Download Induced Polarization (IP)Method

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 & 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.
Induced Polarization Measurements
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
Time-Domain IP
•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
or
α = area beneath overvoltage curve between time interval ta,tb
M = chargeability [mVs/V = ms or millisecond]
Chargeability of
minerals
and earth materials
Frequency-Domain IP
•Measurement of apparent resistivity at 2 or more AC current
frequencies.
•Frequency effect is usually defined as
 f  F
FE 
F
 f = apparent resistivity measured at DC or
at very low AC frequency (0.05-0.5Hz)
 F = apparent resistivity measured at very high AC frequency
(0.1-10Hz)
Frequency-Domain IP
•Percentage frequency effect, PFE
PFE  100(  f   F ) /  F
•Another frequency-domain measure of IP is Metal factor, MF
 f  F
5 FE
3 PFE
MF  2 10
 2 10
 2 10
 f F
f
f
5
•Unit of MF is the same as conductivity [mho/m or or siemen/m ]
Metal factor of earth materials
Field IP Procedures
•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.
AB = current electrodes
MN = potential electrodes
Electrode configuration and movement along survey profile
Data Plotting and Contouring
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.
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.
Homework Exercise
The 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
Potential dipole
10S-9S
9S-8S
8S-7S
7S-6S
6S-5S
5S-4S
4S-3S
3S-2S
2S-1S
n=1
ρa/2π
180
210
270
315
480
330
1091
1200
n=2
MF
28
31
42
39
40
88
46
31
ρa/2π
190
275
280
80
220
1120
1130
1510
n=3
MF
24
36
35
172
17
41
29
27
ρa/2π
280
270
290
72
70
675
1751
1830
1710
MF
27
33
60
219
175
99
61
31
28