used for electrical resistivity, with measurements being made of both the resistivity and chargeability of the subsurface. The survey typically comprises several ground electrodes in deployed an array connected to a computer-controlled multi-channel receiver.
Typically most inversion software for IP data utilises algorithms such as the complex resistivity method which incorporates both apparent resistivity methods which incorporate both apparent resistivity and IP datasets. As such data from IP surveys are commonly presented as one cross-section for resistivity and one for chargeability in the various polarisation index used.
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