Induced Polarization (IP) has been used for many years by Geophysicists in search for mineral and water deposits. In recent years Induced Polarization has also become a popular solution to finding sources of petroleum contamination in water. IP is a valuable source of information in the metal mining industry.
WHAT IS IP?
In rock-fluid systems, electric energy can be stored for short periods of time. An applied current will induce a small polarizing voltage which decays with time after the current is switched off. Induced polarization is more pronounced in mineralized rocks.
Induced Polarization is a geophysical property of formations within the earth that causes received electrical square waves to have a gradual voltage decay rather than the expected immediate transition from one voltage to another. Various theories are used to explain this apparent “chargeability” of the earth. Numerous papers have been written on the fact that water contained in the pore space of rock formations does exhibit an “IP effect”.
Polarization is attributed to the presence of interfaces between ionic and electronic conduction (electrode polarization) and to the presence of unequal ionic transport properties (membrane polarization). Sulphide minerals and graphite are sources of electrode polarization and clays and zeolites are sources of membrane polarization.
The induced polarization effects are important to logging and surface geophysics in two ways. A polarization measurement can detect the presence of conductive minerals even when concentrations are less than 1 percent. In resistivity measurements IP is sometimes considered a nuisance factor. Resistivity measurements made at one frequency will differ from resistivity measurements made at another frequency unless they are corrected for the effect of polarization. The effect becomes beneficial in frequency domain I.P. where percent frequency effect is measured.
WHAT IS THE IP EFFECT?
It is well known that… “Ions are the carriers of electrical current in a liquid.” It is also well known that electronic conductivity in mineral bearing rock formations is responsible for a larger IP effect. Because it takes time for ions to move within a liquid, a “storage delay” occurs when a voltage is applied to a formation containing liquid within its pore space. When a voltage is applied, and suddenly removed, a time domain voltage decay is observed. The effect is that when the voltage has been removed, a gradually reducing transient voltage is still present. This time related effect is known as “Time Domain” IP.
TIME DOMAIN MEASUREMENTS:
IP EFFECT AND PERCENT IP:
The most simple method of measuring the IP effect in time domain IP is comparison of the receiver transient voltage at time (t) after transmitter voltage cutoff v(t) to the steady voltage (vc). The result is given the terms millivolts per volt or percent IP.
IP effect = V(t)/Vc percent IP = 100 V(t)/Vc
DECAY TIME INTEGRAL:
When potential is integrated over a defined period of time of the transient decay, a decay time integral is obtained.
Chargeability (M) is defined as: M = 1/vc x integral of t1 to t2 x v(t) x dt
The resultant chargeability is expressed in milliseconds.
FREQUENCY DOMAIN IP:
When alternating currents of two or more frequencies are applied to a formation, an IP frequency effect is observed. The IP effect related to frequency is referred to as “Frequency Domain” IP. Apparent resistivity (ra) at two frequencies (rdc) and (rac) are used in definition of frequency effect (Fe).
Fe = (rdc-rac)/ rac = (rdc/rac) - 1
Percent Frequency Effect (PFE):
PFE = 100 (rdc-rac)/ rac