Geophysical Mapping: Method Details
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Geoelectrics - Crosshole tomography
Characteristics of hazardous waste
Location of buried materials
Quality / Thickness of aquifer/aquitard
Quality and thickness (Natural resources)
Soil / rock quality
Depth of Overburden-bedrock interface
Foundations of ancient structures
Buildings and Structures
Gravel, clay, limestone, salt exploration
Host sediments, hydogeological settings
Permafrost and ice detection
Porosity / Permeability
'+' = Technique applicable; '0' = Application possible/limited use
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
between the boreholes.
Crosshole tomography; 2-D and 3-D electrical survey; tomographic inversion; resistivity contrast; resistivity distribution; tomogram
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
Main factors that control resolution of tomograms include resistivity contrasts, signal-to-noise ratio, and electrode configurations employed.
is at best of the order of the electrode spacing.
depends on the array type chosen and the borehole separation.
Depth of investigation
is equal to borehole depths.
voltages [mV]. Depending on electrode configurations and currents injected values may range from a few ÁV to several V
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.
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
1 key person, 1-2 assistants
1 - 2 tomographic planes / day (strongly instrument dependent)
Requires 1 - 2 days per acquisition day
Equipment rental costs:
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.
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
Tomogram (2-D Resistivity distribution between boreholes)
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