Sources of Induced Polarization Effects 2 IP effects related in chemical energy storage in rock structures •Membrane/electrolytic Polarization - result of variations in the mobility of ions in fluids through rock structures -may occur in rocks that do not contain metallic minerals -constitutes background or so-called normal IP effect •Electrode Polarization or overvoltage -result of variations between ionic and electronic conductivity where metallic minerals are presented -larger in magnitude than background IP

Membrane/Electrolytic Polarization Membrane effect is a feature of electrolytic conduction. It arises from differences in the ability of ions in pore fluids to migrate through a porous rocks. The minerals in a rock generally have a negative charge at the interface between the rock surface and pore fluid and thus attract positive ions in the pore fluid. The positive ions accumulate on the grain surface and extend into the adjacent pores, partially blocking them.

Membrane/Electrolytic Polarization When an external voltage is applied, positive ions can pass through the cloud of positive charge but negative ions accumulate, unless the pore size is big enough to allow them to bypass the blockage. This causes temporary accumulations of negative ions, giving a polarized ionic distribution in the rock. The ionic build-up takes a short time after the voltage is switched on; when the current is switched off, the ions drift back to their original positions. This process of ion redistribution show a decaying voltage as an IP effect. The membrane IP effect is most pronounced in rocks containing clay minerals in which the pore size is small, and the small clay grains are relatively strongly charged and adsorb ions on their surfaces.

Electrode Polarization Electrode polarization occurs when metallic minerals are present in the rock. The flow of electrons through the metal is much faster than the flow of ions in the electrolyte, so the opposite charges accumulate on facing surfaces of a metallic grain that blocks the path of ionic flow through the pore fluid. An overvoltage builds up for some time after the external current is switched on. The magnitude of the effect is related to the metallic concentration. After the current is switched off, the accumulated ions diffuse back to their original positions and the overvoltage decays slowly. This is similar to the effects on the polarization on electrodes in electrolyte fluid, hence the name

Electrode Polarization Electrolytic flow in upper pore, Electrode polarization in lower pore

Electrode & Membrane Polarizations Electrode polarization as well as membrane polarization is essentially a surface phenomenon. The IP effect decreases with increasing porosity as more alternative paths become available for the more efficient ionic conduction. Saline waters exhibit very poor IP response, because their high conductivity does not allow for any ion accumulation. Almost all sulfides, some oxides such as magnetite, ilmenite, pyrolusite, and cassiterite, and graphite provide good IP effect (electrode polarization). The IP effect is therefore greater if the metallic ore or clay is disseminated rather than compact. Thus IP method is suitable for disseminated ore exploration.

Chargeability of minerals and earth materials

Metal factor of earth materials

Interpretation of IP Data •An IP pseudosection, qualitatively, represents an ‘electrical vertical section’ that reflects both lateral and vertical variations of the IP effect in the ground. •IP pseudosections provide a convenient image of the presence anomalous conductors but does not represent their true lateral and vertical extent. •True IP distribution can be obtained by ‘Inversion’ method like in resistivity method. •Quantitative interpretation for IP data is more complex than resistivity method, so there are only few published studies of inversion of IP field data.