Resistivity methods

Question 1

What is meant by potential difference, current and resistance?

Answer 1

Potential difference/voltage: Comes from a battery or other supply. Symbol V, unit Volt.

Current: Flow of electrical charge from positive to negative potential. Symbol I, unit Ampere.

Resistance: The amount of potential difference required to push a given current is directly proportional to the resistance. Symbol R, unit volt/meter. Ohm’s law R=V/I

Question 2

What does resistance depend on?

Answer 2

1. The material properties – the resistivity of the material.
2. The shape of the material that has current flowing through it.

Question 3

How is resistivity measured?

Answer 3

Apply known potential difference –>
Measure the current –>
Calculate resistance –>
Calculate resistivity

Question 4

What does resistivity depend on?

Answer 4

• Presence of metallic ores.
• Temperature of the subsurface (Geothermal energy)
• Archaeological features
• Amount of groundwater present (Dissolved salts, Contaminants, Porosity and permeability)

Question 5

How is resistors in series’ total resistance calculated?

Answer 5

By simply adding their resistance together.

Question 6

How is resistors in parallell’s total resistance calculated?

Answer 6

R(tot)=1/(1/R1 + 1/R2 + …)

Question 7

What is conductans?

Answer 7

The inverse of resistance. A measurement of a material’s ability to pass current through it.

Question 8

Which two types of conduction is found in rocks?

Answer 8

Electrolytic/ionic: Slow movements of ions in fluid. Salts dissociate into ions in solution and ions move. Motion of cations (+) and anions (-) in opposite directions.

Electronic: Metals allow electrons to move freely

Question 9

Which types of rocks have the highest to the lowest resistivity?

Answer 9

Igneous rocks (magmatiska) the highest. Then metamorphic lower and then sedimentary rocks the lowest.

Older rocks higher resistivity than newer rocks.
This has to due with how much pore space and water there is in the rocks.

Question 10

How much spacing between the electrodes is used during surveying?

Answer 10

Typically the outer electrode spacing is 2x your target depth

Question 11

How many electrodes are used during surveying and what is used to measure the resistivity?

Answer 11

Modern systems use at least four electrodes.
A resistivity meter consists of both a voltmeter and a current meter (ammeter). Most systems directly reports the ratio V/I, the resistance.

Question 12

Mention some electrode arrays?

Answer 12

Wenney array, pole-pole array, pole-dipole array, dipole-dipole array, Schlumberger array

Question 13

What is vertical electrical sounding (VES) done for?

Answer 13

Used to determine the apparent resistivity of several horizontal layers with different resistivities.

Question 14

How is measurements done in VES?

Answer 14

The essence is to expand electrodes from a fixed centre. It causes electricity to penetrate deeper into the ground. The potential is measured with current electrodes (Wenner, Schlumberger)

Question 15

What happens with the current and the apparent resistivity when the electrode spacing is small?

Answer 15

Nearly all current will flow through upper layer. The resistivities of lower levels have negligible effect and therefore the measured apparent resistivity is the resistivity of the upper layer

Question 16

What is meant by apparent resistivity?

Answer 16

It comes when measuring a layered subsurface. The apparent resistivity is the resistivity that uniform ground giving the same V/I with the same electrode separations would have. When changing spacing the apparent resistivity changes due to current flowing into deeper layers.

Question 17

What is meant by current refraction?

Answer 17

Current refracts towards the normal when going into a layer with greater resistivity. Changes the distribution of current in a layered subsurface. is the “opposite” of Snell’s law

Question 18

How is VES data intepretated?

Answer 18

The aim is to know resistivity and thickness of each layer and this is done by computer modelling, both forward and inverse modelling. Previously master curves were used.

Question 19

What are the limitations with VES?

Answer 19

There is a maximum detection rate that depends on electrode spacing and resistivity between layers for example.

Resistivities can vary laterally - then VES should not be done. Use electrical profiling instead.

Layers can have anisotropic resistivities.

Sandwiched thin layers give non-uniqueness due to refraction. Called equivalence

Question 20

What is electrical profiling?

Answer 20

It is done to map lateral changes in resistivity. Simliar arrays as VES can be used. The difference is that here the entire array is moved along a line/profile.

Question 21

What is electrical imaging?

Answer 21

When resistivity may vary both laterally and vertically, neither VES or electrical profiling may give the desired results and electrical imaging is then done. It involves expanding and moving arrays.
Electrical imaging gives a pseudosection that reveals some useful patterns.

Question 22

What unit is resitivity measured in?

Answer 22

Ohm meter

Question 23

Vad är en sonderingskurva?

Answer 23

Skenbar resitivitet som en funktion av avståndet mellan strömelektroderna. När man gör VES kan man plotta en sonderingskurva.

Question 24

How does water effect the resitivity?

Answer 24

Resistivity in water is the measure of the ability of water to resist an electrical current, which is directly related to the amount of dissolved salts in the water. Water with a high concentration of dissolved salts will have a low resistivity, and vice versa