Self Potential Method (Electrical Survey)

Date : June 06, 2020 (Saturday) Time :- 11: 00 A.M. (GMT + 5:45)
APPLIED GEOPHYSICS
ENGINEERING GEOLOGY
Presenter :
Sudhan Kumar Subedi
Self Potential Method
(ELECTRICAL SURVEY)
Topic :
All interested can freely join……..

Self (Spontaneous) Potential Method
June 06, 2020
Self Potential (SP) Method
Background
 Devised in 1830, by Robert Fox who used copper-plate electrodes connected to a galvanometer to detect
underground copper sulphide deposits in Cornwall, England.
 Self-potential or spontaneous polarization method is based
on the surface measurement of natural potentials resulting
from electrochemical reactions in the subsurface.
 Does not require electric currents to be injected into the
ground as in the Resistivity and IP methods.
 The SP method ranks as the cheapest method
of surface geophysical methods in terms of
equipment necessary and amongst the
simplest to operate in the field.

June 06, 2020
Applications
 SP method has been used in base metal explorations, characteristically to detect the presence of massive
ore bodies, in contrast to the induced polarization (IP) method which is used predominantly to investigate
disseminated ore bodies.
 SP method has been increasingly used in
• groundwater and geothermal investigations
• environmental and engineering applications
– mapping the seepage flow associated with dams
- geological mapping
- delineate the shear zones and near surface faults

June 06, 2020
Occurrence of Self Potentials
 The SP method is passive, i.e. differences in natural ground potentials are measured between any two points
on the ground surface.
 The potentials measured can range from less than a millivolt (mV) to over 1 volt. ( < mV to > 1 V)
 The sign (+ve or –ve ) of the potential is an important diagnostic factor in the interpretation of SP anomalies.
 Self-potentials are generated by number of natural sources ( although the exact physical processes by which
some are caused are still unclear).
 Natural ground potentials consists of two components :
1. Mineral Potentials : constant and unidirectional, due to electrochemical processes.
2. Background Potentials : fluctuates with time ; caused by variety of different processes ranging from
alternating currents induced by thunderstorms and by variations in Earth’s magnetic field, to the effects of
rainfall.

June 06, 2020
Types of SP anomalies and their geological sources
Mineral Potentials
Geological Source Types of Anomaly
Sulphide Ore bodies
(pyrite, chalcopyrite, pyrrhotite, sphalerite, galena)
Graphite ore bodies
Magnetite
+ other electronically conducting minerals
Coal
Manganese
Negative ≈ hundreds of mV
Quartz Veins
Pegmatite
Positive ≈ tens of mV

June 06, 2020
Types of SP anomalies and their geological sources
Background Potentials
Geological Source Types of Anomaly
Fluid streaming, geochemical reactions
Bioelectric (plants, trees)
Groundwater movement
Topography
Negative, ≤ 300 mV
Positive + / - Negative ≤ 100 mV
Positive or negative, up to hundreds of mV
Negative up to 2 V

June 06, 2020
Origin of Self Potentials
 Certain natural or spontaneous potentials occurring in the subsurface are caused by electrochemical or
mechanical activity.
 Groundwater is thought to be common factor responsible for SP.
 Potentials are generated by the flow of water, by water reacting as an electrolyte and as a solvent of
different minerals.
 These potentials are associated with
- weathering of sulphide mineral bodies,
- variation in rock properties (mineral content) at geological contacts
- Bioelectric activity of organic material
- Corrosion
- Thermal and pressure gradients in underground fluids and
- Other phenomenon of similar natures

June 06, 2020
Types of Potentials
Electrokinetic Potential
(electrofiltration)
(electromechanical)
(streaming)
Diffusion Potential
(Liquid-Junction)
Nernst Potential
(Shale)
Electrochemical
Potentials
Mechanical
Potential
Variable with time
i.e. Background Potentials
Mineral Potential Constant and Unidirectional

June 06, 2020
Types of Potentials
 Also called as electrofiltration, electromechanical or streaming potentials.
 forms as a result of electrolyte flowing through a capillary or porous medium along the flow path, and is
measured across the end of the capillary ( Ahmad, 1961)
 The effect is believed to be due to electrokinetic coupling between the fluid ions and the walls of capillary.
 The electrokinetic potential (Ek ) generated between the ends of the capillary passage is given by :
Ek =
Ɛµ𝐶 𝐸Δ𝑃
4πη
Where, Ɛ = dielectric constant
µ = Resistivity
CE = electrofiltration coupling coefficient
ΔP = Pressure difference
η = dynamic viscosity

June 06, 2020
Types of Potentials
 Ek gradient is in the same direction as the pressure gradient, i.e. opposite to the direction of the
electrolyte flow.
 Ek normally provides amplitudes of some mV to several hundreds of mV.
 Ek can be found associated with flow of subsurface water and thermal fluids
 Ek effects have been observed over zones of water leakage through fissure in the rock floor of reservoirs,
over terrains with large elevation changes and in geothermal areas.

June 06, 2020
Types of Potentials
Diffusion/Liquid-Junction Potential
 If the concentration of the electrolytes in the ground varies locally, potential differences are set up due to
the difference in mobilities of anions and cations in solutions of different concentrations, called Diffusion
or Liquid-Junction potentials.
 For this mechanism to explain the continued occurrence of such potentials, a source capable of
maintaining imbalances in the electrolytic concentration is needed, otherwise the concentrations
differences will disappear with time by diffusion.
 The diffusion potential (Ed ) is given by :
Ed = −
RT Ia−Ic
Fn Ia−Ic
ln(
C1
C2
)
Where, Ia = mobilities of anions (+ve)
Ia = mobilities of cations (-ve)
R = Universal gas constant = 8.314 JK-1mol-1
T = Absolute Temperature
n = ionic valence
F = Faraday’s constant = 96487 Cmol-1
C1 and C2 = Solution concentrations

June 06, 2020
Types of Potentials
Nernst Potential
 When two identical metal electrodes are immersed in a homogeneous solution, there is no potential
difference between them.
 If, however, the concentrations at the two electrodes are different , there is potential difference, called
Nernst Potential.
 The Nernst potential (En ) is given by :
En = −
RT
Fn
ln(
C1
C2
)
Where,
R = Universal gas constant = 8.314 JK-1mol-1
T = Absolute Temperature
n = ionic valence
F = Faraday’s constant = 96487 Cmol-1
C1 and C2 = Solution concentrations
 The Nernst potential is of particular importance in well logging, so referred as Shale Potential.

June 06, 2020
Types of Potentials
Electrochemical Potential
 Electrochemical Potential = Diffusion Potential + Nernst Potential
 Electrochemical Potential is directly dependent on the concentration differences (C1/C2) and temperature.
 One of the most common natural electrolytes is NaCl.
 For this reason, the measurement of self potential is important in exploration of geothermal resources,
where temperatures are obviously elevated and concentration of salts within groundwater are also likely to
be high.
For NaCl, Ia / Ic = 1.49 and at 25°C,
Ed = -11.6 log (C1/C2 )
And For n = 1 and T = 298K,
En = -59.1 log (C1/C2 )
Thus,
For NaCl, at T°C, Electrochemical Self- Potential is given by :
Ec = −70.7
(T+273)
273
ln(
C1
C2
)
When concentrations are in the ration 5:1, Ec = ± 50 mV at 25 °C

June 06, 2020
Types of Potentials
Mineral Potential
 Mineral potential is the most important in mineral exploration of SP associated with massive sulphide ore
bodies.
 Large negative (-ve) SP anomalies (100-1000mV) can be observed particularly over deposits of pyrite,
chalcopyrite, pyrrhotite, magnetite and graphite.
 The potential are almost invariably negative over the top of the deposit and are quite stable in time.
 Sato and Mooney (1960) have provided the most complete explanation of the electrochemical processes
which cause the observed SP anomalies
 However, this hypothesis does not explain all the occurrences of the SP indicates that the actual physical
processes are more complicated and no yet truly understood.

June 06, 2020
Types of Potentials
Mineral Potential
Schematic model of origin of the self potential anomaly of the orebody. The mechanism depends on the
differences in oxidation potential above and below the water table.

June 06, 2020
Types of Potentials
Mineral Potential
Physicochemical model proposed by Sato and Mooney (1960) to
account for the self-potential process in massive sulphide orebody.

June 06, 2020
Measurement of Self-Potentials
 Simple and Inexpensive
 Two non-polarizable porous-pot electrodes are connected to a precision multimeter with an impedance
greater 108 Ω and capable of measuring at least 1 mV.
 Each electrode is made up of a copper electrode dipped in a
saturated solution of copper sulphate which can percolate
through the porous base to the plot in order to make electrical
contact to the ground.
 Alternatively , zinc electrode in saturated zinc sulphate solution
or silver in silver chloride can be used.
Source : Reynolds, J.M. (2011)
 Maximum Depth of sensitivity of SP method = ≈60 – 100 m
depending on depth of ore body and nature of overburden.
Porous-pot electrodes

June 06, 2020
Field Methods
There are two different methods of conducting a Self-Potential Survey :
1. Potential Amplitude Method (Fixed Base Method )
2. Potential Gradient Method
1. Potential Amplitude Method (Fixed-Base Method)
 The fixed-base method keeps a single electrode in one place without moving it. This electrode is called the
base station and acts as the reference point for all other measurements.
 To follow the fixed-base SP method, we simply move the other electrode a certain distance forward (say,
10 meter) along a line and take a reading.
 We, then move it another 10 meter forward, so it’s 20 meter from the base station , and repeat this
process until the end of the line.
 The result of the survey is plotted against the distance from the base station.
 Both method are carried out at right-angles to the suspected strike of the geological target.

June 06, 2020
Field Methods
1. Potential Amplitude Method (Fixed-Base Method)
 The fixed-base method does a good job
detecting small anomalies and is more
accurate than the gradient method.
 But, the disadvantage is that we need a really long wire.
 If we our measurement line is 1000 m , then we need a 1000 m wire to the base station.
 This is cumbersome and heavy, and we have to be careful to ensure that the insulation of the wire doesn’t
get damaged or that it is pulled too hard and broken.
 Strong negative anomalies indicate mineralization.

June 06, 2020
Field Methods
2. Potential Gradient Method (dipole/leap frog configuration)
 The gradient method starts with fixed separation of two electrodes :
- One at the starting point (i.e. Point Zero)
- And the other at whatever spacing we’ve determine (say, 10 meter)
 The measured potential difference between two electrodes is divided by electrode separation to give
Potential gradient (mV/V).
 We then move each electrode in steps (leap-frogged) :
- The electrode at 10 meters move to 20 meters , and
- the electrode at point zero moves out 10 meters and so on.
Along traverse with care of correct polarity of potential recorded.
 The point to which this observation applies is the midpoint between two electrodes.

June 06, 2020
Field Methods
2. Potential Gradient Method
 The benefit to the gradient method is that even if we measure 1000 m , we only need a 10 meter wire
between two electrodes.
 The issue with the gradient method is that we get an error with each reading.
 So, when we sum all of the gradient measurements up, it adds to error. This tends to hide any small anomalies.

June 06, 2020
Interpretation of Self-Potential Data
 SP anomalies are often interpreted qualitatively by :
- Profile Shape
- Amplitude
- Polarity ( +ve or –ve)
- Contour pattern
 Top of ore body is assumed to lie directly beneath the position of minimum potential.
 For quantitative interpretation, it is possible to calculate the potential distributions around the polarized
bodies of simple shape, such as sphere, ellipsoid, and dipole, by making some simplifications and
assumptions concerning the potentials on the surface of the sources.

June 06, 2020
Source : Telford et. al, (1990)
Figure : Typical SP profile and
contours over sulphide ore body

June 06, 2020
Figure : (A) Weiss SP anomaly in Ergani , Turkey, with the causative orebody shown schematically in (B). Note
that the axis of polarization is inclined uphill.
Source : After Yungul (1950)
 If the axis of polarization (i.e. the axis between the cathode and anode on the orebody) is inclined
from the vertical , the shape of the profile will become asymmetrical with the steepest slope and
positive tail both lying in the downdip side.

June 06, 2020
Figure : Two SP minima with different causes : one
produced by electrochemical process associated
with mineralized graphitic phyllites and one caused
by electrokinetic processes due to flow of water in
permeable disintegrated conglomerates.
Source : Nayak (1981)
 Complications arise when two or
more geological features give rise to
superimposed SP anomalies.

June 06, 2020
 However, if similar sized bodies are present but with different dips, the resultant anomaly can be used to
resolve between them.
Figure : SP anomalies due to (A)- two graphite bodies with axes of polarization inclined away from each other (in syncline)
and (B). Inclined towards each other (in anticline)
Source : Meiser (1962)

June 06, 2020
Source : Meiser (1962)
Observed Profile
SP anomaly across a single graphite body in gneiss
Individual model SP anomalies for each of four graphite ore bodies in gneiss

June 06, 2020
 Approximation of shape of ore body to one of known geometry, usually a sphere, or inclined rod or plate
Source : Telford et. al, (1990)
Figure : SP anomalies associated with
(a). Sphere and
(b). a dipping plate Source : Parasnis (1986)
Figure : SP anomalies associated with
a dipping rod

June 06, 2020
Source : Parasnis, (1996)
Figure : Self potential map of
Kimheden pyrite orebody in northern
Sweden
(Map contours are in mV)
Figure : SP anomaly profile along A-B

June 06, 2020
Source : Coleman, (1991)
Figure : SP anomaly over a closed landfill, showing the typically larger anomalies associated with the landfill
boundaries compare with those observed in the interior.

June 06, 2020
Source : Semenov, (1980)
Figure : SP profile along a pegmatite dykes in gneiss

THANK YOU ALL FOR
YOUR KINDLY
ATTENTION !!!

Self Potential Method (Electrical Survey)

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