erth2020 introduction to geophysics

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ERTH2020 Introduction to Geophysics

The Self Potential Method(or Spontaneous Potential)

“The ugly duckling of environmental geophysics” 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).

c.f. Revil & Jardani, 2013, pp. 14

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Self Potential Method

Revil & Jardani, 2013

Applications:

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 …

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Revil & Jardani, 2013, p. 2

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• 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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• 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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Jardani, A. et al., 2008; c.f. http://en.wikipedia.org/wiki/Aquifer_test

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).

I. Data obtained prior to pumping.II. Transient phase during pumpingIII. Steady-state phaseIV. Recovery phaseV. Steady state phase

thermal drift

The SP response is sensitive to ground water flow triggered

through the pumping test

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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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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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Data processing.

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Telford et al, 1991, pp. 283

Mechanisms governing the occurrence of SP signals can be classified as follows:

All mechanisms are fundamentally electrochemical in nature

• Diffusion potentials (liquid-junction potential)

• Shale Potentials (Nernst potential)

• Bioelectric potentials• Mineral potentials• Streaming potentials (zeta potential)

Background/Noise

Geophysical Exploration

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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.

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 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, , is given by:

anion/cation mobilities; electric charge/ion, universal gas constant; is the temperature; is the Faraday constant; , solution concentrations

via Nernst-Planck equation

flux density, : diffusion coefficient, ionic species and

electric field .

𝐸𝑑=− 𝑅𝑇𝑛𝐹

( 𝐼𝑎− 𝐼𝑐 )( 𝐼𝑎+𝐼 𝑐 )

ln (𝑐1𝑐2 )


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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.

In general, Diffusion/Nernst potentials can create anomalies in the tens of millivolts, and is just a source of noise in most SP surveys.

𝐸𝑛=− 𝑅𝑇𝑛𝐹 ln (𝑐1𝑐2 ) (Nernst potential)


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Self Potential Method• Bioelectric potentials

– ion selectivity and water pumping action of plant roots can create SP anomalies.

• 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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Lowrie, 1997, p. 209

• Mineral potentials– arise from geochemical oxidation-reduction

(redox) reactions, equivalent to a ‘battery’.

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Nyquist and Corry, 2002; Revil & Jardani, 2013, p.72

• Mineral potentials– arise from geochemical oxidation-reduction


After Sato & Mooney (1960):• Cathodic reaction above the water table

o 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

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Nyquist and Corry, 2002;

• Mineral potentials ()– arise from geochemical oxidation-reduction


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

Calculated via the Nernst Potential

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• Mineral potentials ()

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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Reynolds, 2011, pp. 363


SP anomaly across a sulfide ore-body at Sariyer, Turkey.

Pyrite & chalcopyrite occur in varying concentrations within a massive deposit, hosted in Andesite and below Devonian schist.

The area shows steep topography, shifting the SP anomaly downhill

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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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• 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 and viscosity is forced through a capillary or porous medium.

The resultant potential difference between the ends of the passage is

𝐸𝑠=− 𝜁 𝜀𝜌4𝜋 𝜂 ( ∆ 𝑃 )

adsorption (zeta) potential pressure difference: 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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Reynolds, pp. 351

• Streaming potentials.

C i=ζ i

i. A vertical boundary with upwelling from the right

ii. Pumping from a well.

iii. Horizontal boundary flow along different interfaces .

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• Streaming potentials.

Arrows denote points at which faults cross the SP survey line.

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.

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The Electric Double layer

http://en.wikipedia.org/wiki/Zeta_potential

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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The Electric Double layer

Costar et al., 2008, pp. 15

The -potential develops across boundaries between a fluid electrolyte and mineral grains in fractured rock and porous media.

The more negative the -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 -Potential.

Electric double-layer

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Application: Sinkhole Detection

Revil & Jardani, 2013, p.161

Picture of Sinkhole A1, which has a diameter of 10 meters. The depth of

the depression is about 2 m

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DC Resistivity Survey

SP Stations (+)

Visible Sinkholes

(Interpreted) “Crypto “Sinkholes

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SP Contour Map & DC Resistivity Section

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References 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

Telford, W.M, Geldart, L.P., Sheriff, R.E.: “Applied Geophysics”, 1991, Cambridge University Press

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 self-potential (SP) methods”, 2003, Environmental Geology, 43, pp. 782

erth2020 introduction to geophysics

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selfpotential method

selfpotential measurements

terms self potential

mv streaming potential

v mineralisation potential

sampled electrical potential

natural potential differences

nonpolarizing electrodes