GEOMAGNETISM II Magnetization · GEOMAGNETISM II Magnetization Induced magnetization, Ji. When a material is exposed to a magnetic eld H, it acquires an induced magnetization. These

GEOMAGNETISM II

Magnetization

Induced magnetization, Ji. When a

material is exposed to a magnetic field

H, it acquires an induced

magnetization. These are related

through the magnetic susceptibility, χ.

Ji = χH

Remanent magnetization, Jr. This

remanent magnetization is a recording

of past magnetic field that have acted

on the material.

Factors affecting the magnetic

susceptibility:

◦ The spin of the electron

◦ Number of electrons in the outer

shell - pair or odd?

Actually, each nucleus can be thought

as a small magnetic dipole!

Three types of magnetic materials:

◦ Paramagnetic

◦ Diamagnetic

◦ Ferromagnetic

(a) Diamagnetic substance.

◦ Acquisition of SMALL induced

magnetization OPPOSITE to the

applied field.

◦ The magnetization depends linearly

on the applied field and reduces to

zero on removal of the field

This form of magnetism is a

fundamental property of ALL materials.

(b) Paramagnetic substance.

◦ The susceptibilities of paramagnetic

substances are SMALL and

POSITIVE.

◦ The magnetization depends linearly

on the applied field and reduces to

zero on removal of the field

Paramagnetism can only be observed at

relatively low temperatures. The

temperature above which

paramagnetism is no longer observed is

called the Curie Temperature.

(c) Ferromagnetic substance.

◦ The path of the magnetization as a

function of the applied field is

non-linear and is called hysteresis

loop.

◦ Magnetization that can be orders of

magnitude larger than for the

paramagnetic solids.

◦ Upon removal of the magnetizing

field, magnetization does not return

to zero but retains a record of the

applied field.

◦ Like paramagnetism, ferromagnetism

is observed only at temperatures

below the Curie temperature.

Natural Remanent Magnetization, J

(NRM). In situ magnetization of rocks

is the vector sum of two components:

J = Ji + Jr

REMANENTINDUCED

NRM

NRM is the remanent magnetization

present in a rock sample prior to

laboratory treatment. It depends on

the geomagnetic field and geological

processes during rock formation and

during the history of the rock.

Question: For a rock to acquire

permanent magnitization, what type of

materials must be present?

NRM = primary NRM + secondary

NRM

Three forms of primary NRM:

Thermoremanent magnetization,

acquired during cooling from high

temperature.

Chemical remanent magnetization,

formed by growth of ferromagnetic

grains below the Curie temperature.

Detrital remanent magnetization,

acquired during accumulation of

sedimentary rocks containing detrital

ferromagnetic minerals (see cartoon on

next page).

Secondary NRM:

Results from chemical changes affecting

ferromagnetic minerals, exposure to

nearby lighting strikes, or long-term

exposure to the geomagnetic field

subsequent to rock formation.

Sampling

The first step of paleomagnetic survey

is to collect oriented cores. The

information of each sample includes

coordinates, azimuth and dip (or hade).

Measurement of NRM

NRM is measured with a special devise

called magnetometer.

Display of Data

Vector directions are described in

terms of inclination and declination.

This information is then projected

onto a stereographic plot.

Rotation of the sample coordinates to

geographic direction

Bedding-tilt correction

Geological applications

Fold Test

Synfolding

Conglomerate Test

Flow direction in volcanic dikes

While the magma is flowing in the dike,

elongate particles become imbricated

against the chilled margins. In the ideal

case, the flow directions from the two

margins are distinct and fall on either

side of the dike plane.

The fact that the western margin data

plot on the western side and the eastern

margin data plot in the eastern side

suggests that the flow was upward.

Field survey, a few simple examples

In conclusion, it is more difficult to

visually interpret magnetic anomalies

than gravity anomalies. These visual

problems, however, present no problem

for the computer modeling algorithms

used to model magnetic anomalies.

Temporal variations

Magnetic readings taken at the same

location at different times will NOT

yield the same results.

It is useful to classify temporal

variations into one of four types

depending on their rate of occurrence

and source:

Polarity reversal: 103− 106 years

Secular Variations: years

Diurnal Variations: hours-days

Magnetic Storms: minutes-hours

Polarity reversal

Reversals occur at irregular intervals

over time. The current sense of polarity

is called normal and the opposite is a

reversal.

Secular Variations

Slow changes in magnetic north over

time. Shown below is the declination

and inclination of the magnetic field

around Britain from the years 1500

through 1900.

Since secular variations change slowly

with respect to the time it takes us to

complete our exploration magnetic

survey, this type of temporal variation

is of little importance to us.

Diurnal Variations

These are variations occur over the

course of a day and are related to

variations in the Earth’s external

magnetic field. Shown below is the

typical variations in the magnetic data

recorded at a single location (Boulder,

Colorado) over a time period of two

days.

Can be on the order of 20 to 30 nT per

day and should be accounted for when

conducting exploration magnetic

surveys.

Magnetic Storms

Occasionally, magnetic activity in the

ionosphere will abruptly increase.

These storms correlates with enhanced

sunspot activity. The magnetic field

observed during such times is highly

irregular and unpredictable.

In this example, the magnetic field has

varied by almost 100 NT in a time

period shorter than 10 minutes!!

Exploration magnetic surveys should

not be conducted during magnetic

storms.

Strategies for dealing with temporal

variations

◦ Unlike the gravitational field, the

magnetic field can vary quite

erratically with time.

◦ Most investigators conduct magnetic

surveys using two magnetometers.

One is used to monitor temporal

variations of the magnetic field

continuously at a chosen base

station, and the other is used to

collect observations related to the

survey proper.

◦ Unlike gravimeters, magnetometers

show no appreciable instrument

drift.

◦ By recording the times at which each

magnetic station readings are made

and subtracting the magnetic field

strength at the base station recorded

at that same time, temporal

variations in the magnetic field can

be eliminated. The resulting field

then represents relative values of the

variation in total field strength with

respect to the magnetic base station.