Geophysical Mapping: Method Details
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Method Name: DC geoelectrics - Sounding
Method Type:   Electrical Methods
Assigned Problems:
+ Depth of Overburden-bedrock interface Civil Engineering
+ Gravel, clay, limestone, salt exploration Natural Resources
+ Groundwater table Groundwater
+ Host sediments, hydogeological settings Hazardous Waste
+ Ice thickness Natural Hazards
+ Landslides Natural Hazards
+ Permafrost and ice detection Natural Hazards
+ Quality / Thickness of aquifer/aquitard Groundwater
+ Quality and thickness (Natural resources) Natural Resources
+ Quantity/ Thickness Hazardous Waste
0 Aquifer pollution Groundwater
0 Location of buried materials Hazardous Waste
0 Porosity / Permeability Groundwater
0 Soil / rock quality Civil Engineering
0 Temporal variations Groundwater
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   Electrical sounding provides a one-dimensional (1-D; assuming a horizontally stratified Earth) vertical profile of the electrical resistivity distribution with depth.
Keywords:   Vertical electrical sounding (VES); 1-D resistivity profile; resistivity distribution; inversion; sounding curves; 1-D resistivity-depth functions
Prerequisites:  
  • Target must be characterized by a resistivity contrast.
  • Buried wires, metal pipes, metal fences may influence measurements.
  • Urban areas may cause high noise levels (e.g. stray currents).
  • Measurements during rain should be avoided.
  • A thin high-/low resistivity top layer may obscure deeper targets.
  • In some areas electrode coupling may be poor (e.g., asphalt, gravel, dry sand).
  • Prohibited use: pronounced 2 - or 3 D dimensional subsurface geometry.
  • Subsurface must be approximately horizontally layered (maximum dip of layers: 10░).
Resolution:   Typical values of resolution are (Depth range / Target thickness): 0 - 10 m / 1 m to several m; 10 - 50 m / tens of m; > 50 m / several tens of meters. Maximum of about 6 layers can be resolved reasonably.

They depth of investigation is at most 1/3 of the largest electrode layout (maximal several 100 m), but mostly much less. It depends on the separation between the electrodes, the contrasts of resistivity of the various formations and the thickness of the prospected targets. Typical depth of investigation < 50 m.

Expected Results:  
  • Measured parameter: voltages [mV]. Depending on electrode configurations and currents injected values may range from a few ÁV to several V.
  • Data analysis: inversion algorithms are employed to produce 1-D models from the measured sounding curves. Layer suppression: relatively thin layers between two layers of moderate resistivity may not contribute to the measured data and therefore remain hidden. Pitfalls: Non-uniqueness: many underground models may predict observed data equally well.
  • Interpretation: resistivity-depth functions are associated with geological units. Additional geological or geophysical surface data may be required for reliable interpretation. A priori information (layer thickness and / or resistivity values) are helpful to constrain the models.
Combination with other Methods:  
  • Required additional information: rough estimates of target depths.
  • Related add-on information: electromagnetic data; Electrical data; Knowledge of layer thicknesses (e.g., borehole logs, seismic methods) and/or resistivity values (e.g., electrical logs).
  • Independent additional information: georadar data; seismics data
Operating Expense:  
  • Crew size: 1 key person, 1-2 assistants
  • Acquisition speed: small arrays (< 250 m): 12 - 15 soundings per day; large arrays (> 250 m): 5 - 12 soundings per day.
  • Processing: requires 1 - 2 days per acquisition day
  • Equipment rental costs: low
Parameters to specify:  
  • Array type: Either the Schlumberger or, less effectively, the Wenner array is used for sounding, since most commonly available interpretation methods and interpretation aids are based on these arrays.
  • Array size: Total length of array: 3 to 10 times the depth of investigation; typically more then six times the depth of investigation.
  • Array orientation: Should be parallel to the strike of geological structures.
  • Array orientation: should be parallel to predominant direction of groundwater flow.
  • Electrode position: should be logarithmically spaced. Six points per decade is the minimum recommended number. Usually 8 points are used.
QC Documents:  
  • Documentation of accuracy of transmitted currents and voltages.
  • Measurements of natural potentials and transition resistances between electrodes and ground.
  • Measurements of reproducibility (measurements of reciprocal or redundant configurations).
  • Optional: Map of buried cables, roads, crossed sounding (to assure the 1-D structure, otherwise profiling or tomography is recommended).
Products:  
  • 1-D resistivity-depth functions
  • Sounding curves
  • Interpretation
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