Induced Polarization (IP) applies the same basic operation and array configuration as resistivity method that If you apply the current into the ground via current electrodes , the voltage can be measured at potential electrodes which is non - polarizable in IP method.
The Earth accumulates the charges of applied current at some time, but when the applied current is switched off, the overvoltage will tend to decay slightly (slowly) from its peak value to zero after a certain time interval. This is the capacitive property of the Earth's ground. You can measure the IP effect by using either chargeability or apparent resistivity.
Let's see through this post how these two (2) measurements can be deployed to describe IP effect.
What are the two (2) commonly applied methods in IP measurements?
Here below are two (2) common IP measurement methods,
i/ Time domain (Pulse Transient)
ii/ Frequency domain
1: time domain ip measurement
It measures Overvoltage as a function of time.
When the current is applied to the ground and switched off a few moments later an overvoltage decay results. This is due to the capacitive nature of the Earth's ground.
It monitors the decaying voltage after the current is switched off.
Figure 1: The graph of overvoltage decay against time (Reynolds,1997)
Measure of Induced Polarization (IP) Effect.
Chargeability (M): Is the ratio of Overvoltage (Vp) to Initial voltage (Vo).
i.e M = Vp/Vo
Chargeability is expressed as milliVolts per volt (mV/V) or percent (%).
N.B: It is very difficult and virtually impossible to measure true chargeability in the field as each layer will have its own chargeability.
It then advised to measure Apparent chargeability (Ma) as a fraction of the steady - state voltage.
If you consider the figure 1 above, Chargeability is defined by the area A, traced when the voltage decay between time interval (t1 to t2)
Such that,
Ma = A/∆Vo= 1/∆Vo ∫ Vp(t)dt
limit --> t1 to t2.
Apparent chargeability is expressed in unit of time such as Millisec (Ms)
N.B: Major advantage of integration and normalizing by dividing by V is that noise from cross coupling background potentials is reduced.
2: frequency domain ip measurement
Apparent Resistivity (A.R) is measured at two (2) frequencies lower than 10 Hz. Example between 0.1 and 5 Hz, 0.3 and 2.5 Hz or 0.1 and 10 Hz
Electrode array is the same as in the time domain.
Since Apparent Resistivity at low frequency is greater than Apparent Resistivity at high frequency.
Hence, At short charging time the measured overvoltage is apparently lower than obtained for longer charging time.
Such as shorter charging time, gives higher frequency and vice versa is true.
Since, frequency varies inversely with time
f = 1/T
frequency effect (FE)
FE = [A.Ro - A.R1]/A.R1
Where
A.Ro and A.R1 are Apparent Resistivities at low and high frequency respectively.
Since A.Ro is greater than A.R1
or A.Ro is approximately equals to Resistivity due to Direct current and
A.R1 is approximately equals to Resistivity due to Alternative current
N.B: Frequency Effect (FE), in frequency domain is the same as chargeability in time domain for weakly polarizable Medium.
IP measurement has gotten much attention especially in exploring mineral deposits with high conductivity such as disseminated metallic ores that are difficult to be detected by Electromagnetic and Resistivity methods.
I hope this will help you!
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