Induced Polarization (IP) effects were first reported as early as 1912 and the method has been in use since the late 1940s. IP is used primarily for mineral exploration, and is also being developed as a tool for geothermal, hydrological and environmental applications. Essentially IP is an extension of traditional resistivity surveying. The ground is characterised not only by its resistivity, but also by its chargeability (how well it holds electrical charge). This means that the geophysicist has at least two diagnostic tools for the ground, making interpretation of the subsurface more certain.
The IP method involves passing a current through the ground through two current electrodes (‘C’) and measuring a voltage across two potential electrodes (‘P’). The same configurations of electrodes, or arrays, can be used as in resistivity surveying, but the most popular are the dipole- dipole and pole-dipole arrays. The depth of investigation for any electrode spacing “a” is increased by using greater electrode pair separation n values.
When a current is injected into the ground, the ground charges up, or polarises, like a capacitor. When the current is turned off, the induced charge takes a finite time to dissipate. The amount of charge that the ground will hold and the time it takes to acquire and dissipate that charge is diagnostic of the materials in the ground. In IP surveying, non-polarisable electrodes are used (electrodes that do not retain any charge) so that all the signal observed comes from the ground itself. IP surveying can be carried out in vertical sounding, horizontal profiling or 2D and 3D tomography modes. The majority of IP surveys are carried out using either time domain or frequency domain IP techniques.
Applications:
- Landfill Investigation
- Mapping and monitoring leachate plumes
- Mapping and monitoring of groundwater pollution
- Determination of depth to bedrock
- Locating sinkholes / cave systems
- Locating buried channels
- Mapping buried dykes and other ore bodies
- Locating fissures, faults and mineshafts
- Buried foundation mapping
- Time-lapse infiltration studies
- Assessment of aquifer heterogeneity
