Induced Polarization (IP) is a geophysical method which indirectly measures the chargeability of the subsurface by using voltage decay of a produced current. Injecting a direct current into the ground and then abruptly turning that current off, the induced voltage will decay over some time. Measuring the decaying voltage as a function of time it is possible to calculate the apparent chargeability. If the subsurface has a high chargeability, the decay time will be large (the ground will remain electrically charged longer). On the other hand, if the subsurface is not very chargeable, the decay time will be very short (fast decay). Many metals or sulfides have high chargeability while dry igneous/metamorphic rocks will have very low chargeability. As a result, this method is most often used in mineral exploration.
The Induced Polarization method exploits this property. It consists in the injection of a DC electric current transmitted into the ground like in the ERT method (injection time windows are larger). Then, the current is switched off and the decaying voltages at various electrodes are measured. As a result, in addition to the resistivity measurements, capacitive properties of the subsurface material are determined as well.
There is also a frequency-domain version of IP known as Spectral IP which is slightly different but the main goal — finding chargeability of the subsurface — remains the same.