Uw-Mil Geophysics 316 (Magnetism)

Front 1

What is active geophysical methods

Back 1

Active methods artificially produce an electrical, electromagnetic, or acoustic signal that is transmitted into the ground. Variations in the subsurface interact with and modify these signals according to their specific physical properties, producing an altered return signal that is measured by instrumentation at the surface. Resistivity methods inject an electrical current, soil conductivity meters transmit radio energy, and ground penetrating radar sends a microwave pulse; in each case the instrumentation measures a response to these actions.

Front 2

What is passive geophysical methods

Back 2

Passive methods detect variations within the natural fields of the earth. They include magnetic and gravitational fields, and certain native electrical properties within the soil. Magnetometry is the principal passive method employed in archaeo-geophysics.

Front 3

What are the three sources for the magnetic field that we measure at Earth’s surface and what are the timescales of variation for each?

Back 3

1) external field: from the magnetosphere (minutes to days)

2) internal core field: from dynamo action in the outer core (years to geologic time scales)

3) crustal field: from remenant magnetization (geologic time scales)

Front 4

How do we describe magnetic field or magnetization?

Back 4

Magnetic field is a vector and as such it has both a direction and an intensity. At any point, the field can be described by declination, inclination and intensity.

Front 5

What is Declination?

Back 5

Angular deviation from true north in the horizontal plane

Front 6

What is Inclination?

Back 6

angular deviation from horizontal (defined as positive down)

Front 7

What is Intensity?

Back 7

the strength or magnitude of the field

Front 8

What is the Geocentric Axial Dipole (GAD) assumption?

Back 8

The approximate dipole is located at the center of the Earth and aligned with the Earth’s spin axis.

Front 9

If we assume GAD, how does inclination vary with latitude?

Back 9

actual inclination does not exactly match the dipole equation

Front 10

Why does Earth’s field deviate from GAD over short time scales?

Back 10

Earths dipole flips

Front 11

What is Diamagnetism?

Back 11

Precession of electrons in the presence of a magnetic field. Generates a field appeasing the inducing field.

Front 12

What is Paramagnetism?

Back 12

Becomes magnetized in the presence of a field, but magnetization is zero in the absence of a field. It has no remanent magnetization.

Front 13

What is Ferromagnetism?

Back 13

Retains a magnetization even in the absence of a field. Has remanent magnetization.

Front 14

What is Remanent magnetization?

Back 14

the (permanent) magnetization present in a material

Front 15

What is Induced magnetization?

Back 15

is the (temporary) magnetization present in a material only as long as a field is applied.

Front 16

What are 4 processes by which rocks can become permanently magnetized?

Back 16

1) thermo remanent
2) detrital remanent
3) isothermal remanent
4) chemical remanent

Front 17

What is thermo remanent magnetization?

Back 17

a magnetization acquired on cooling through a rock's Curie temperature(s) and blocking temperatures

Front 18

What is detrital remanent magnetization?

Back 18

a magnetization acquired by sediments during deposition

Front 19

What is isothermal remanent magnetization?

Back 19

re/magnetization in the presence of a large magnetic field, such as a lightning bolt strike

Front 20

What is chemical remanent magnetization?

Back 20

magnetization acquired when a magnetic crystal forms in a magnetic field below its Curie temperature.

Front 21

What is the fold test? Baked contact test? Conglomerate test?

Back 21

Tests to find out when magnetization was acquired before or after folding, before or after intrusion, before or after emplacement of conglomerate.

Front 22

What is a virtual geomagnetic pole (VGP) and how is it related to apparent polar wander?

Back 22

The point on the Earth's surface at which a magnetic pole would be located if the observed direction of remanence at a particular location was due to a magnetic dipole at the centre of the Earth.

Front 23

What is apparent polar wander?

Back 23

the perceived movement of the Earth's paleo-magnetic poles relative to a continent while regarding the continent being studied as fixed in position.

Front 24

When we measure magnetic anomalies produced by a magnetic body below Earth’s surface, how and why do we care about the direction of Earth’s field at our measurement location?

Back 24

We measure magnetic anomalies as deviations from the surrounding (ambient) Earth’s field. We must know direction of Earth’s field at our measurement location in order to calculate the anomaly.

Front 25

Imagine that Earth's field is (largely) caused by a bar magnet in the center of the Earth. Let's refer to magnetic north as the location where this imaginary bar magnet would pierce Earth's surface. What is the present angular difference between true north (Earth's spin axis) and magnetic north?

Back 25

11.5°

Front 26

If the Earth's field could be represented by a perfect dipole (like a bar magnet), inclination would vary smoothly from equator to pole. What would the inclination be at the following latitudes (45°N, North Pole, Equator).

Back 26

45°N = +63° inclination
North pole (+90°) = +90° inclination
Equator (0°) = 0° inclination

Front 27

Paleomagnetism involves..

Back 27

rocks that retain a magnetism acquired long ago

Front 28

True or False - Magnetite has a stronger magnetic remanence than hematite?

Back 28

True