Geophysical Mapping: Method Details
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Method Name: Geoelectrics - Crosshole tomography
Method Type:   Electrical Methods
Assigned Problems:
+ Aquifer pollution Groundwater
+ Cavity detection Civil Engineering
+ Characteristics of hazardous waste Hazardous Waste
+ Contaminant plumes Hazardous Waste
+ Fractures Groundwater
+ Location of buried materials Hazardous Waste
+ Monitoring Hazardous Waste
+ Quality / Thickness of aquifer/aquitard Groundwater
+ Quality and thickness (Natural resources) Natural Resources
+ Soil / rock quality Civil Engineering
0 Depth of Overburden-bedrock interface Civil Engineering
0 Foundations of ancient structures Buildings and Structures
0 Gravel, clay, limestone, salt exploration Natural Resources
0 Groundwater table Groundwater
0 Host sediments, hydogeological settings Hazardous Waste
0 Permafrost and ice detection Natural Hazards
0 Porosity / Permeability Groundwater
0 Quantity/ Thickness Hazardous Waste
0 Temporal variations Groundwater
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   DC geoelectric crosshole tomography is a method in which the source electrodes (current injection points) and the potential electrodes (measuring points) are placed downhole in two separated boreholes and moved over a range of depths. It is able to yield detailed information on the variation of resistivity between the boreholes.
Keywords:   Crosshole 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)
  • Changing waether conditions may influence measurements
  • Electrode coupling in cased boreholes is difficult and mostly impossible, electrode need to be attached outside of casing
  • Topography effects need to be considered
Resolution:   Main factors that control resolution of tomograms include resistivity contrasts, signal-to-noise ratio, and electrode configurations employed. Vertical resolution is at best of the order of the electrode spacing. Horizontal resolution depends on the array type chosen and the borehole separation.

Depth of investigation is equal to borehole depths.

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: 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: surface-based geoelectrical data; electromagnetic data
  • Independent additional information: crosshole seismics; crosshole georadar
Operating Expense:  
  • Crew size: 1 key person, 1-2 assistants
  • Acquisition speed: 1 - 2 tomographic planes / day (strongly instrument dependent)
  • Processing:Requires 1 - 2 days per acquisition day
  • Equipment rental costs: intermediate
Parameters to specify:  
  • Array type Dipole-dipole ore pole-pole type configurations are typically employed for cross-hole tomography.
  • Electrode spacing: usually between 0.5 m and a few 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 and voltages
    • Measurements of reproducibility (measurements of reciprocal or redundant configurations)
    • Sensitivity matrices, resolution matrices
    • Optional: Map of buried cables
    Products:  
    • Tomogram (2-D Resistivity distribution between boreholes)
    • Interpretation
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