13-08-2012, 03:07 PM
magnetic measurement
[attachment=31305]
Fluxgate Magnetometer
The fluxgate magnetometer is based on what is referred to as the magnetic saturation
circuit. Two parallel bars of a ferromagnetic material are placed closely together. The
susceptibility of the two bars is large enough so that even the Earth's relatively weak
magnetic field can produce magnetic saturation in the bars.
Each
Zeeman-Effect
Zeeman discovered that in a strong magnetic field a single atomic spectrum line is split
into 3 lines whose separation distance is a measure of the field-strength magnitude.
Viewed at right angles to the field, the middle (original position) line is polarized parallel
to the field, whereas the two symmetric outer lines are polarized at right angles to the
field. Viewed parallel to the field, only the 2 outer lines are seen, actually they are
circularly polarized in opposite directions.
Optical detection of the Zeeman effect for magnetic field measurement is restricted to
very strong fields. This method has been applied to study the splitting of the FeI iron
lines near 5250.216 A from solar surface emission to detect the magnetic field there.
Optical-pump Magnetometer
In the presence of the magnetic field, the energy level of an alkaline vapor (e.g.85Rb)
splits into fine energy levels of magnetic quantum number m according to the Zeeman
effect. The energy difference between the split energy states are small. Under normal
conditions, atoms have equal probability in occupying any one of the split ground
states.
This situation can be changed by optically pumping the atoms so that one of the higher
energy level of the split ground state (say the forbidden level m=3) become
overpopulated. This can be achieved if we irradiate the sample with a circularly
polarized beam (e.g. from a 85Rb lamp) from which the spectral line for m=3 to the
excited level is removed. Then all the atoms in the other split ground states will
become excited to the higher energy level. At the higher energy level, they may decay
back to the ground state, including the forbidden level. So eventually, the forbidden
level become overpopulated.
Once over population of the forbidden state is achieved, a radio-frequency wave can be
sent in to unload the forbidden state. The magnetic field can now be determined from
by measuring the frequency of this radio-wave since they are proportional to each
other.
Planning Magnetic Surveys:
· Airborne - rapid coverage, generally high accuracy, cheap, both Total Field and its
gradients can be determined, can be combined with other geophysical surveys.
Limited by flying speed (sampling rate), terrain clearance requirements, navigational
accuracy (positioning & track spacing)
· Marine - good rate of coverage, adequate sampling rates, can be combined with
marine seismic surveys. Limitations: 1) difficult to position a magnetic base station
for the correction of temporal variations (solution: 2 magnetometer systemgradiometers);
2) ship is itself magnetic, so the sensors has to be towed hundreds
of meters astern; 3) higher cost.
· Ground - advantage: sampling spacing down to 1 meter is feasible, Limitations:
slow, land access problems, cost. Normally used for detailed follow-up work.
[attachment=31305]
Fluxgate Magnetometer
The fluxgate magnetometer is based on what is referred to as the magnetic saturation
circuit. Two parallel bars of a ferromagnetic material are placed closely together. The
susceptibility of the two bars is large enough so that even the Earth's relatively weak
magnetic field can produce magnetic saturation in the bars.
Each
Zeeman-Effect
Zeeman discovered that in a strong magnetic field a single atomic spectrum line is split
into 3 lines whose separation distance is a measure of the field-strength magnitude.
Viewed at right angles to the field, the middle (original position) line is polarized parallel
to the field, whereas the two symmetric outer lines are polarized at right angles to the
field. Viewed parallel to the field, only the 2 outer lines are seen, actually they are
circularly polarized in opposite directions.
Optical detection of the Zeeman effect for magnetic field measurement is restricted to
very strong fields. This method has been applied to study the splitting of the FeI iron
lines near 5250.216 A from solar surface emission to detect the magnetic field there.
Optical-pump Magnetometer
In the presence of the magnetic field, the energy level of an alkaline vapor (e.g.85Rb)
splits into fine energy levels of magnetic quantum number m according to the Zeeman
effect. The energy difference between the split energy states are small. Under normal
conditions, atoms have equal probability in occupying any one of the split ground
states.
This situation can be changed by optically pumping the atoms so that one of the higher
energy level of the split ground state (say the forbidden level m=3) become
overpopulated. This can be achieved if we irradiate the sample with a circularly
polarized beam (e.g. from a 85Rb lamp) from which the spectral line for m=3 to the
excited level is removed. Then all the atoms in the other split ground states will
become excited to the higher energy level. At the higher energy level, they may decay
back to the ground state, including the forbidden level. So eventually, the forbidden
level become overpopulated.
Once over population of the forbidden state is achieved, a radio-frequency wave can be
sent in to unload the forbidden state. The magnetic field can now be determined from
by measuring the frequency of this radio-wave since they are proportional to each
other.
Planning Magnetic Surveys:
· Airborne - rapid coverage, generally high accuracy, cheap, both Total Field and its
gradients can be determined, can be combined with other geophysical surveys.
Limited by flying speed (sampling rate), terrain clearance requirements, navigational
accuracy (positioning & track spacing)
· Marine - good rate of coverage, adequate sampling rates, can be combined with
marine seismic surveys. Limitations: 1) difficult to position a magnetic base station
for the correction of temporal variations (solution: 2 magnetometer systemgradiometers);
2) ship is itself magnetic, so the sensors has to be towed hundreds
of meters astern; 3) higher cost.
· Ground - advantage: sampling spacing down to 1 meter is feasible, Limitations:
slow, land access problems, cost. Normally used for detailed follow-up work.