Induced Polarization is a phenomenon where the potential difference did not drop instantaneously to zero when the current was turned off. Instead, the potential difference dropped sharply at first, then gradually decayed to zero after a given interval of time. This phenomenon is called induced polarization effect or I.P effect.
In 1950’s onwards, induced polarization surveys were carried out using conventional 4-electrode systems for mineral exploration, particularly for conductive sulfide ore bodies.
How It Works?
I.P. effect is caused by two main mechanisms, the membrane polarization and the electrode polarization effects. The membrane polarization effect is largely caused by clay mineral present in the rock or sediment. Therefore, I.P survey is suitable to differentiate between clay and water in groundwater exploration.
Meanwhile, the electrode polarization effect is caused by conductive minerals in rocks such that the current flow is partly electrolytic (through groundwater) and partly electronic (through the conductive mineral). This effect make I.P survey suitable for metallic mineral exploration such as disseminated sulphides.
I.P. measurements are made in time-domain or frequency domain. The time domain I.P. unit (chargeability) is usually given in millivolt per volt (mV/V) or milliseconds (ms).
Recent development in the modern multi-electrode system, the resistivity meter has capability to measure induced polarization. Fortunately, data acquisition (field setup) for induced polarization is similar to the electrical resistivity imaging (ERI). The measurement for induced polarization take place after resistivity measurement is taken. Typically, resistivity meter used by us measures the I.P data in time domain format.
Advanced data processing using specialist software is used to produce two-dimensional (2-D) cross-sectional model of induced polarization. Typically, for measurement using time-domain format, the model is deliver in term of chargeability variations with depth. Figure below is an example of 2-D I.P profile or model.
From here, our geophysicists will analyst the results and provide our client with detail interpretation of the ground.