1. Lecture N. 5: Spontaneous (self) Potential methods
H. SAIBI
October 30st , 2014

2. Electrical methods Electrical methods: Earth Resistivity
Self Potential (SP)
Induced Potential (IP)

3. Self potential (SP)
SP method is based on measuring the natural potential differences which exist between any two points on the ground
Potential differences, partly constant / partly fluctuating, are associated with electric currents in the ground

4. SP
Values normally range from a fraction of mVolts to a few tens of mVolts.
Occasionally SP in the order of a few hundreds mVolts occur.
Often (but not always) related to:
Sulphide and graphite ore bodies
Magnetite + other conductive minerals
Coal and manganese deposits

5. SP
Self-potentials are surprisingly stable in time. Several cycles of seasons (snow, rain, drainage) do not affect the pattern of self-potentials

6. SP – field procedure
The measurement of self-potentials can be easily done with an electrode and any mVoltmeter
Electrodes must be non-polarizable, polarizing effects of electrodes might be stronger than SP effect!

7. SP – field procedure Two standard procedures: Advance together
Electrodes on fixed distance, moving together along staked lines
Advance separate from each other
One electrode connected to end of cable
Use stationary electrode as base station
Advance second electrode along cable

8. SP - origin
Origin of self-potentials is not fully solved yet. However, a few main mechanisms have been identified:
Electrofiltration
Diffusion (concentration differences)
Adsorption
Mineral potentials

9. SP - origin 1. Electro filtration
An electric potential difference is developed between the ends of a capillary tube through which an electrolyte is flowing
Field is in same direction as pressure gradient, opposite to direction of electrolyte flow
Can be found associated with the flow of water through sand, porous rocks, etc.

10. SP - origin 1. Electro filtration
High ground is generally more negative than low ground
Indicator that electric current due to these potentials tends to flow uphill
Agreement with theory
Only flow of water not enough: resistivity of electrolyte, dielectric constant electrolyte, streaming potential and pressure need to vary

11. Table: Types of SP anomalies and their geological sources.
Table: Types of electrical potentials.

12. Electrical potentials
Electrokinetic: ,  and  are the dielectric constant, resistivity and dynamic viscosity of the electrolyte respectively; P is the pressure difference; and CE is the electrofiltration coupling coefficient.
Diffusion potential:
Ia and Ic are the mobilities of the anions (+ve) and cations (-ve) respectively; R is the Universal Gas Constant (8.314 JK-1 mol-1); T the absolute temperature (K);  is ionic valence; F is Faraday’s Constant (96487 C mol-1); C2 and C2 are the solution concentrations.
Nernst potential: when Ia = Ic in the diffusion potential equation.

13. SP – origin (Electrokinetic)
1. Electro filtration - examples
Horizontal flow across vertical boundary
Pumping from well
Vertical flow across horizontal boundary
Figure: Idealised electrofiltration SP profiles and maps.

14. SP – origin (Electrokinetic)
1. Electro filtration – applications
- Leakage spots on submerged (earth) slopes of water reservoirs
- Detection of concealed karstic cavities, springs
- Effect of pumping on groundwater table
Figure: SP anomaly produced by pumping from a well.

15. (Top) Schematic illustration of streaming-potential residual anomalies associated with groundwater recharge and discharge in karst terrain. (Bottom) The arrows indicate groundwater-flow direction, while the positive and negative symbols indicate equivalent charge polarization.

16. SP – origin (Electrochemical)
2. Diffusion (concentration differences)
Differences in concentration of electrolytes in the ground from place to place produce electric potential differences
Works like a battery:

17. Figure: An SP profile across pegmatite dykes in gneiss.
SP - origin
3. Adsorption
Adsorption of pos + neg ions on surface of veins (quartz, pegmatite). Electromechanical origin unclear
Figure: An SP profile across pegmatite dykes in gneiss.

18. SP - origin 4. Mineral potentials also called ‘sulphide potentials’
Observed on electronically conducting materials (e.g. pyrite has one of highest potentials)
Figure: Physicochemical model to account for the SP process in a massive sulphide orebody.

19. Earth Resistivity
Direct or low-frequent alternating current is introduced into the ground by means of two electrodes
Resulting potential distribution on the ground is mapped by two probes and yields information about the electrical resistivity below the surface

20. Resistivity profiling
In profiling, observations are made at regular intervals along a survey line.
The observation points are at regular intervals.
The electrode separations are fixed.
This example shows the effect of resistive dyke intrusions in Botswana
(Pala Road Project)

21. Resistivity mapping
This procedure consists of a series of more or less parallel
profile lines, generating a grid of stations.

22. Interpretation
Figure: (A) Weiss SP anomaly in Ergani, Turkey, with the causative orebody shown schamatically in (B). Note that the axis of polarization is inclined uphill (Yungul, 1950)

23. Interpretation
Figure: Two SP minima with different causes: one produced by electrochemical processes associated with mineralised graphite phyllites, and one caused by electrokinetic processes due to the flow of water in permeable desintegrated conglomerates (Nayak, 1981).

24. Interpretation (Different Dips)
Figure: SP anomalies due to (A) two graphite bodies with axes of polarisation inclined away from each other (in syncline), and (B) inclined towards each other (in anticline) (Meiser, 1962).

25. Interpretation (Shape of the Orebody)
Figure: Self-potential anomalies associated with (A) a sphere, (B) a dipping plate (Parasnis, 1986), and (C) a dipping rod (Telford et al., 1990).

26. (a) Polarized sphere model as an electric dipole
(a) Polarized sphere model as an electric dipole. (b) The SP response of a buried polarized sphere for different angles of polarization.

27. (a) A polarized, inclined sheet modeled as electric line charges
(a) A polarized, inclined sheet modeled as electric line charges. (b) The SP response of a buried polarized sheet for different angles of inclination.

28. Applications and Case Histories: Geothermal
Figure: Thermal gradient and SP profiles over a Dome Fault Zone, Roosevelt Hot Springs, USA. Arrows denote points at which mapped faults cross the SP survey line
(Corwin and Hoover, 1979).

29. Applications and Case Histories: Location of massive sulphide ore bodies
Anomalies are invariably negative with respect to a point far away from the mineral occurrence
Figure: SP anomaly across a pyrite orebody at Sariyer, Turkey. The borehole is located at the location of the topographically corrected SP minimum (Yungul, 1950).

30. Applications and Case Histories: Landfills
Figure: SP anomaly over a closed landfill, showing the typically larger anomalies associated with the landfill boundaries compared with those observed in the interior (Coleman, 1991).

31. SP signals and 2-D electrical resistivity structure at Stromboli volcano, Italy. C=conductive zone; R=resistive zone.

32. (Left) The continuous wavelet transform of a buried long cylinder for various values of s, with the symbols marking the maxima. (Right) Straight lines joining the CWT maxima point toward the actual depth of burial of the cylinder.

33. Depth determination to SP sources using CWT analysis

34. SP profiles and co-located 2-D ERT images at an active landslide in northeast England; WMF=Whitby mudstone formation; SSF=Staithes sandstone and Cleveland ironstone formation.