Geophysical Mapping: Method Details
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Method Name: Self potential
Method Type:   Electrical Methods
Assigned Problems:
+ Monitoring Hazardous Waste
+ Quality / Thickness of concrete Buildings and Structures
0 Aquifer pollution Groundwater
0 Cavity detection Civil Engineering
0 Host sediments, hydogeological settings Hazardous Waste
0 Landslides Natural Hazards
0 Porosity / Permeability Groundwater
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   The spontaneous or self potential (SP) method is a passive electrical technique that involves measurement of naturally occurring ground potentials. The two main sources of SP signals important in environmental and engineering studies are streaming potentials, due to movement of water through porous subsurface materials, and diffusion potentials resulting from differing concentrations of electrolytes within the groundwater.
Keywords:   Spontaneous or self potential (SP); natural potential; diffusion potentials; electrokinetic effects; maps (2-D) of SP distribution; SP tomograms
  • Natural Potential caused by, e.g., groundwater flow; variation in electrolyte concentration in the ground, must exist.
  • Buried wires, metal pipes, metal fences may influence measurements.
  • Urban areas may cause high noise levels (e.g. stray currents).
  • High-voltage power lines, railways and antennas may influence measurements.
  • Measurements during rain should be avoided.
  • Changing waether conditions may influence measurements.
  • Ground coupling/surface conditions around electrodes influence measurements.
  • In some areas electrode coupling may be poor (e.g., asphalt, gravel, dry sand).
  • Topography effects need to be considered.
  • SP measurements are very sensitive to time variant noise (e.g., telluric currents; bioelectric potentials; poor electrode coupling).
  • Profiles > 1 km may need special corrections for laterally varying telluric currents.
Resolution:   Since data interpretation is mostly qualitative, measures of resolutions are difficult to specify. At best horizontal resolution is of the order of electrode spacing in gradient array configuration.

Typical depth of investigation: significant depth interval usually includes the uppermost few meter to a few tens of meters.

Expected Results:  
  • Measured parameter: electrical potential differences in mV. Values for groundwater studies are within several tens of mV. Over graphite or some sulfides deposits, anomaly can reach more than some hundreds of mV.
  • Data analysis: voltages are plotted in form of apparent resistivities as profiles or contour maps. Results are typically displayed in profiles or contour maps. Forward modelling or SP tomography can provide a semi-quantitative interpretation.
  • Interpretation: the origin of natural potentials is not entirely understood yet; some role of thumb where potentials occur are: Natural potential increase with increasing hydraulic gradient and groundwater flow through formations with different hydraulic conductivity are normally associated with natural potentials.
Combination with other Methods:  
  • Required additional information: knowledge of the gross resistivity subsurface structure.
  • Related add-on information: electrical data; induced polarization data; magnetics data; thermal data; seismic data
  • Interpretation: NA
Operating Expense:  
  • Crew size: 1 key person, 1-2 assistants
  • Acquisition speed: 200 - 500 measurements per day, depending on the electrode spacing and terrain.
  • Processing:requires 1 - 2 days per acquisition day.
  • Equipment rental costs: low
Parameters to specify:  
  • Array type (gradient or total field array): The gradient array is faster in use but more susceptible to errors then the total field array. Total length of array can be up to several kilometres..
  • Electrode spacing: Usually between 1 and several tensof m.
QC Documents:  
  • Measurements of reproducibility (measurements of reciprocal or redundant configurations).
  • Optional: Map of buried cables, roads.
  • Maps (2-D) of self-potential distribution
  • Interpretation
  • Optional: self-potential tomograms (2-D self-potential distribution with depth).
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