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Introduction to Geophysics
The Self Potential Method
(or Spontaneous Potential)
“The ugly duckling of environmental geophysics”
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Nyquist & Corry, 2002
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Self Potential Method
 Passive geophysical method (like gravity/magnetics)
• One of the oldest geophysical methods.
o First measurement by Fox (1830) in Cornwell, UK, over sulphide vein
mineralisation.
• Frequently used since the 1920s as a (secondary) tool for base-metal
exploration and also for detecting subsurface fluid-flow.
• Involves measurement of electric potentials (voltages) at specific
points on the surface or downhole (self-potential stations).
o Required: volt-meter, non-polarising electrodes.
• Natural potential differences generally exist between any two points on
the ground (associated with electrical currents in the subsurface).
• Mostly used qualitatively due to lack of quantitative models but this is
changing rapidly (complex causative sources of self-potential signal).
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c.f. Revil & Jardani, 2013, pp. 14
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Self Potential Method
Applications:
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for mineral exploration
geothermal applications
groundwater investigation
formation evaluation in the oil and gas industry
to detect fluid flow in fractured rocks and gas reservoirs
engineering applications to detect dam fractures and seepage
and others …
Revil & Jardani, 2013
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Self Potential Method
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Revil & Jardani, 2013, p. 2
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Self Potential Method
• Development of non-polarising electrodes
(porous-pot) in 1865 by M.C. Matteucci,
Greenwich Observatory.
• Measurements in mV (streaming potential)
to several V (mineralisation potential).
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Revil & Jardani, 2013, p. 2
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Self Potential Method
• High Impedance Potentiometer (Voltmeter).
• Impedance (Resistance) has to be at least 10x higher than the ground
between the electrodes to avoid current leakage in the voltmeter.
• Impedance range from 105Ohm.m to 1012 Ohm.m for very resistive ground
(ice, permafrost, crystalline rock)
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Revil & Jardani, 2013, p. 2
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Self Potential Method
SP-Response associated with an aquifer test.
• Water was pumped from one well and injected into another.
• Time variation of measured SP data due to ground water flow
associated with pumping and injection tests (at one SP station).
The SP response is sensitive to
ground water flow triggered
through the pumping test
thermal drift
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I.
II.
III.
IV.
V.
Data obtained prior to pumping.
Transient phase during pumping
Steady-state phase
Recovery phase
Steady state phase
Jardani, A. et al., 2008; c.f. http://en.wikipedia.org/wiki/Aquifer_test
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Self Potential Method
Data acquisition.
• Also used is the “star-approach” where first the potential
differences between a set of base stations is determined.
• Subsequently, each base is used as the local reference of profiles
which are radially distributed about this station.
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Revil & Jardani, 2013, p. 4
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Self Potential Method
Data acquisition.
• Large-scale mapping frequently uses a loop network approach
• One base station is chosen as the reference and measurements
are taken with scanning electrodes at SP stations.
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Revil & Jardani, 2013, p. 4
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Self Potential Method
Data processing.
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Revil & Jardani, 2013, p. 6
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Self Potential Method
Mechanisms governing the occurrence of SP signals can be
classified as follows:
Background/
Noise
Geophysical
Exploration
•
•
•
•
•
Diffusion potentials (liquid-junction potential)
Shale Potentials (Nernst potential)
Bioelectric potentials
Mineral potentials
Streaming potentials (zeta potential)
All mechanisms are fundamentally electrochemical in nature
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Telford et al, 1991, pp. 283
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Self Potential Method
• Diffusion potentials (liquid-junction potential)
– associated with gradients in concentrations of ionic species in the ground that
set up diffusion potentials.
• Shale Potential (Nernst potential)
‒ (special case of diffusion potential) electrodes are immersed in a homogeneous
solution but with different concentrations at the electrodes.
• Bioelectric potentials
– ion selectivity and water pumping action of plant roots can create SP anomalies.
• Mineral potentials
– (apparently) arise from geochemical oxidation-reduction (redox) reactions,
equivalent to the galvanic cell defined in electrochemistry.
• Streaming potentials (zeta potential)
– arise when water or other fluids flow through sand, porous rock, moraines,
basalts, etc.
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Nyquist and Corry, 2002; Telford et al, 1991, pp. 283
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Self Potential Method
• Diffusion potentials
– associated with gradients in concentrations of ionic species in the ground that
set up diffusion potentials.
• Anions & Cations with different mobilities result in different
diffusion rates
𝑉𝑒 + > 𝑉𝑒 − ⟶ electric potential
(faster moving ions of one charge will begin to outpace the ions of the opposite
charge. The resultant electric field is just what is required to speed up the slower
moving ions and maintain electro-neutrality).
• In equilibrium, the diffusion potential, 𝐸𝑑 , is given by:
𝑅𝑇 𝐼𝑎 − 𝐼𝑐
𝑐1
𝐸𝑑 = −
ln
𝑛𝐹 𝐼𝑎 + 𝐼𝑐
𝑐2
𝐼𝑎 , 𝐼𝑐 anion/cation mobilities; 𝑛 electric charge/ion, 𝑅 universal gas constant;
𝑇 is the temperature; 𝐹 is the Faraday constant; 𝐶1 , 𝐶2 solution concentrations
→ via Nernst-Planck equation
𝐟𝑖 = −𝐷𝑖 𝛻𝑐𝑖 +
𝐹
𝑛𝑐𝐄
𝑅𝑇 𝑖 𝑖
𝐟𝑖 : flux density, 𝐷𝑖 : diffusion
coefficient, 𝑖: ionic species and
electric field 𝐄.
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Nyquist and Corry, 2002; Telford et al, 1991, pp. 283
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Self Potential Method
• Shale Potential (Nernst potential)
‒ when two identical electrodes are immersed in a homogeneous solution but with
different concentrations at the electrodes.
Sandstone and Shale, marlimillerphoto.com
• Shale potential develops at the boundary
between shale and sandstone because shale is
more permeable to Na+ ions than Cl- ions.
• The net effect is that voltages recorded
adjacent to shale are higher than voltages
recorded adjacent to sandstone.
𝑅𝑇
𝑐1
𝐸𝑛 = −
ln
𝑛𝐹
𝑐2
(Nernst potential)
In general, Diffusion/Nernst potentials can create anomalies in the
tens of millivolts, and is just a source of noise in most SP surveys.
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Nyquist and Corry, 2002; Telford et al, 1991, pp. 283
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Self Potential Method
• Bioelectric potentials
– ion selectivity and water pumping action of plant roots can create SP anomalies.
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landviser.net
landviser.net
• Bioelectric anomalies can reach hundreds of millivolts.
• Abrupt changes in SP have been noted in the field when the vegetation
changes (commonly associated with changes in soil composition).
• Background/Noise in conventional geophysics, but useful to map electrical
potential gradients which governs water and nutrients uptake by plants.
Nyquist and Corry, 2002
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Self Potential Method
• Mineral potentials
– arise from geochemical oxidation-reduction
(redox) reactions, equivalent to a ‘battery’.
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Lowrie, 1997, p. 209
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Self Potential Method
• Mineral potentials
– arise from geochemical oxidation-reduction
(redox) reactions, equivalent to a ‘battery’.
After Sato & Mooney (1960):
• Cathodic reaction above the water table
Chemical reduction  electron gain
• Anodic reaction at depth below water table
o
Chemical oxidation  electron loss
• The ore body itself functions only to
transport electrons from anode to cathode
• SP anomaly associated with ore bodies can
be in the order of a few hundreds of
millivolts to over 1 V
o
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Nyquist and Corry, 2002; Revil & Jardani, 2013, p.72
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Self Potential Method
• Mineral potentials (𝑬𝒉 )
– arise from geochemical oxidation-reduction
(redox) reactions, equivalent to a ‘battery’.
Calculated via the
Nernst Potential
Used very successfully in base metal exploration.
Note: the Sato & Mooney (SM) “battery” model
cannot fully explain all observed phenomena:
 Large amplitudes > 800 mV
• (Max SM model ~ 800 mV)
 Large measured voltage gradients
• (SM model predicts smooth gradients)
 Anomalies of ore bodies completely below the water table
 Lack of positive pole
• Measured data always negative for completely drilled body
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Nyquist and Corry, 2002;
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Self Potential Method
• Mineral potentials (𝑬𝒉 )
 Typical contour map and profile over an
ore body producing a large SP anomaly
 The negative maximum lies directly
over the sulphide mass
 Over steep topography, the centre will
usually be displaced
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Telford et al, 1991, pp. 298
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Self Potential Method
• Mineral potentials (𝑬𝒉 )
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SP anomaly across a sulfide orebody at Sariyer, Turkey.
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Pyrite & chalcopyrite occur in
varying concentrations within a
massive deposit, hosted in
Andesite and below Devonian
schist.
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The area shows steep
topography, shifting the SP
anomaly downhill
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Reynolds, 2011, pp. 363
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Self Potential Method
• Mineral potentials (𝑬𝒉 )
Each of the various mineralisation zones
may be represented by a sphere whose
SP anomaly contributes to the total
anomaly observed
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Reynolds, 2011, pp. 363
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Self Potential Method
• Streaming potentials (electrokinetic or zeta potential)
– arise when water or other fluids flow through sand, porous rock, moraines,
basalts, etc.
 This is observed when a solution of electrical resistivity 𝜌 and viscosity 𝜂
is forced through a capillary or porous medium.
 The resultant potential difference between the ends of the passage is
𝜁𝜀𝜌
𝐸𝑠 = −
∆𝑃
4𝜋 𝜂
𝜁: adsorption (zeta) potential
∆𝑃: pressure difference
𝜀: solution dielectric constant
 In areas of high rainfall, steep topography and porous rock, streaming
potentials can be of large amplitude.
 E.g. A 2693-mV SP anomaly on Agadak Volcano (Adak Island, Alaska) is
attributed to streaming potentials
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Telford et al, 1991, pp. 283
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Self Potential Method
• Streaming potentials.
i. A vertical boundary with
upwelling from the right
ii. Pumping from a well.
iii. Horizontal boundary flow
along different interfaces .
Ci = ζi
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Reynolds, pp. 351
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Self Potential Method
• Streaming potentials.
Thermal gradient and SP profiles over the Dome Fault
Zone, Roosevelt Hot Springs (Utah) associated with
Mineral, Streaming and Diffusion Potentials.
Correspondence of broad SP
anomaly and thermal gradient
profile suggest a thermal
origin for the SP anomaly.
Pos. anomaly
(geothermal activity)
Neg. anomaly
(Alunite & Pyrite)
The geothermal SP anomaly
results from Streaming
Potentials driven by
convection cells, but also due
to Diffusion Potentials due to
temperature gradient.
Arrows denote points at which faults
cross the SP survey line.
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Reynolds, 2011, pp. 359
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The Electric Double layer
 The Zeta potential is the potential drop across the mobile part of the double layer:
i.e. it is the electric potential in the interfacial double layer (DL) at the location of the
slipping plane versus a point in the bulk fluid away from the interface.
ζ is positive if the potential increases from the
bulk of the liquid phase towards the interface.
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http://en.wikipedia.org/wiki/Zeta_potential
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The Electric Double layer
Electric double-layer
The 𝜁-potential develops across boundaries
between a fluid electrolyte and mineral
grains in fractured rock and porous media.
The more negative the 𝜁 -potential the more
positive ions are transported with the flow
and thus the greater the net transport of
negative charge ions.
 Aggregation of excess charge on each side of the interface
 electrical double layer.
 The mobile part of the electrical double layer is dragged along with the fluid-flow
 transport of electric charge with the flow.
 The amount of charge transported is directly related to the 𝜁 -Potential.
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Costar et al., 2008, pp. 15
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Application: Sinkhole Detection
Picture of Sinkhole A1, which has a
diameter of 10 meters. The depth of
the depression is about 2 m
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Revil & Jardani, 2013, p.161
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Application: Sinkhole Detection
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Revil & Jardani, 2013, p.162
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Application: Sinkhole Detection
SP Stations (+)
DC Resistivity Survey
Visible Sinkholes
(Interpreted) “Crypto “Sinkholes
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Revil & Jardani, 2013, p.164
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Application: Sinkhole Detection
SP Contour Map & DC Resistivity Section
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Revil & Jardani, 2013, p.165
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References
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Revil, A., Jardani, A.: “The Self-Potential Method", 2013, Cambridge.

Nyquist, J. E., Corry, E. C., “Self-potential: The ugly duckling of environmental geophysics”,
2002, The Leading Edge, pp. 446.

Jardani, A. et al., “Reconstruction of the Water Table from Self-Potential Data: A Bayesian
Approach”, 2008, Ground Water, pp. 213
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Telford, W.M, Geldart, L.P., Sheriff, R.E.: “Applied Geophysics”, 1991, Cambridge University
Press
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Lowrie, W. “Fundamentals of Geophysics”, 1997, Cambridge University Press

Reynolds, J.M., "An Introduction to Applied and Environmental Geophysics", 2011, John Wiley
& Sons
•
Costar A., Heinson G., Wilson T., Smit, Z.,: “Hydrogeophysical mapping of fracture orientation
and groundwater flow in the Eastern Mount Lofty Ranges, South Australia”, 2009, DWLBC
Report, Gov. South Australia
Fagerlund F., Heinson G., “Detecting subsurface groundwater flow in fractured rock using selfpotential (SP) methods”, 2003, Environmental Geology, 43, pp. 782
•
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