Home
Tutorial Top


Induced Polarization

Polarizable Ground

Current injected into the ground causes some materials to become polarized. There are two microscopic causes of this macroscopic effect. The phenomenon is called induced polarization, and the physical property that is measured is often called chargeability. The following figure illustrates. Note how the measured potential exhibits a delayed response when ground is chargeable. Chargeable ground may take several seconds to return to equilibrium after it has been polarized with a current source.
.
.

Two microsopic effects that cause ground to be chargeable:

1) Membrain polarization is described with the next three figures.
 
Membrain polarization occurs when pore space narrows to within several boundary layer thicknesses.
Charges accumulate when an electric field is applied.
Result is a net charge dipole which adds to any voltage measured at the surface.
2) Electrode polarization is described with the next two figures.
 
Electrode polarization occurs when pore space is blocked by metallic particles. Again charges accumulate when an electric field is applied.
The result is two electrical double layers which add to the voltage measured at the surface.

Measured IP data

Not that this material is similar to that provided on the IP page of the inversion methodology tutorial.

Different types of IP data [top]

Two types of time domain data [top]

  1. Dimensionless, where  M = (s) / (m),  using parameters from the adjacent waveform diagram. However this form is less common since it is impractical to measure.
  2. The most commonly measured form of time domain IP is the area under the decay curve, specified by the following equation, using parameters specified in the figure.

Two types of frequency domain data [top]

  1. Data with units known as percent frequency effect (PFE) require the response to be measured at two frequencies. At higher frequencies, the ground has less time to respond, therefore the signal is expcted to be smaller. Below is the equation providing PFE, and a figure illustrating how the data are gathered.
  2. Data with units of phase are gathered by maintaining careful synchrony between transmitted sine wave and the received signal. Then the phase difference between the source and received signals is recorded as a measure of chargeability. Units are usually millradians. The following figure illustrates:


Home | Tutorial top
Email Webmaster