Geophysical Mapping: Method Details
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Method Name: Induced polarization - Tomography (surface-based)
Method Type:   Electrical Methods
Assigned Problems:
+ Aquifer pollution Groundwater
+ Contaminant plumes Hazardous Waste
+ Gravel, clay, limestone, salt exploration Natural Resources
0 Characteristics of hazardous waste Hazardous Waste
0 Location of Ancient Structures Cultural Heritage
0 Quality and thickness (Natural resources) Natural Resources
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   The purpose of induced polarization surveys is to determine the lateral and vertical subsurface chargeability distribution.
Keywords:   Surface based tomography; 2-D and 3-D electrical survey; tomographic inversion; chargeability contrast; chargeability distribution; tomogram
Prerequisites:  
  • Target must be characterized by a chargeabilty 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 chargeability 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 penetration depths up to several 100 m are possible.

Expected Results:  
  • Measured parameter: chargeability [mV/V] (and usually also the apparent resistivity [Ωm]).
  • 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; Resistivity distribution should be approximately known.
  • 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 2 - 3 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.
  • Electrode type, if possible use non-polarizing electrodes.
  • 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 and voltages.
  • Measurements of natural potentials and transition resistances between electrodes and ground.
  • Measurements of reproducibility (measurements of reciprocal or redundant configurations).
  • Sensitivity matrices, resolution matrices.
  • Optional: Map of buried cables, roads.
Products:  
  • Tomogram (2-D cahrgeabilty-depth slice along a profile or 3-D resistivity bodies).
  • Interpretation
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