Lecture 3 - Magnetism

Question 1

What are the significance of magnetic surveys?

Answer 1

• Measure the strength of the Earth’s magnetic field at different places using a magnetometer
• Can map and interpret magnetic anomalies
• Provides information on the geometry and
  properties of magnetised rock units in the
  subsurface

Question 2

Describe and explain a magnetic property

Answer 2

• Earth’s magnetic field varies in both direction and magnitude.
• The shape of a magnetic anomaly measured over a subsurface body will therefore depend on the direction which the Earth’s field is being measured

Question 3

Describe the Earth’s magnetic field

Answer 3

Magnetization is a vector so the contrast between rock units varies in magnitude and direction. This has a strong effect on the shape of the magnetic anomaly measured over the contact. Different magnetic anomalies don’t necessarily indicate different bodies – it all depends on characteristics of the local magnetic field, as to what you record in your survey

Question 4

In what unit is the magnetic field measured?

Answer 4

Tesla

Question 5

What is Tesla defined as?

Answer 5

The field that exerts a torque of 1
Newton-metre on a current loop of 1 m2
, in which a current of 1A is passed

Question 6

What is meant by torque?

Answer 6

The tendency of a force to rotate an object around an axis

Question 7

What does the Earth’s magnetic field look like and what causes this?

Answer 7

The Earth is a magnetic dipole inclined at 11.5 degrees and resembles a bar magnet.The angle of inclination varies with latitude due to convection currents in the Earth’s molten iron core

Question 8

What are solar winds?

Answer 8

Very high energy, high speed particles that will erode the atmosphere on a planet e.g. Mars. The Earth’s magnetic field deflects solar wind

Question 9

What is meant by palaeomagnetism?

Answer 9

If the rock contains magnetism when it was formed this will be aligned parallel to the ancient magnetic field.

Question 10

What can cause disturbances in the magnetic field?

Answer 10

Utilities, power lines, buildings, metallic debris

Question 11

What is induced magnetism?

Answer 11

temporary magnetism imposed on rock because it is permeated by the Earth’s magnetic field

Question 12

What is Natural Remnant Magnetism?

Answer 12

Is permanent magnetism ‘frozen’ into rock at the time of its formation (e.g. in a lava when it cools)