Geophysical Mapping: Method Details
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Method Name: Geoelectrics - Tomography (surface based)
Method Type:   Electrical Methods
Assigned Problems:
+ Aquifer pollution Groundwater
+ Cavity detection Civil Engineering
+ Contaminant plumes Hazardous Waste
+ Depth of Overburden-bedrock interface Civil Engineering
+ Foundations of ancient structures Buildings and Structures
+ Fractures Groundwater
+ Gravel, clay, limestone, salt exploration Natural Resources
+ Groundwater table Groundwater
+ Host sediments, hydogeological settings Hazardous Waste
+ Landslides Natural Hazards
+ Location of Ancient Structures Cultural Heritage
+ Location of buried materials Hazardous Waste
+ Monitoring Hazardous Waste
+ Permafrost and ice detection Natural Hazards
+ Quality / Thickness of aquifer/aquitard Groundwater
+ Quality and thickness (Natural resources) Natural Resources
+ Quantity/ Thickness Hazardous Waste
+ Soil / rock quality Civil Engineering
+ Temporal variations Groundwater
0 Characteristics of hazardous waste Hazardous Waste
0 Ice thickness Natural Hazards
0 Porosity / Permeability Groundwater
0 Quality / Thickness of concrete Buildings and Structures
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   The purpose of geoelectric surveying is to determine the lateral and vertical subsurface resistivity distribution.
Keywords:   Surface based tomography; 2-D and 3-D electrical survey; tomographic inversion; resistivity contrast; resistivity distribution; tomogram
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)
  • Topography / surface dips > 10░ may require topographic corrections
Resolution:   Main factors that control resolution of tomograms include resistivity contrasts, signal-to-noise ratio, and electrode configurations employed. Horizontal resolution is at best of the order of the electrode spacing. Vertical resolution depends on the array type chosen.
Typical values of vertical resolution are (Depth range / Target thickness): 0 - 10 m / 1 m to several m; 10 - 50 m / several m to 10 m; > 50 m / several tens of m or more

Typical depth of investigation: < 50 m, but up to several 100 m are possible

Expected Results:  
  • Measured parameter: voltages [mV]. Depending on electrode configurations and currents injected, values may range from a hundred ÁV to several V.
  • Data analysis: tomographic inversion of voltages. Resulting tomograms show 2D resistivity distributions along profiles or between boreholes. Pitfalls: Non-uniqueness problem: many underground models may predict observed data equally well.
  • Interpretation: individual resistivities are related to different geological units and water content. 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: geological information for reliable interpretation
  • Related add-on information: electromagnetic data
  • Independent additional information: georadar data ; seismics data
Operating Expense:  
  • Crew size: 1 key person, 1-2 assistants
  • Acquisition speed: around 1500 - 5000 measuring points (or 250 - 500 m profile length) per day
  • Processing: Requires 1 - 2 days per acquisition day
  • Equipment rental costs: intermediate (requires multi-electrode system)
Parameters to specify:  
  • Array type: Wenner, dipole-dipole or Schlumberger type configurations are typically employed for tomographic inversions.
  • Array orientation: Should be perpendicular to the strike for a maximum response of geological structures (if only measured along profiles).
  • Electrode spacing: Usually between 1 and 10 m.
  • Data should be acquired with a multi-electrode system, which allows large amounts of data to be collected efficiently.
QC Documents:  
  • Documentation of accuracy of transmitted currents.
  • Measurements of natural potentials and transition resistances between electrodes and ground.
  • Measurements of natural potentials and transition resistances between electrodes and ground.
  • Measurements of reproducibility (measurements of reciprocal or redundant configurations).
  • Sensitivity matrices, resolution matrices.
  • Documentation of accuracy of transmitted currents.
  • Optional: Map of buried cables.
Products:  
  • Tomogram (2-D Resistivity-depth slice along a profile or 3-D resistivity bodies)
  • Interpretation
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