Magnetics - Part 1

Question 1

How does magnetic strength and direction vary between the equator and the poles?

Answer 1

Equator strength - ~30, 000 nT
Pole strength - ~60, 000 nT
Direction is near horizontal at equator and near vertical at the poles

Question 2

What forms the magnetic field?

Answer 2

Motions of liquid metal at core e.g. Fe (98%) and the sun (2%)

Question 3

What is the magnetic field composed of?

Answer 3

Vectors, with magnitude AND direction

Question 4

What is meant by the magnetic inclination?

Answer 4

The angle that a compass needle points

Question 5

How does magnetic inclination vary at the north and south magnetic pole?

Answer 5

At the north magnetic pole the needle points directly to the ground and at the south pole the needle points straight upwards.
At the magnetic pole the inclination is zero

Question 6

Why is it important to know tendency of materials towards magnetisation and how magnetisation locally affects the Earth’s field?

Answer 6

Earth’s magnetic field is perturbed by materials that are capable of being magnetised

Question 7

What relationship are measurements of the Earth’s magnetic field dependent on?

Answer 7

Relationship between Earth’s total magnetic field (F), magnetisation (J) induced within a body of magnetic susceptibility (X)

Question 8

What equipment is typically used by magnetic survey to measure magnitude (intensity) of the total field vector? What can thios not be used to measure?

Answer 8

Proton precession magnetometer

Can’t be used to measure direction

Question 9

What is the units for magnetic intensity, magnetic susceptibility and magnetic induction?

Answer 9

Magnetic field intensity and magnetic induction (magnetisation) measure by nanotesla (nT = 10^-9 T)
Magnetic susceptibility is dimensionless

Question 10

What is the magnitude and direction of magnetisation induced within a material dependent on?

Answer 10

Magnitude and direction of the external (ambient field) and the ability of the material to be magnetised

Question 11

What is the equation for the induced magnetic field of material (J)?

Answer 11

J = XFamb
Where:
J - induced magnetic field of material
X - magnetic susceptibility
Famb - magnitude and direction of ambient (external) field

Question 12

What is magnetic susceptibility?

Answer 12

A measure of the degree to which a substance may be magnetised. The overall susceptibility of a rock is roughly equivalent to the susceptibility of the magnetic mineral divided by 100

Question 13

Name three different kinds of magnetic behaviour and give there corresponding X value

Answer 13

Diamagnetism - X=10^-5
Paramagnetism - X=10^-4
Ferromagnetism - X=10^-1

Question 14

Describe what is meant by diamagnetism

Answer 14

diamagnetic minerals (e.g. halite) have a negative magnetic susceptibility acquiring an induced magnetisation opposite in the direction to an applied external field. The weak magnetisation results from alteration of electron orbitals as force from the external field is applied to the material. Susceptibilities of only about 10^-5 mean that magnetisation is on the order of 1/100,000th the strength of the external field

Question 15

Describe paramagnetism

Answer 15

The magnetic susceptibility of paramagnetism is positive; they acquire a magnetism parallel to an external field. The magnetisation occurs as magnetic moments of atoms are particularly aligned in the presence of the external field. Most magnetic minerals show weak magnetic behaviour

Question 16

Describe ferromagnetism

Answer 16

In some metallic minerals rich in iron, cobalt or nickel, atomic magnetic moments align strongly with an external field. Susceptibilities on the order of 10^-1 indicate that magnetisation is in the same direction as and about 1/10 the magnitude of the ambient field. In some cases induced magnetisation may remain in ferromagnetic materials, even after ambient field is removed