Current Refraction
•Because refraction changes the distribution of current
in a layered subsurface
–The ratio of V/I changes
–We can therefore measure changes in resistivity with depth
Uniform subsurface Layered subsurface
12
ρρ
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Current Refraction - Seismology - Lecture Slides, Slides of Geology
Following are the Fundamentals of these Lecture Slides : Current Refraction, Refraction Changes, Distribution of Current, Layered Subsurface, Resistivity, Layered Subsurface, Uniform Subsurface, Apparent Resistivity, Ratio Changes, Electrodes
Typology: Slides
2012/2013
1 / 7
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Current Refraction
- Because refraction changes the distribution of current
in a layered subsurface
- The ratio of V/I changes
- We can therefore measure changes in resistivity with depth
Uniform subsurface (^) Layered subsurface
ρ 2 > ρ 1
Apparent Resistivity
- In a VES survey the ratio V/I is measured with increasing
electrode spacing…
- The ratio changes for two reasons:
- Layers of differing resistivity are encountered
- The electrodes are now farther apart
- Causes measured resistance to decrease!
- To determine #1, we must first correct for #
- To account for the effects of changes in electrode
spacing, the apparent resistivity is found as:
- Here, α is a “geometrical factor”
- equal to a/l for a rod (see previous slides)
- The geometrical factor varies depending on array configuration / type
- I’ll show some common array types later
- For reasons that you will soon see, apparent resistivity
, ρa , is what is typically used
ρ (^) a α
V
I
Apparent Resistivity
Wenner Arrays
- Pronounced “Venner”. This is the most commonly used in the U.S.
- All four electrodes are equally spaced. Spacing = a
- Geometrical correction factor = 2πa
- Measure resistance (V/I)
- Calculate apparent resistivity
- Repeat for a range
of spacings
I
V
ρ a = 2 π a
Wenner VES Survey
- Results of ρa are plotted as log 10 ρa versus log 10 a
- Use logs to help accommodate the large range in values For a simple two layer scenario: (multiple layers are more complex)
- The first few spacings:
- Electrical current mostly flows in the upper layer
- so the apparent resistivity is the actual resistivity of the upper layer
- At spacings that are large compared to layer 1’s thickness:
- Most of the length that the current travels is in the lower layer
- So the apparent resistivity is the resistivity of the lower layer How do we determine layer thickness?