Induced Polarization (IP)Method

•IP is a current-stimulated electrical phenomenon observed as a delayed voltage response in earth materials.

•It has been used extensively in the search for disseminated mineralization in base-metal and to a minor extent in groundwater search.

•In recent decades the IP method has found increasing applications in groundwater and environmental studies.

IP Principles• An Illustration of IP can be obtained with a standard 4-electrode

dc resistivity spread.

• When the current is abruptly interrupted, the voltage across the potential electrodes does not drop to 0 immediately but decays slowly after an initial large decrease of its steady-state value .

•This decay time is of the order seconds or even minutes.•Conversely, if the current is switched on again, the potential increases suddenly at first and then gradually reaches the steady- state value.•The slow decay and growth of part of the signal are due to “Induced Polarization”.

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 effect is like a membrane, which selectively allows the passage of one type of ion.

• This causes temporary accumulations of negative ions, giving a polarized ionic distribution in the rock.

Membrane/Electrolytic Polarization

• 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.

• The magnitude of IP effect varies in different types of clay, being low in montmorillonite and higher is kaolinite.

Electrode Polarization• Electrode polarization occurs when metallic minerals are

present in the rock.

• The current flow is partly electronic (through groundwater), partly electrolytic (through conductive mineral).

• The metallic grains conduct charge by electronic conduction, why electrolytic conduction take places around them.

• However, 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.

Electrode Polarization

(a) Electrolytic flow in upper pore, (b) Electrode polarization in lower pore

• 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.

Electrode & Membrane Polarizations

IP effect measurement may be made as as a function of time or frequency.

1. Time-Domain IP—measure decay voltage as a function of time after current switched off

2. Frequency-Domain IP– measure apparent resistivity at 2 or more frequencies generally below 10 Hz

Induced Polarization Measurements

•Measurements are made by sending a DC current into the ground, the magnitude of IP is expressed as

V(t)/Vc

V(t) = residual voltage = the voltage remaining at time t after the current is switched off

Vc = steady voltage = the voltage that existed when the current was flowing

V(t)/Vc is expressed as mV/V (millivolts per volt), or as a percent.

Time interval t may vary between 0.1 and 10 s of switching current on and off.

Time-Domain IP

•Commercial IP outfits generally register the decay voltage (Vt) over a definite time interval (ta,tb).

•The result is expressed by the time-integral measure of IP as

Time-Domain IP

or

α = area beneath overvoltage curve between time interval ta,tb

M = chargeability [mVs/V = ms or millisecond]

Chargeability of mineralsand earth materials

•Measurement of apparent resistivity at 2 or more AC current frequencies.

•Frequency effect is usually defined as

Frequency-Domain IP

F

FfFE

f

F

= apparent resistivity measured at DC or at very low AC frequency (0.05-0.5Hz)

= apparent resistivity measured at very high AC frequency (0.1-10Hz)

Frequency-Domain IP

•Percentage frequency effect, PFE

FFfPFE /)(100

•Another frequency-domain measure of IP is Metal factor, MF

•Unit of MF is the same as conductivity [mho/m or or siemen/m ]

ffFf

Ff PFEFEMF

355 102102102

Metal factor of earth materials

•IP equipment is similar to that used in resistivity surveys but is more bulky and elaborate.

•Field procedures for making IP measurements are identical to those employed for resistivity measurements.

•Theoretically, any standard electrode arrays used in resistivity surveys can be employed for IP measurements.

•Dipole-dipole array is used the most for IP surveys due to–Easy to use in the field routine

–The current and potential dipole cables are separated from one another,

spurious signals due to electromagnetic coupling are effectively reduced.

•Multi-electrode & roll-along system as used for resistivity surveys is also valid in IP survey to reduce time of operation and to provide more data coverage.

Field IP Procedures

Electrode configuration and movement along survey profile

AB = current electrodesMN = potential electrodes

Data Plotting and Contouring

•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.

Interpretation of IP Data

IP response from various theoretical models (a) Sphere (b) &(c) Ellipsoid (d) Two beds(c) Vertical contact (f) & (g) Vertical dike (h) to (j) Dipping dike

Advantages and Disadvantages of IP Surveys

•IP Method suffers from the same disadvantages as resistivity surveying.•The sources of significant IP anomalies are often not of economic importance, e.g. water-filled shear zones and graphite-bearing sediments can both generate strong IP effects.•Field operations are slow and is consequently is more expensive than other ground geophysical techniques such as gravity survey

•In spite of its drawbacks, the IP method is extensively used in base-metal 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 non-conducting and therefore are not easily detectable by electromagnetic mehtods.

•IP is by far the most effective geophysical method that can be used in the search for such targets.

Potential dipole n=1 n=2 n=3

ρa/2π MF ρa/2π MF ρa/2π MF

10S-9S

9S-8S

8S-7S

7S-6S

6S-5S

5S-4S

4S-3S

3S-2S

2S-1S

-

180

210

270

315

480

330

1091

1200

-

28

31

42

39

40

88

46

31

-

190

275

280

80

220

1120

1130

1510

-

24

36

35

172

17

41

29

27

280

270

290

72

70

675

1751

1830

1710

27

33

60

219

175

99

61

31

28

Homework ExerciseThe results in Table below were obtained using frequency-domain IP in a survey over suspected sulfide mineralization in northern New Brunswik, Canada. The dipole-dipole array was used with dipole separations of 100 ft and n=1,2,3. Resistivity values are in the form

ρa/2π Ω−ft. The grid line is roughly N-S with stations every 100 ft. In all cases the potential dipole was south of the current dipole.

Prepare pseudosection plots for ρa/2π and MF ; draw contours and interpret the results