http://www.redorbit.com/images/pic/29380/earths-magnetic-field/
The magnetic field
and magnetic measurements
Einar Ragnar Sigurðsson
27th November 2013
Slide no:
Outline of presentation
1. The magnetic field of Earth:
▫ The dipole moment
▫ Reversals
2. The rock magnetization:
Induced and remanent
3. The remanent
magnetization such as
TRM, DRM, VRM
4. Different methods for
measuring the rock
magnetization
5. The master’s thesis: Rock
magnetization in Icelandic
volcanic zone
Einar Ragnar Sigurðsson - 27.11.2013
The Earth’s magnetic field during reversal.
Ref: Glatzmaier, G.A. & Coe, R.S. (2007)
Paleo-magnetism and magnetic measurements
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The magnetic field
of Earth
• Generated be an electric
current in the liquid outer
core.
Magnetic dipole generated from
current in a loop
• The dipole magnetic moment
can describe 80-90% of the
magnetic field of Earth.
• Now, there is a magnetic south
pole in northern hemisphere
and magnetic north pole in the
south hemisphere.
• Simplification since 10-20% is
left and how the current
actually is, is not well known
(Kristjansson L., 1985)
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Paleo-magnetism and magnetic measurements
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The rotation axis of Earth and the magnetic
dipole don’t fit -> Declination
• Since the dipole doesn’t
correspond to the
geographical poles we
get the declination and
from the nature of the
dipole moment we get
inclination angle.
▫ B: The total magnetic
field
▫ H: The horizontal
component of the field
▫ Z: The vertical
component of the field
▫ I: Inclination angle, the
dip of the magnetic field
▫ D: Declination. The
angle between the (true)
geographical north and
the magnetic north
• (Kearey et al, 2002)
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
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The IGRF11 model
(NOAA – National Geophysical Data Center, 2013)
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Paleo-magnetism and magnetic measurements
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The unstable magnetic field
• Diurnal variations
• Source of change is solarwind and
thunderstorms close to equator
• Can disturb magnetic
measurements but is also the key
to some electromagnetic
resistance measurements (MT
measurements)
• Secular variations
• Slow changes with time, both in
intensity and location of the poles
• Source of change is assumed to
be changes in the current
generating the Earth magnetic
field
• Measurable through paleomagnetic measurements
Einar Ragnar Sigurðsson - 27.11.2013
• Can led to reversals of the
magnetic poles
• Major time periods with the same
direction of the magnetic field:
- Brunhes: 0-0.78 Ma (normal)
- Matuyama: 0.78-2.6 Ma (reversed)
- Gauss: 2.6-3.6 Ma (normal)
- Gilbert: 3.6-6.0 Ma (reversed)
• Measurable through paleo-magnetic
measurements
Paleo-magnetism and magnetic measurements
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The geomagnetic
reversal
• During the geomagnetic
reversals the dipole field
wanders down to lower
latitudes.
• In same time it weakens a lot
• Then it strengthens again
when going to higher
latitudes again finishing the
reversal, if there is a reversal.
• Based on number of lavas the
dipole field is most often
close to the true north and
south.
(Kristjánson, L)
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Paleo-magnetism and magnetic measurements
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The rock it self can be magnetized : Induced
and remanent
• The induced magnetization, Ji is
induced by the present external
magnetic force
▫ Is in same direction as the
external field
▫ Ji depends on the material
property called susceptibility (k)
𝐽𝑖 = 𝑘𝐻
▫ Will disappear when external
field is “turned off”
• The remanent magnetization, Ji
is not dependent on the
present external magnetic field.
It depends on:
▫ Properties of the material - the
minerals
▫ The paleo-magnetic field from
the time of formation and the
rock and later as well
▫ It is permanent in the rock and
the present external field doesn’t
have any measurable effect on it.
Einar Ragnar Sigurðsson - 27.11.2013
Vector diagram showing how an external
magnetizing force H (that could be from the
Earth magnetic field) induces a magnetic field Ji
that is added to the remanent magnetic field Jr
and hence giving the total magnetic field J.
(Kearey et al, 2002)
Paleo-magnetism and magnetic measurements
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The remanent magnetization
Primary
Secondary
• Thermo Remanent
Magnetization, TRM
• Chemical Remanent Magnetization,
CRM
• In igneous rock that solidifies and
cools through the Curie
temperature
• Magnetic minerals:
• Magnetite (Fe3O4)
• Ulvöspinel (Fe2TiO4)
• Depositional Remanent
Magnetization, DRM
• magnetic particles are deposited
by sedimentation and they align
with the current magnetic field
Einar Ragnar Sigurðsson - 27.11.2013
• Magnetic minerals recrystallize or
grow during metamorphism
• Viscous Remanent Magnetization,
VRM
• Remanent magnetism is acquired
over time if a rock is subjected to an
external field different from the
original field
• Isothermal Remanent
Magnetization, IRM
• The magnetization can changes in
extremely high magnetic field such
as from a lightning
Paleo-magnetism and magnetic measurements
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Different kinds of magnetic measurements:
Ground magnetic survey
• Detailed survey of limited area
• Measurement of magnetic field to find out residual magnetic field
• Residual magnetic field anomalies are caused by different magnitude
and/or direction of magnetization (remanent + induced) of objects.
• The purpose is to locate and find out magnetic properties of hidden
objects underground
(Photo: Einar Ragnar Sigurðsson)
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
(Kearey et al, 2002)
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Different kinds of magnetic measurements:
Aeromagnetic survey – Marine magnetic survey
• Survey covering large area
• Magnetic anomalies caused by different magnitude and/or
direction of magnetization (remanent + induced) of objects.
• The purpose is among other things mapping of magnetic
anomaly pattern related to plate movements and magnetic
reversals.
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Paleo-magnetism and magnetic measurements
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Different kinds of magnetic measurements:
Susceptibility measurement of a sediment core
• The method is convenient and nondestructive.
• The core is put into a magnetic field and
the induced magnetization in the core is
measured.
• The outcome of the measurement is the
susceptibility of the material in the
sediment core.
• High susceptibility value as a material
property may indicate:
▫
▫
▫
▫
Particulate pollution
Ancient forest fires
Floods
Bands of volcanic tephra
• See graph from a 12m
core piece of sediment
in a lake in Washington
State, U.S.
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Paleo-magnetism and magnetic measurements
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Measurements of
remanent rock magnetization
copyright of figure: Leó Kristjánsson
•
•
•
•
•
Sample (lava) from the field
Preparation of the sample: Demagnetization of VRM
Measurement in a laboratory
Calclation of paleo-declination and inclination
Paleo virtual dipole
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Paleo-magnetism and magnetic measurements
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Drilling:
Taking the core
• Originally a chain saw.
• Made in USA
• An exchangeable front end with
industrial diamond crown
• Can last for 100 lavas
• Diameter 25mm
• Approximately 1 minute to drill one
core
• Common to take have 4 samples for
each lava
• Unsuccessful drilling requires on
average drilling 6 cores
• Use of cooling water -> difficulties
when temperature is below 0°C
• 1 liter for one lava
• Centrifugal coupling
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
Photo: Einar Ragnar Sigurðsson
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The orientation of the
core
• Richard Doell method, 50 years old!
• 1” tube and Brunton compass
• The direction to the sun or other
known place is taken with the compass
̵ Use of GPS if no such place
• For orientation of the specimen: The
upper top end of the core is marked
with a wire.
• Finally: Break the core from the lava
Photos: Leó Kristjánsson and Einar Ragnar Sigurðsson
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Paleo-magnetism and magnetic measurements
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The core
• Diameter 24.5 mm
• Length originally ca 50 mm
• Sawed to appropriate length to have the
cylinder as symmetric as possible, that is a
spear like cylinder with
𝐿 = 0.5𝐷 3 ≈ 21 𝑚𝑚
• Orientation of the specimen is very
important:
̵ The z axis as the axis of the cylinder
̵ The y axis is perpendicular to the z axis and was
horizontal in the original rock
̵ The x axis is perpendicular to the two other axis
and sloping upwards 90° from the slope of the
core
Photos: Einar Ragnar Sigurðsson
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
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The magnetic
measurement
• Measured in all possible
directions and al orientations:
̵ In x, y, z direction
̵ Positive and negative direction
̵ For all four orientations
̵ Total: 3 x 2 x 4 = 24
measurements
• The equipment is
perpendicular to Earth
magnetic field and as well
surrounded with a coil to
minimize the effect of external
magnetic field.
• Very old equipment from
institute dr Förster in Germany
̵ Tube amplifier from 1978 and
design from 1968!
Photos: Einar Ragnar Sigurðsson and Leó Kristjánsson
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
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The data processing
• The output from the
magnetometer is mV for all 24
measurements goes to a computer
program, see figures.
• Average for each of x, y and z
• Inclination and declination of the
specimen
• 𝐴𝑚𝑝𝑙 = √(𝑥 2 + 𝑦 2 + 𝑧 2 )
• Calibrated with known coil (see the
small figure) to change the
measured amplitude in mV to the
magnetization of the specimen in
A/m (J0 in the figure)
Photos: Einar Ragnar Sigurðsson
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
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Demagnetization:
“Washing” the specimen
• The specimen is put in to a
“magnetic washing machine”
where the specimen is rotated for
a short time in a strong magnetic
field.
• Rotated to get effect on the VRM
in all possible directions
Photos: Einar Ragnar Sigurðsson
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
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The theory behind the magnetic field
demagnetization
Ref: Kristjansson, L., 1985
• The VRM is softer than TRM so it will disappear first in a strong external
alternating magnetic field
• Magnetic field goes up to strengths such as: 10-15-20-25-30-35 mT and down
again
• Note: 10 mT is approximately 200 times larger than magnetic field of the Earth
• The washing is repeated until the change in declination and/or inclination goes
down to 1 or 2°
• Stop washing before all TRM has been washed out so there will be some
magnetic field left for the final measurement
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
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Some interesting points regarding the
magnetic field demagnetization
Ref: Kristjansson, L., 1985
• The first steps of washing can increase the dipole moment as for the GL
lavas.
• The washing will not have so much effects on lavas younger than 700
thousand years.
• In that case both TRM and VRM are normal or positive magnetized
• The washing will have very large effect on a 800 thousand years old lava.
• That lava should have reversed TRM but most of the VRM is normal magnetized.
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
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The master’s thesis
• Measurements of rock
magnetization inside the
volcanic zones
• Previous measurements mostly
outside the volcanic zones
̵ Difficulties with hyaloclastite
̵ Difficulties with more complicated
geology than flat lavas
̵ Recognizing the magnetic reversal
important
• Rock magnetization
measurements in Icelandic table
mountains
• Find out changes in declination
from the eruptions:
̵ In the pillow lava
̵ In the lavashield on top of the
mountain
̵ For how long time was the
eruption lasting?
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
(Náttúrufræðistofnun ísladns)
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History of magnetic field in Reykjavik
0
52500
-5
Declination [degrees]
-15
51500
-20
51000
-25
Total magnetic field [nT]
52000
-10
-30
50500
Declination
-35
Total Field
-40
1900
50000
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
Year
Declination and total magnetic field in Reykjavik (# Latitude: 64.14 degrees North,
Longitude: 21.9 degrees East) from 1900 in IGRF 11.
Calculations from NOAA – National Geophysical Data Center web page.
Einar Ragnar Sigurðsson - 27.11.2013
Paleo-magnetism and magnetic measurements
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Rock-magnetic
measurements in
table mountains
Change of declination in China
• Some disadvantages:
̵ Sometimes the declination can
stay stable
̵ The declination goes up and
down
̵ Not very precise – we need very
long eruptions to see any change
in declination
(Merrill & McElhinny, 1983)
• Has been done at least once in
Iceland
̵ Hlöðufell south of Langjökull
̵ Did not show difference in
declination
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Paleo-magnetism and magnetic measurements
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References
• Glatzmaier, G.A. & Coe, R.S., 2007. Magnetic polarity
reversals in the core. Treatise on Geophysics, Volume 8, Core
Dynamics, Chp. 9, eds. P. Olson and G. Schubert (Elsevier),
pp. 283-297.
• Kearey, P., Brooks P.,Hill, I (2002). An Introduction to
Geophisycal Exploration. Malden: Blackwell publishing.
• Kristjansson L., 1985. Bergsegulmælingar – nytsöm tækni við
jarðfræðikortlagningu. Náttúrufræðingurinn, 54 (3-4), p.
119-130
• Merrill, R.T. & McElhinny, M.W., 1983. Earth's Magnetic Field:
Its History, Origin and Planetary Perspective (International
Geophysics Series). Academic Press Inc
• NOAA – National Geophysical Data Center, 2013. Magnetic
Field Calculators, retrieved from
http://www.ngdc.noaa.gov/geomag on 15.11.2013.
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Paleo-magnetism and magnetic measurements
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Alternative method
for demagnetization
• Heating the specimen to
successively higher
temperatures in zero field
• Measure remanent magnetic
field at room temperature
after cooling in zero field
• Disadvantages:
̵ The magnetic minerals are often
altered during the heating.
̵ Time consuming
• Advantages
̵ Works better on specimens from
sedimentary rocks than
demagnetization employing
alternating field
Einar Ragnar Sigurðsson - 27.11.2013
Photo: www.kochi-core.jp/en/facilities_and_equipment/magnetism.html
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Other methods
• The spinner magnetometer
̵ The specimen is rotated at
constant rate by a motor, close
to a pick-up coil or fluxgate
probe.
̵ Electronic circuits are used to
amplify only a signal of the same
frequency as that of the rotation
̵ More time consuming method
̵ better result with weaker
magnetization
Photo showing a spinner
• uperconducting quantum
interference device (SQUID,
cryogenic) magneto- meters
̵ Can be used for very weakly
magnetic rocks, even limestones
Photo showing SQUID equipment
(ref www.kochi-core.jp/en/facilities_and_equipment/magnetism.html)
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SQUID
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History of magnetic field in USA –
Washington DC
0
Declination [degrees]
-2
-4
-6
-8
-10
-12
1750
1800
1850
1900
1950
2000
Year
Declination in Washington DC (# Latitude: 38.89 degrees North, Longitude: 76.59 degrees
West) from 1900 in IGRF 11.
Calculations from NOAA – National Geophysical Data Center web page.
Einar Ragnar Sigurðsson - 27.11.2013
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History of magnetic field in China
(Merril & McElhinny, 1983)
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History of magnetic field in London
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