resistivity

Question 1

What is Ohm’s Law?

Answer 1

The relationship between applied current and the resulting potential
–ΔV = IR
–(ΔV / L) = ρ (I / A)

Question 2

What is resistivity?

Answer 2

A material’s ability to oppose electric flow
- inverse of conductivity

Question 3

What is the conservation of electrical charge?

Answer 3

a physical law that states the change in the amount of charge in a fixed volume is equal to the amount that flows into the volume less the amount that flows out of that volume

Question 4

What is the flow pattern for current flow lines

Answer 4

Electric current flow through a material from a source (+) electrode to a sink (-) electrode.

Question 5

What are equipotential surfaces

Answer 5

the potential field associated with current flow lines
- intersect flow lines at 90 degrees

Question 6

What types of rocks have low resistivity?

Answer 6

Rocks with metallic minerals, porous rocks/water saturated rocks

Question 7

Describe electrical conduction

Answer 7

Free electrons that exist in metals and graphite act as the charge carriers. These electrons move very easily when a voltage difference is applied to these materials.

Rocks containing metallic ores or graphite can have very low resistivity due to electronic conduction.

Question 8

Describe ionic conduction (electrolytic conduction)

Answer 8

In porous rock or soil containing water, electrical current can be transported through the pore space using the dissolved ions in the water as charge carriers.

Depends on concentration & mobility of dissolved ions, porosity & water saturation

Question 9

Describe interfacial conduction (surface conduction)

Answer 9

Electrical structures called diffuse double layers occur along solid/pore water interfaces. Electrical current is conducted along the surface in these structures.

• independent of electrolytic conduction component

Question 10

What is the least resistive rock type?

Answer 10

Sedimentary
(due to lower porosity and lack of pore connectivity in igneous/meta)

Question 11

Are unconsolidated or consolidated rocks more resistive?

Answer 11

Consolidated.

Question 12

Does fracturing and other permeability enhancing processes (e.g., jointing, weathering, dissolution) increase or decrease resistivity?

Answer 12

Decrease

Question 13

How do surface resistivity methods work? (i.e., general 4 electrode array)

Answer 13

measurement of electrical potential/voltage at points along the Earth’s surface which result from the electric flow between the current electrodes

Source and sink A B
Potential electrodes M N
A M N B

Question 14

What is apparent resistivity pa

Answer 14

Apparent resistivity is calculated under the assumption that Earth is homogenous. Actual resistivity can vary from this as Earth is heterogeneous

Question 15

How can we determine the resistivity of the individual layers from surface resistivity measurments

Answer 15

Inversion.

Question 16

If ρ1 > ρ2, then θ1 < θ2, which way do current flow lines bend?

Answer 16

Away from the normal

Question 17

If ρ1 < ρ2, then θ1 > θ2, which way do current flow lines bend?

Answer 17

Towards the normal

Question 18

Describe vertical electrical sounding (VES)

Answer 18

• vertical variation where bedding is horizontal
• performed by systematically expanding array dimensions while the array remains centered at a fixed midpoint
• As the array expands, current flow lines penetrate deeper into the Earth.

Question 19

Describe electric profiling / constant separation transversing (CST)

Answer 19

• horizontal variations
• systematically moving an array with fixed electrode separations along a profile line

Question 20

Describe electrical resistivity tomography (ERT)

Answer 20

• horizontal and vertical data
• regularly spaced electrodes

Question 21

What is the Wenner array?

Answer 21

equally spaced electrode where the inner electrodes are the potential electrodes; the outer electrodes are the current electrodes.
A M N B

Question 22

What is the Schlumberger array?

Answer 22

The inner electrodes are the potential electrodes; the outer electrodes are the current electrodes.
- Spacing of inner is small
A. M N. B

Question 23

What is the Dipole-dipole array

Answer 23

the current and potential are paired to form a current dipole and a potential dipole at the ends of the array

B. A M N

Question 24

What is the principle of equivalence

Answer 24

A basin shaped curve would cause it to appear that there is a relatively more/less resistive layer at depth

Question 25

What is the the principle of suppression

Answer 25

inability to detect intermediate layers due to sounding curve indicating a progressive increasing or decreasing resistivity with depth

Question 26

Applications of resistivity methods

Answer 26

Near-surface stratigraphy and aggregate exploration
Bedrock surface mapping
Engineering & geotechnical issues (e.g., landslide monitoring & characterization)
Groundwater resource management
Groundwater salinity & contamination
Agricultural application (e.g., monitoring soil moisture, soil salinity)
Archaeology