Electrical Resistivity Lab

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Electrical Resistivity Lab

• HWR 532
• Josh Clemons

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The following graphics were generated using
Model 41 consisting of 41 electrodes spaced 1
meter apart with 4 nodes per electrode spacing.
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Buried TargetSmall target, shallow depth
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Buried TargetLarge target, shallow depth
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Buried TargetSmall target, intermediate depth
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Buried TargetLarge target, intermediate depth
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Buried TargetSmall target, deep depth
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Buried TargetLarge target, deep depth
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2 Targets, Different rSmall, high r target above
large, low r target
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Note how the following two graphics are almost
identical, although the underground situation is
different. The small target below the large
target is virtually invisible.
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2 Targets, Different rSmall, high r target below
large, low r target
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1 TargetLarge, low r target
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1 TargetSmall, high r target
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Values Used for Clay/Sand Slides

• EC of water 800 mmho/cm
• Porosity of sand 0.4
• Porosity of clay 0.45
• Archie equation
• ECbulk (ECwater)(water content)2(porosity)1.2

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1 m sand above clayq 0.05
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2 m sand above clayq 0.05
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3 m sand above clayq 0.05
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4 m sand above clayq 0.05
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1 m sand above clayq 0.15
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2 m sand above clayq 0.15
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3 m sand above clayq 0.15
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4 m sand above clayq 0.15
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1 m sand above clayq 0.25
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2 m sand above clayq 0.25
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3 m sand above clayq 0.25
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4 m sand above clayq 0.25
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1 m sand above clayq 0.35
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2 m sand above clayq 0.35
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3 m sand above clayq 0.35
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4 m sand above clayq 0.35
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Notice in the following four slides that it is
difficult to tell the difference between various
water contents in the same beds.
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2 m sand above clayq 0.05
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2 m sand above clayq 0.15
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2 m sand above clayq 0.25
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2 m sand above clayq 0.35
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Inversion ModelingWenner array, 1 m spacing
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Inversion ModelingWenner array, 1 m spacing

• RMS error after 3 inversions is 29.7 - not very
  good.
• The model indicates a relatively resistive
  formation or group of formations just below the
  surface, underlain by less resistive layers.

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Inversion ModelingWenner array, 3 m spacing
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Inversion ModelingWenner array, 3 m spacing

• RMS after 3 inversions is 35.7 - even worse than
  for the 1 m spacing.
• Drawing conclusions from this model would be
  risky. However, it does appear that there is
  something relatively resistive just below the
  surface, underlain by less resistive material.

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Inversion ModelingWenner array, 5 m spacing
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Inversion ModelingWenner array, 5 m spacing

• RMS after 3 inversions is 64.1 - the worst yet.
  The RMS has increased with electrode spacing for
  these models.
• As with the 3 m spacing, drawing a conclusion is
  risky. However, the apparent relatively
  resistive body appears again at right.