Offset - Seismology - Lecture Slides, Slides of Geology

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

Uploaded on 07/19/2013

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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.
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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

be misleading.