Magnetics - Part 2

Question 1

In what two ways can magnetisation occur in rocks?

Answer 1

1. Induced by Earth’s present magnetic field

2. Formed sometime in the past as rock lithified

Question 2

What is induced magnetisation?

Answer 2

Magnetism as a result of an external field, induced magnetisation may be lost when taken out of field. Magnitude and direction of induced field dependent on ambient field and magnetic susceptibility of the rock

Question 3

What is remanent magnetisation?

Answer 3

when rocks form some minerals (magnetite) align orientate themselves in direction of ambient field. As rock lithifies, orientation of the magnetic domains may be frozen into rock. Remanent magnetisation remains after ambient field has gone. Commonly 5 times greater than induced magnetisation from present field

Question 4

What is the total magnetic field the sum of in the absence of remanent magnetisation?

Answer 4

F = Famb + Find
Where:
F - total magnetic field
Famb - Earth's ambient magnetic field in region
Find - induced magnetic field

Question 5

How can subsurface magnetic susceptibility be calculated?

Answer 5

Find = F - Famb

Question 6

How is the total field anomaly (^F) calculated?

Answer 6

^F = F-Famb

Subtracting magnitude of ambient field from F

Question 7

What are aeromagnetic surveys used to measure?

Answer 7

Magnitude of total magnetic field

Question 8

How can forward modelling of magnetic anomalies be used?

Answer 8

Interpret distribution of magnetic susceptibility contrasts (^X) in the subsurface

Question 9

What mineral content results in high levels of magnetism?

Answer 9

High Ferromagnetic mineral content

Question 10

What are the main sources of magnetic anomalies?

Answer 10

Crystalline basement rocks, as more magic than overlying sediments

Question 11

What are indicators of shallow basement?

Answer 11

Regions with high magnetic anomaly amplitudes, short wavelengths and steep gradients

Question 12

What indicated deep basement?

Answer 12

More subdued magnetic anomalies

Question 13

How do minerals with strong ferromagnetic behaviours at low temperatures respond to being heated up?

Answer 13

They display weaker paramagnetic properties.

Deeper into the subsurface rocks are heated and can lose strong magnetisations

Question 14

What is the Currie temperature?

Answer 14

600 degrees in most rocks

Question 15

What can the Currie temperature be used to map?

Answer 15

Approximate depth where rocks reach 600 degrees