Geophysics ( Geomagnetism )

Geomagnetism: Lecture 2

Sources:1. http://www.earthsci.unimelb.edu.au/ES304/2. http://geomag.org/index.html3. Gubbins, Nature, 2008.

http://www.earthsci.unimelb.edu.au/ES304/

http://geomag.org/index.html

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, .

Factors affecting the magnetic susceptibility include:• The electron spin.• Number of electrons within the outer shell - pair or odd?

Remnant magnetization, Jr. The remnant of past magnetic field that have acted on the material.€

Ji = χH.

Magnetization

Three types of magnetic materials:

• Paramagnetic

• Diamagnetic

• Ferromagnetic

Magnetization

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.

Magnetization

Paramagnetic substance:

• The susceptibilities of paramagnetic substancesare SMALL and POSITIVE.

• The magnetization depends linearly on the applied field and reduces to zero on removal of the field

Can only be observed at relatively low temperatures. The temperature above which paramagnetism is no longer observed is calledthe Curie Temperature.

Magnetization

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.

Magnetization

Ferromagnetic substance (continue):

• 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 Remnant Magnetization (NRM)

In situ magnetization of rocks is the vector sum of two components:

€

J = Ji + Jr .

remnantinduced

J

NRM is the remnant 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.

Ji

Jr


Question: for a rock to acquire remnant magnetization, what type of material must be present?

€

Jr = Jrprimary + Jr

secondary .

Three forms of primary NRM:

• Thermo-remnant magnetization: acquired during cooling from high temperature.

• Chemical-remnant magnetization: formed by growth of ferromagnetic grains below the Curie temperature.


• Detrital-remnant magnetization: acquired during accumulation of sedimentary rocks containing detrital ferromagnetic minerals.


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.

NRM: Practical issues

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:

NRM is measured with a special devise called magnetometer.


Progressive demagnetization


Projection of demagnetization

• 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.

NRM: Geological applications

Fold and conglomerate tests:

(with black arrows indicating directions of NRM)

Question: was NRM acquired prior to or after the conglomerate formation?

Solution: random distribution of NRM indicate that NRM was acquired prior to the conglomerate.


Fold and conglomerate tests:

(with black arrows indicating directions of NRM)

Question: was NRM acquired prior to or after folding?

Solution: improved grouping of NRM upon restoring the limbs of the fold indicate that NRM was acquired prior to folding.


Fold test:

Before unfolding:

After unfolding:


Pre-folding versus syn-folding induced magnetization:


Pre-folding versus syn-folding induced magnetization:

Crosses: 0% unfoldingCircles: 50% unfoldingSquares: 100% unfolding


Inference of flow direction in dikes:

During magma flow in dikes, elongated particles imbricate against the chilled margins. In such case, the NRM directions along the two margins are distinct and fall on either side of the dike plane.

Question 1: What is the mechanism by which the NRM is being acquired here?Question 2: What would you see if the flow direction was at an opposite direction?


Inference of flow direction in dikes:

That the western margin data plot on the right side and the eastern margin data plot on the left side suggests that the flow was upward.

Field survey

Strength of magnetic field above an anomaly in the North Pole.

Field survey

Strength of magnetic field above an anomaly in the equator.

Field survey

Strength of magnetic field above an anomaly in the latitude 45 degrees.

Field survey

Strength of magnetic field above an anomaly in the mid-latitude.

Field survey

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.

Temporal variations are classified according to the rate of occurrence and source:

• Polarity reversal: 103 - 106 years

• Secular variations: years

• Diurnal variations: hours-days

• Magnetic storms: minutes-hours

Temporal variations: Polarity reversal

Reversals occur at irregular intervals over time. The current sense of polarity is called normal and the opposite is called reversed.

Temporal variations: Polarity reversal

The Cretaceous Superchron

Temporal variations: Secular variations

Slow changes in magnetic north over time. Shown below are the declination and inclination of the magnetic field around Britain from the years 1500 through 1900.

Temporal variations: Diurnal variations

These variations occur over the course of a day, and are related to changes 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 corrected for when conducting exploration magnetic surveys.

Temporal variations: 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.

Temporal variations: Practical implications

• 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.

Temporal variations from CHAMP satellite

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are needed to see this picture.

Strength of the magnetic field at the Earth's surface in 2006, as given by the main field model POMME-3.0



Temporal variations from CHAMP satelliteContinuous measurements of the magnetic field by satellites can be used to estimate the present changes in the magnetic field.

• The 1st time derivative gives the secular variation. It shows that the field strength is decreasing in most parts of the World. The strongest decrease is seen the Caribbean. But there are also areas of increasing field strength, such as in the Indian Ocean.

• The 2nd time derivative is called the secular acceleration. Note the westward movement of the Indian Ocean high and the Caribbean low.




•The general tendency of magnetic field features to move westward is called the westward drift.

• Another way to see the field westward is to split the 2000-2008 data interval into two windows and into compute 2nd time derivative to each window separately.




About 95% of the field strength observed at the Earth's surface is due to the geodynamo process in the fluid outer core. Thus, after making some simplifying assumptions (e.g., that the flow is horizontal), Maus (2008) has invert the observed secular variation for flow at the surface of the outer core.




• Secular acceleration of the main field in 2006

• Inferred flow at the surface of the outer core.

Earth magnetic field and the geodynamo model

• Projection of the surface magnetic field down to the core surface reveals a complex structure.

• The vertical field is strongest not at the geomagnetic pole, but in two areas some 200 away from it.

Only northern Hemisphere is shown (Gubbins, 2008)

Simulation by Glatzmaier and Roberts suggests this pattern to be the effect of fluid flow within the inner core being intense around a cylinder that aligned with the geographic axis.

Yellow shows the area where the fluid flow is the greatest, the blue mesh marks the core-mantle boundary, and the red mesh mash the inner core boundary.


• Magnetic field generated by a dynamo model in which heat flow from the core surface matches that estimated from the temperature in the solid mantle immediately above it.

• The four locations of strongest fields lie very close to the corresponding locations in the Earth magnetic field.


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Geophysics ( Geomagnetism )