1. The Crossley Heath School, Halifax
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Electromagnetic Methods: Resistivity
17/04/2013
Will Brown
The Crossley Heath School, Halifax

2. Overview Resistivity: EM Methods: Making measurements
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Resistivity:
EM Methods:
Making measurements
Electromagnetic (EM)
“Apparent” resistivity
Equipment
Ground Penetrating Radar (GPR)
Looking deeper
Looking further
Spontaneous Potential (SP)
Sections (ERT)
Examples
Induced Polarisation (IP)
Impacts
Practical

3. EM Electromagnetic methods rely on two main principles:
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Electromagnetic methods rely on two main principles:
Measuring directly the electrical properties of the Earth
Measuring the response of different materials to an EM field or waves
Geophysical methods use a wide range of this spectrum above and below visible light. I’m mainly talking about geoelectrics today but we’ll get onto more general EM methods later.
Active or passive methods.

4. Resistivity l A Ohm’s Law: V = Voltage I = Current R = Resistance
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Ohm’s Law:
V = Voltage
I = Current
R = Resistance
ρ = Resistivity
l = Length
A = Cross-section area A l
Resistance is a fundamental material property, resistivity is an object-dependent property. Resistance is a constant for a material, resistivity gives you much more detail about porosity, fluid type, etc...

5. Conductivity l A Ohm’s Law: V = Voltage I = Current G = Conductance
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Ohm’s Law:
V = Voltage
I = Current
G = Conductance
σ = Conductivity
l = Length
A = Cross-section area A l
Conductivity is the inverse of resistivity.

6. Units
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7. Properties
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All materials have a resistivity, some have a specific value, but many have characteristic range. This can be quite large.

8. Measurements l A In the earth, things are a bit more complicated....
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In the lab:
we input a current
we measure the voltage
we calculate the resistance
we measure the geometry
we calculate the resistivity
In the earth, things are a bit more complicated....
Very complex (heterogeneous)
Strange geometries
Very high contact resistance

9. Measurements In the earth, things are a bit more complicated....
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Homogeneous – uniform, evenly distributed, the same everywhere.
Heterogeneous – unevenly distributed, properties vary in different places.
In the earth, things are a bit more complicated....
Very complex (heterogeneous)
Strange geometries
Very high contact resistance

10. Measurements An earth resistivity measurement:
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An earth resistivity measurement:
We input a current just like we would in the lab, but measure the voltage somewhere else. (because of high contact resistance)
In the lab:

11. Measurements An earth resistivity measurement:
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An earth resistivity measurement:
We input a current just like we would in the lab, but measure the voltage somewhere else. (because of high contact resistance)
We can approximate the geometry
In the lab:
In the field:
Geometrical factor

12. Geometrical factor: Why 2πa?
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Geometrical factor: Why 2πa?
Current lines
Equi-potentials

13. Geometrical factor: Other arrays.
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Geometrical factor: Other arrays.

14. What do we actually measure?
“Apparent ρ”
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What do we actually measure?

15. What do we actually measure?
“Apparent ρ”
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What do we actually measure?

16. What do we actually measure?
“Apparent ρ”
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What do we actually measure?
We measure an “apparent” resistivity

17. Equipment Why are they all different distances apart? Electrodes
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Why are they all different distances apart?
Electrodes

18. Seeing vertically – a sounding
Soundings
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Seeing vertically – a sounding
The wider our spacing, the deeper we measure.
Spacing
Is a combination of
and
Tends towards layer resistivities at ends. Currents concentrated around a depth equal to electrode spacing (a). So the wider the spacing, the deeper we see. If you want to see deep you need long cables...know you know why we use different arrays.

19. Seeing vertically – a sounding
Soundings
17/01/2019
Seeing vertically – a sounding
The wider our spacing, the deeper we measure.
Spacing
Is a combination of
and
Tends towards layer resistivities at ends. Currents concentrated around a depth equal to electrode spacing (a). So the wider the spacing, the deeper we see. If you want to see deep you need long cables...know you know why we use different arrays.

20. Seeing vertically – a sounding
Soundings
17/01/2019
Seeing vertically – a sounding
The wider our spacing, the deeper we measure.
Spacing
Is a combination of
and
Tends towards layer resistivities at ends. Currents concentrated around a depth equal to electrode spacing (a). So the wider the spacing, the deeper we see. If you want to see deep you need long cables...know you know why we use different arrays.

21. Geometrical factor: Other arrays.
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Geometrical factor: Other arrays.
If A and B have to be connected together and M and N, what is the main difference between these arrays? Remember width of array implies depth of survey.

22. Soundings
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Theoretical curves give resistivity of second layer not independently measured.
Old versions are log-log scaled, harder to plot on but easier read off rho2 and h with overlay. Plotting K allows rho2 calculation and linear plotting. (a/h) is used since a layer is sampled when the spacing is approximately equal to the depth. Curves based on ratio of layer resistivities.
Positive K is resistivity increase with depth, negative K is decrease with depth.

23. There’s a whole book of master curves for 3 layers!
Soundings
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There’s a whole book of master curves for 3 layers!
The more layers there are, the more complex to curve and the more permutations of resistivity contrasts.

24. Seeing laterally – a profile
Profiles
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Seeing laterally – a profile
Distance along profile

25. Combining a profile and sounding: a section
Sections
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Combining a profile and sounding: a section

26. Measurements We see CONTRASTS
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We see CONTRASTS
This means that resistivity surveys can be used for:
Archaeology
Finding fresh water
Locating chemical contaminations
Determining subsurface structure/geology
Plus many more...

27. Examples
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Archaeology ?

28. Examples Contaminant Plumes Lower resistivity plume of contaminants
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Contaminant Plumes
Lower resistivity
plume of contaminants

29. Examples Landfills Monitoring old landfill sites High resistivity
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Landfills
Monitoring old landfill sites
High resistivity
Low resistivity

30. What do all these examples have in common?
Impacts
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What do all these examples have in common?
i.e. Really good for environmental purposes, does no damage and leaves no trace.

31. What do all these examples have in common?
Impacts
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What do all these examples have in common?
Buried materials with contrasting properties.
Unknown locations.
Large areas.
Impractical to measure directly – can’t dig it all up!
i.e. Really good for environmental purposes, does no damage and leaves no trace.

32. What do all these examples have in common?
Impacts
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What do all these examples have in common?
Buried materials with contrasting properties.
Unknown locations.
Large areas.
Impractical to measure directly – can’t dig it all up!
Resistivity is non-invasive, silent, quick and cheap but requires contrasts in properties, especially good for fluids
i.e. Really good for environmental purposes, does no damage and leaves no trace.