Induced polarization (IP) is part of electrical methods. In addition, it is part of the geophysical methods of prospecting and geophysical science, so its study is considered important.
Induced polarization (IP) principles
When using a standard four-electrode resistivity dispersion in a direct current mode, if the current is abruptly disconnected, the voltage between the potential electrodes does not immediately drop to zero.
After a large initial decrease, the voltage undergoes a gradual decrease and may take many seconds to reach a zero value.
A similar phenomenon is observed as the current is connected.
After an initial voltage surge, the voltage gradually increases over a discrete time interval to a steady-state value.
The earth acts as a capacitor and stores the electric charge, that is, it becomes electrically polarized.
If, instead of using a direct current source to measure resistivity, an alternating current source of variable low frequency is used, it is found that the measured apparent resistivity of the subsurface decreases as the frequency increases.
This is due to the ability of the earth to inhibit the passage of direct currents, but it transmits alternating currents with greater efficiency as the frequency increases.
The capacitive property of the earth causes both the transient drop of a residual voltage and the variation of the apparent resistivity as a function of frequency.
The two effects are representations of the same phenomenon in the time and frequency domains, and are linked by the Fourier transform.
These two manifestations of the ground capacitance property provide two different study methods for investigations of the effect.
The measurement of a decreasing voltage in a certain interval of time is known as the method of induced polarization (IP) in the time domain.
The measurement of apparent resistivity at two or more frequencies of low alternating current is known as surveying. induced polarization (IP) in the frequency domain.
Induced polarization mechanisms.
Laboratory experiments indicate that electrical energy is stored in rocks primarily through electrochemical processes.
This is accomplished in two ways.
The passage of current through a rock as a result of an externally imposed voltage is accomplished primarily by electrolytic flow in the pore fluid.
Most rock-forming minerals have a net negative charge on their outer surfaces in contact with the pore fluid and attract positive ions onto this surface.
The positive ion concentration extends approximately 100 mm into the pore fluid, and if this distance is of the same order as the diameter of the pore throats, the movement of ions in the fluid resulting from the impressed voltage is inhibited.
Negative and positive ions accumulate on both sides of the lock, and by removing the imprinted voltage, they return to their original locations for a finite period of time, causing a gradual drop in voltage.
This effect is known as membrane polarization or electrolytic polarization.
It is more pronounced in the presence of clay minerals where the pores are particularly small.
The effect decreases with increasing salinity of the porous fluid.
When metallic minerals are present in a rock, there is an alternative electronic path available for current flow.
When a voltage is applied to each side of the pore space, positive and negative charges are imposed on opposite sides of the grain.
Negative and positive ions then accumulate on each side of the bead as it tries to release electrons into the bead or accept electrons conducted through the bead.
The speed at which electrons are conducted is slower than the rate of exchange of electrons with ions.
Consequently, the ions accumulate on both sides of the grain and cause a buildup of charge.
When the impressed voltage is removed, the ions diffuse slowly to their original locations, causing a transient decreasing voltage.
This effect is known as electrode bias or overvoltage.
All minerals that are good conductors (for example, metal sulfides and oxides, graphite) contribute to this effect.
The magnitude of the electrode bias effect depends on both the magnitude of the impressed voltage and the mineral concentration.
It is most pronounced when the mineral is disseminated throughout the host rock, since the surface area available for ion-electron exchange is at its maximum.
The effect decreases as the porosity increases as more alternative pathways become available for more efficient ion conduction.
In the prospecting of metallic minerals, the interest is obviously in the effect of polarization (overvoltage) of the electrode.
Membrane polarization, however, is indistinguishable from this effect during temperature measurements. induced polarization (IP).
Consequently, the polarization of the membrane reduces the effectiveness of the inspections of induced polarization (IP) and causes geological “noise” that can be equivalent in magnitude to the surge effect of a rock with up to 2% metallic minerals.
Applications of induced polarization survey.
Despite its drawbacks, the method of induced polarization (IP) is widely used in base metals exploration as it has a high success rate in locating low-grade ore deposits such as disseminated sulphides.
These have a strong IP effect, but are not conductive and therefore not easily detectable by electromagnetic methods.
The induced polarization (IP) is by far the most effective geophysical method that can be used in the search for such targets (disseminated sulfides).
Although the deposit is low grade and contains less than 2% conductive minerals, the chargeability anomaly is well defined and centered over the ore body.
In contrast, the corresponding apparent resistivity profile reflects the large resistivity contrast between the ancient red sandstone and the dolomitic limestone, but gives no indication of the presence of mineralization.
Another example of the method of induced polarization (IP) is a traverse over a porphyry copper body in British Columbia, Canada.
The crosses of induced polarization IP and resistivity were performed at three different electrode spacings of a pole-dipole array.
The Constant separation traversing CST results show little variation in the body, but the IP (chargeability) profiles clearly show the presence of mineralization, allow to determine its boundaries and provide estimates of depth.