Self potential (SP) Method
Self potential (SP) method is the one of passive form amongst electrical geophysical techniques. This is because Self-Potential signals originate naturally in the ground.
This means that the SP signals are recorded with no need of applying electrical signals into the subsurface. The Self- Potential is generated itself within the Earth's ground due to various electrochemical processes such as electrical filtration, diffusion, adsorption and others as I will describe later in details.
However the technique is based on a simple model where the generated natural ground Potential difference (P.d) is measured in millivolts (mV) between two (2) points.
Origin (Mechanism) of Self - Potential
How is Self-potential (SP) generated?
1. Electrokinetic potential
Flowing of fluid (electrolyte) through a capillary or porous medium generates potentials along the flow path. The potentials are alternatively called electrofiltration, electromechanical or streaming potentials.
The effect is believed to be due to electrokinetic coupling between the fluid ions and the walls of the capillary.
The electrokinetic potential (Ek) generated between the ends of the capillary passage is given by, Ek = ERCE∆P/ 4Ï€n
E, R and n are dielectric constant, Resistivity & dynamic viscosity of the electrolyte respectively.
∆P - pressure differences.
CE - Electro - filtration Coupling coefficient.
2. Thermoelectric potential
Potential gradients will appear across a rock sample if a temperature gradient is maintained across the rock sample.
Thermoelectric coupling coefficient (TEC) is defined as the ratio of the voltage to the temperature difference such as TEC = ∆V/∆T, TEC values of rocks vary from ‐0.09 to + 1.36 mV/°C, average ~ 0.27 mV/°C
SP generated from TE potentials are of smaller amplitudes than SP generated from TE potentials are of smaller amplitudes than usually seen in geothermal areas.
More concentrated areas of high temperature at shallow depth, such as thermal fluids in a fault zone, could give rise to anomalies of greater amplitude.
Boundaries of SP anomalies measured in several geothermal areas appear to correlate with zones of known anomalous high heat flow; the portion of anomalies is generated by TE mechanism.
3. Electrochemical 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 liquid‐junction or diffusion 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 concentration differences will disappear with time by diffusion.
The well model and widely accepted mechanism for electrochemical potential was given by Sato and Mooney (1960).
Figure 1: Mechanism of electrochemical potential (after Sato and Mooney, 1960)
Electrical potential is also generated when 2 identical metal electrodes are immersed in solutions of different concentrations called Nernst potential.
Diffusion + Nernst potentials = Electrochemical or static self potentials.
Ed = RT (Ia - Ic)In (C1/C2)/nF (Ia + Ic)
where,
C1 & C2 - Are the soln concentrations
T - Temperature (K)
R - Universal Gas Constant, 8.314
n - Ionic valence
F - Faradays' constant, 96500
Ia & Ic - Mobilities of (-ve) anions and cations (+ve)
When, Ia = Ic in diffusion potential equation.
It obeys Nernst potential
En = ( RT/nF)In (C1/C2).
At 25°c and 1 atm, It occurs when there is a potential difference between 2 electrodes immersed in a homogeneous solution and at which the conc. of the solutions are locally different.
4: Adsorption (Zeta) Potential.
Adsorption can simply be defined as interaction between the liquid and the surface of the solid.
Due to adsorption of solid anions onto the surface of the material.
Adsorption potential may account for the observed anomalies over clays where the solid - liquid double layer may generate a potential.
Why are SP anomalies more positive over pegmatites and dykes?
Ans. Adsorption potential.
N.B: Don't mix adsorption with absorption.
Adsorption is the collection of one substance on the surface of another.
Absorption is the penetration of one substance into the surface of another substance. e.g water is absorbed by a sponge.
Self-potential Field Techniques
Let us move to the following two (2) field techniques that are applied during measurements of Self Potential in the field.
First of all you have to ensure that these two field methods are carried out at right angles (90°) to the suspended strike of a geological target. Such that there should be a series of parallel lines which are at 90° to the strike.
As the Rule of thumb, electrodes used should be contained in a porous pot with salt solution of the same nature such as if it is Cu within CuSO4 salt solution or Zn electrodes within ZnSO4 salt solution in order to make electrical contact with the ground.
1. Potential Amplitude method.
Here, you have to fix one of the two electrodes at a base station while moving the second electrode along the predefined traverse.
The base station must be kept on unmineralized ground where its electrode potential is zero. The potential difference (mV) is recorded between the fixed electrode at the base station and the moving electrode.
Because the base is fixed and has zero electrode potential, the Potential difference measured will represent the electrode of the moving electrode. This technique can also be known as Fixed Base Procedure.
One of the advantages of this technique is that it is difficult to confuse the polarity and accumulating errors.
However you have to take care of the temperature of the electrolyte in a mobile pot so that it does not differ significantly from that in the reference electrode. This can be done by using pots which are top capped.
It depend on type of nature of electrode for example temperature coefficient for Cu, CuSO4 0.5 mV/°c and 0.25 mV/°c for Silver AgCl
Figure 2: Fixed Base Procedure.
2. Potential gradient method
This applies two electrodes which are separated at fixed distance typically 5 m or 10 m, between which the Potential difference (P.d) is measured at the mid-point of the electrode separation to give Potential Gradient (mV/m).
It is also known as Leap Frog Technique due to the fact that both two electrodes are taken from one point to another like how the frog is moving. It is also known as Gradient Model technique.
Figure 3: Potential Gradient technique.
Here you have to take care for the correct polarity of the potential which is measured.
Applications of Self - potential method
Mineral exploration
Such as sulfide orebodies may produce negative anomalies of several hundred millivolts see figure 4.
Mineral Potential is Important in mineral exploration, associated with massive sulfide orebodies, ranges from 100 - 1000's mV. Large negative Sp anomalies can be observed particularly over pyrites and chalcopyrites. See figure 4
It was originally thought that these potentials were maintained by oxidation of the ore itself, but is now generally agreed that the the ore acts as a passive conductor focussing currents produced by the oxidation-reduction reactions that take place across the water table (figure 1)
Figure 4: SP anomaly profile A - B over sulfide orebody.
Delineation of Landfills
The main contribution of SP anomaly in landfill is due to ions imbalance caused by Leachate. As ions move from one point to another may cause generation of potential such as electrochemical potential. Also Leachate flows, however this will depend on the direction of flow such as if the Leachate flow is outward a streaming potential may be generated.
Then the high SP anomaly contributed by these two potentials would be present at the landfills boundaries. The good example regarding this case is given by Coleman (1991) see figure 5 below.
Figure 5: SP anomaly over the Landfill with high SP amplitudes observed at landfills boundaries
Geothermal exploration
Geothermal Resources are associated with high temperature and high heat with a complex mixture of saline water. Since water bodies can have highly differing temperatures and salinities and be highly mobile (Cioni et al, 1992), consequently the streaming potential may be well developed and hence may be measured using SP method.
Boundaries of SP anomalies measured in several geothermal areas appear to correlate with zones of known anomalous high heat flow portion of anomalies is generated by Thermal Electric (TE) mechanism.
Hydrogeology
SP anomalies shapes can be used to characterize the groundwater table, a detailed example was given by Fournier (1989), such as SP anomaly maximum may indicate the depression of the water table due to better drainage, high horizontal SP gradient may indicate lateral limit of an unconfined aquifer.
Location of Leakage in subsurface structures such as dams
When groundwater flows through structures by finding the path with lowest resistance the electrokinetic streaming potentials may be generated having detectable SP anomaly magnitude. See figure 6 below
Figure 6: SP anomalies generated by seepage through earth dams Butler and Liopis (1990)
More detail associated with leak is given by Butler and Liopis (1990), Jansen et al (1994).
All in all the signs such as positive (+) and negative (-) of Potential difference (P.d) are important factors to be taken into consideration during interpretation of Self potential (SP) anomalies. Such as if the sign of Potential difference (p.d) is Positive (+) this may imply Lower ground while the negative (-) Potential difference (p.d) may imply higher ground.
References
Butler D.K and Liopis, J.L (1990), Assessment of anomalies seepage conditions. Geotechnical and Environmental Geophysics, vol. 2, Environmental and Groundwater, Tulsa, Society of Exploration Geophysicists, 153 - 173
Cioni, R. Fanelli, G., Guidi, M, Kinyariro, J.K, and Marini, L. (1992), Lake Bogoria hot springs (Kenya), geochemical features and geothermal implications, Journal of Volcanology and Geothermal Research, 50(3): 231 - 246.
Coleman, A.R (1991), The use of self - potential method in the delineation of a reclaimed landfill site, University of Birmingham, The Midlands Geotechnical Society.
Fournier, e. (1989), Spontaneous Potential and resistivity surveys applied to Hydrogeology in volcanic area, case history of the chaine des Puys, France. Geophysical Prospecting, 37(6): 647 - 668.
Jansen, J., Billington, E., Snider, F., and Jurcek, P. (1994). Marine SP surveys for dam seepage investigations, evaluation of array geometries through modeling and field trials.
Sato, M. & Mooney, H.H (1960), The electrochemical mechanism of sulfide self potential, Geophysics, 25, 226 - 49.
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