GEOMAGNETISM II Magnetization · GEOMAGNETISM II Magnetization Induced magnetization, Ji. When a...
Transcript of GEOMAGNETISM II Magnetization · GEOMAGNETISM II Magnetization Induced magnetization, Ji. When a...
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.