The BGS Offset Wenner Array System
•Multi-electrode arrays are now commonly used.
–A computer-controlled switch box turns electrodes on-off
–Can get a lateral and vertical data in one step
–Can also assess error and lateral variations.
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In these Lecture Slides, the Lecturer has tried to illustrate the following key points : Offset, Wenner Array System, Multi-Electrode Arrays, Computer-Controlled Switch Box, Vertical Data, Lateral Variations, Step, Electrodes On-Off, Detection of Small, Layers Result
Typology: Slides
2012/2013
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The BGS Offset Wenner Array System
• Multi-electrode arrays are now commonly used.
- A computer-controlled switch box turns electrodes on-off
- Can get a lateral and vertical data in one step
- Can also assess error and lateral variations.
VES Limitations
- Maximum depth of detection depends on:
- Electrode spacing (rule of thumb depth = ½ C electrode spacing)
- Resistivity contrasts between layers
- Limits of detection of small ΔV
- Low-resistivity layers result in ΔV becoming very small
- Large spacings cause ΔV to become small
- Layers may have spatially-variable resistivities
- If so, electrical profiling may be a better choice
- If not, you can interpolate lateral continuity
Electrical Profiling
• Lateral changes in resistivity can be effectively mapped
using electrical profiling.
- Can use similar arrays to VES
- Patterns vary depending on what array is used
- Patterns are complicated because electrodes may be in zones
of different properties.
Electrical Imaging
- Because resistivity may vary both laterally and vertically, neither
VES or electrical profiling may give the desired results.
- To image lateral and vertical changes, electrical imaging is used
- Involves expanding and moving arrays
- produces a pseudosection
- pseudosections do not reveal the actual properties, but do show useful patterns
Final Remarks
• Like all geophysical techniques resistivity:
- Produces non-unique results
- Data should be compared to known geological data (e.g. boreholes)
- Similar rocks have a wide range in resistivities depending on water content
- Lithology changes do not necessarily correspond to a resistivity change
- Resistivity changes to not necessarily correspond to a lithology change
- So, without sound geological knowledge, resistivity data may