|
|
|
|
Magnetometer
|
| |
|
| |
A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place and differences in Earth's magnetic field (the magnetosphere) can be caused by the differing nature of rocks and the interaction between charged particles from the Sun and the magnetosphere of a planet. Magnetometers are often a frequent component instrument on spacecraft that explore planets.
etometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits, airplanes like the Shrike Commander has been used .
Discussion
Ask a question about 'Magnetometer' Start a new discussion about 'Magnetometer' Answer questions from other users |
Encyclopedia
A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument. Magnetism varies from place to place and differences in Earth's magnetic field (the magnetosphere) can be caused by the differing nature of rocks and the interaction between charged particles from the Sun and the magnetosphere of a planet. Magnetometers are often a frequent component instrument on spacecraft that explore planets.
Uses
Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic field variations caused by the deposits, airplanes like the Shrike Commander has been used . Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects. Magnetic anomaly detectors detect submarines for military purposes.
They are used in directional drilling for oil or gas to detect the azimuth of the drilling tools near the drill bit. They are most often paired up with accelerometers in drilling tools so that both the inclination and azimuth of the drill bit can be found.
Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the Earth's magnetic field, which is published on the K-index.
In space exploration
A three-axis fluxgate magnetometer was part of the Mariner 2 and Mariner 10 missions.A dual technique Magnetometer is part of the Cassini-Huygens mission to explore Saturn. This system is composed of a vector helium and fluxgate magnetometers. Magnetometers are also a component instrument on the Mercury MESSENGER mission. A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of a planet's or moon's magnetic field.
Types
Magnetometers can be divided into two basic types:
- Scalar magnetometers measure the total strength of the magnetic field to which they are subjected, and
- Vector magnetometers have the capability to measure the component of the magnetic field in a particular direction.
The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined. Examples of vector magnetometers are fluxgates, superconducting quantum interference devices (SQUIDs), and the atomic SERF magnetometer. Some scalar magnetometers are discussed below.
A magnetograph is a special magnetometer that continuously records data.
Rotating coil magnetometer
The magnetic field induces a sine wave in a rotating coil. The amplitude of the signal is proportional to the strength of the field, provided it is uniform, and to the sine of the angle between the rotation axis of the coil and the field lines. This type of magnetometer is obsolete.
Hall effect magnetometer
The most common magnetic sensing devices are solid-state Hall effect sensors. These sensors produce a voltage proportional to the applied magnetic field and also sense polarity.
Proton precession magnetometer
One type of magnetometer is the proton precession magnetometer, also known as the proton magnetometer, which measures the resonance frequency of protons (hydrogen nuclei) in the magnetic field to be measured, due to Nuclear Magnetic Resonance (NMR).
A direct current flowing in an inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with that field. The current is then interrupted, and as protons are realigned with Earth's magnetic field they precess at a specific frequency. This produces a weak alternating magnetic field that is picked up by a (sometimes separate) inductor. The relationship between the frequency of the induced current and the strength of Earth's magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 hertz per nanotesla (Hz/nT).
Because the precession frequency depends only on atomic constants and the strength of the external magnetic field, the accuracy of this type of magnetometer is very good. Magnetic impurities in the sensor and errors in the measurement of the frequency are the two causes of errors in these magnetometers.
If several tens of watts are available to power the aligning process, these magnetometers can be moderately sensitive. Measuring once per second, standard deviations in the readings in the 0.01 nT to 0.1 nT range can be obtained.
The strength of the Earth's magnetic field varies with time and location, so that the frequency of Earth's field NMR (EFNMR) for protons varies between approximately 1.5 kHz near the equator to 2.5 kHz near the geomagnetic poles.
The measurement of the precession frequency of proton spins in a magnetic field can give the value of the field with high accuracy and is widely used for that purpose. In low fields, such as the Earth's magnetic field, the NMR signal is expected to be weak because the nuclear magnetization is small, but special devices can enhance the signal 100 or 1000 times. Incorporated in existing portable magnetometers, these devices make them capable of measuring fields to an absolute accuracy of about one part in 106 and detecting field variations of about 0.1 nT. Apart from the direct measurement of the magnetic field on Earth or in space, these magnetometers prove to be useful to detect variations of magnetic field in space or in time, caused by submarines, skiers buried under snow, archaeological remains, and mineral deposits
Fluxgate magnetometer
/inclinometer]]
A fluxgate magnetometer consists of a small, magnetically susceptible, core wrapped by two coils of wire. An alternating electrical current is passed through one coil, driving the core through an alternating cycle of magnetic saturation, i.e., magnetised - unmagnetised - inversely magnetised - unmagnetised - magnetised. This constantly changing field induces an electrical current in the second coil, and this output current is measured by a detector. In a magnetically neutral background, the input and output currents will match. However, when the core is exposed to a background field, it will be more easily saturated in alignment with that field and less easily saturated in opposition to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which this is the case will depend on the strength of the background magnetic field. Often, the current in the output coil is integrated, yielding an output analog voltage, proportional to the magnetic field.
Fluxgate magnetometers, paired in a gradiometer configuration, are commonly used for archaeological prospection. In Britain the most common such instruments to be used are the Geoscan FM series of instruments and the Bartington GRAD601. Both are capable of resolving magnetic variations as weak as 0.1 nT (roughly equivalent to one half-millionth of the Earth's magnetic field strength).
A wide variety of sensors are currently available and used to measure magnetic fields. Fluxgate magnetometers and gradiometers measure the direction and magnitude of magnetic fields. Fluxgates are affordable, rugged, compact and very low-power making them ideal for a variety of sensing applications. Fluxgate magnetometer sensors are manufactured in several geometries and recently have made significant improvements in noise performance, crossfield tolerance and power utilization
The typical fluxgate magnetometer consists of a "sense" (secondary) coil surrounding an inner "drive" (primary) coil that is wound around permeable core material. Billingsley Aerospace & Defense, Inc. currently manufactures four types of sensors: ring core, rod / Förster, racetrack and the recently developed Single Domain. Each sensor has magnetic core elements that can be viewed as two carefully matched halves. An alternating current is applied to the drive winding, which drives the core into plus and minus saturation. The instantaneous drive current in each core half is driven in opposite polarity with respect to any external magnetic field. In the absence of any external magnetic field, the flux in one core half cancels that in the other and the total flux seen by the sense coil is zero. If an external magnetic field is now applied, it will, at a given instance in time, aid the flux in one core half and oppose flux in the other. This causes a net flux imbalance between the halves, so that they no longer cancel one another. Current pulses are now induced in the sense winding on every drive current phase reversal (or at the 2nd, and all even harmonics). This results in a signal that is dependent on both the external field magnitude and polarity.
There are additional factors that affect the size of the resultant signal. These factors include the number of turns in the sense winding, magnetic permeability of the core, sensor geometry and the gated flux rate of change with respect to time. Phase synchronous detection is used to convert these harmonic signals to a DC voltage proportional to the external magnetic field.
Fluxgate magnetometers were invented in the 1930s by Victor Vacquier at Gulf Research Laboratories; Vacquier applied them during World War II as an instrument for detecting submarines, and after the war confirmed the theory of plate tectonics by using them to measure shifts in the magnetic patterns on the sea floor.
Overhauser magnetometer
The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This NMR effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field, i.e., generated from an RF source.
RF magnetic fields are ideal for use in magnetic devices because they are transparent to the Earth's DC magnetic field and the RF frequency is well out of the bandwidth of the precession signal, i.e., they do not contribute noise to the measuring system.
The unbound electrons in the special liquid transfer their excited state, i.e., energy, to the hydrogen nuclei, i.e., protons. This transfer of energy alters the spin state populations of the protons and polarizes the liquid — just like a proton precession magnetometer — but with much less power and to much greater extent.
The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons. The constant of proportionality is known to a high degree of accuracy and is identical to the proton precession gyromagnetic constant. As with the proton magnetometer, magnetic impurities and inaccuracies in frequency measurement are two causes of error in the measurement. The Overhauser magnetometer may have an additional error because the frequency produced can be changed slightly by an interaction between the protons and the coil used to detect the magnetic field.
Overhauser magnetometers achieve some 0.01 nT/vHz noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.
The Overhauser magnetometer, with its unique set of features, represents a pillar of modern magnetometry of the Earth’s magnetic field. Its sensitivity matches costlier and less convenient cesium magnetometers, for example. The Overhauser magnetometer also offers superior omnidirectional sensors; no dead zones; no heading errors; or warm-up time prior to surveys; wide temperature range of operation (from -40 to 55 degrees Celsius standard and -55 to 60 degrees Celsius optional); rugged and reliable design; and virtually no maintenance during its lifetime. Other advantages include high absolute accuracy, rapid speed of operation (up to five readings per second), and exceptionally low power consumption.
Overhauser magnetometers use proton precession signals to measure the magnetic field, but that’s where the similarity with the proton precession magnetometer ends.
Overhauser magnetometers were introduced by GEM Systems, Inc., following development in the 1980s and 1990s, and are the standard for magnetic observatories, long term magnetic field monitoring in volcanology, geophysical ground and vehicle borne exploration, and marine exploration.
Operating principles
The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field, i.e., generated from an RF source.
RF magnetic fields are ideal for use in magnetic devices, because they are “transparent” to the Earth’s “DC” magnetic field and the RF frequency is well out of the bandwidth of the precession signal, i.e., they do not contribute noise to the measuring system.
The unbound electrons in the special liquid transfer their excited state, i.e., energy, to the hydrogen nuclei, i.e., protons. This transfer of energy alters the spin state populations of the protons and polarizes the liquid — just like a proton precession magnetometer — but with much less power and to much greater extent.
The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons. The constant of proportionality, is known to a high degree of accuracy and is identical to the proton precession gyromagnetic constant.
Overhauser magnetometers achieve some 0.01 nT/vHz noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.
Advantages over proton precession and other quantum magnetometers
To summarize, some of the main differences between Overhauser and proton precession magnetometers are:
- More than an order of magnitude greater sensitivity even in the lowest of Earth’s fields. This reflects the fact that Overhauser systems offset a basic weakness of proton magnetometers, i.e., deterioration of signal quality in low magnetic flux density, by creating a small auxiliary magnetic flux density while polarizing.
- Sensitivity that virtually matches cesium sensitivity.
- This is the only quantum magnetometer that offers continuous or sequential operation. With Overhauser magnetometers, it is possible to measure continuously or sequentially due to the use of an RF polarization field. The RF field is transparent to the measurement of magnetic field and can therefore, be enabled at all times.
- Cycling speed. Since the liquid can be polarized while the signal is being measured, the sampling rate is higher (as high as 10 Hz possible).
- Energy efficiency. Overhauser magnetometers are significantly more efficient than any other quantum magnetometer due to the low power required for RF signal generation. Power consumption can be optimized to as low as 1 W for continuous operation.
- Omnidirectional sensors. No dead zones, virtually no heading errors and no warm-up time.
There are also other advantages related to the manufacturing process. of less interest to users, such as relative simplicity, reliability of design, low manufacturing cost relative to sensitivity, weight and power consumption benefits.
Potassium vapor magnetometers
Physical overview of quantum magnetometers
Some subatomic particles, in particular electronic and nuclei of some elements process spin; rotation and they have an accompanying mechanical moment.
Since the particles with spin have charge, they also possess a magnetic moment related to the mechanical. In an applied magnetic field, such as the Earth’s, magnetic moments can only assume discrete orientations governed by the spin number, I.
There can be only 2 I + 1 permitted states.
For I = ½ for electrons and protons there are only two allowed energy states, permitted angles between the two vectors are +450. In an assembly of spins the distribution of populations of the two levels is regulated by
- e-E/kT = e µH/kT
so the higher energy level is less populated. The result is a slight paramagnetism of the assembly of particles with spin due to spin and the magnetic moment. Individual spinning particles precess around the magnetic field with the angular frequency
- ?o = ?H,
where ? is a gyromagnetic constant.
Since there are many particles spinning incoherently, there is no macroscopic effort of it, i.e., the magnetization due to spins appear static. However, if one applies a rotating magnetic field of the angular frequency ?o in the plane perpendicular to the magnetic field, the vector of magnetization will be deflected off the direction of magnetic field and will precess around it with the same frequency. Precessing or rotating magnetization will induce a voltage in a coil suitably wound around the assembly of spins. Frequency of the detected voltage is proportional to the applied field to a great precision.
In a weak magnetic field such as Earth’s, the induced voltage is far too small for direct detection. Instead various means are applied in order to polarize the sensor spin assembly, i.e., to increase the macroscopic magnetization due to spins.
There are three principally different groups of quantum magnetometers:
- Proton magnetometers use strong DC magnetic fields to increase protons magnetization.
- Overhauser effect is based on a mixture of electrons and protons. Electrons are manipulated to transfer their polarization to protons.
- Some alkali metals and He-3 and He-4 can be optically pumped to increase the magnetization due to their electron spins.
We will be concerned only with the third group.
Optical pumping of alkali vapors
Only unpaired and free electrons exhibit spin with the features described above. Vapors of the alkali group of elements have a single, unpaired electron in their valence shell and they can be readily used as sources of electrons with spins. Helium gas in the other hand needs to be ionized in order to eliminate one electron from the valance shell; the remaining electron then behaves as an unpaired electron.
In ground state 2 S1/2 the electron has 2 energy levels, or -1/2 or +1/2 spins. To polarize it we need to depopulate one level and overpopulate the other. This is done by applying a light beam with special characteristics. Gas discharge lamps of the elements in question are used as sources of polarizing light. Photons of two spectral lines D1 and D2 can lift the electrons from either energy level of the background state into metastable state. There will be very little in polarization if we allow both D1 and D2 to act their polarizations are opposite and we need to eliminate or suppress one. This is done by an interference filter.
Next we need to circularly polarize the D1 light. Then, only electrons with -½ spin will be able to absorb the quantum of light and be lifted into metastable 2 P1/2. There is a natural decay from metastable levels back into background levels, but eventually the -½ spin level will be depleted, and the sensor will become more transparent, not absorbing photons any more.
If we now apply rotating magnetic field around the sample and in the plane perpendicular to the applied magnetic field, there will be a precession of the magnetization due to electron spins. Depending on the phase of this precession the ability of the spins to absorb protons of D1 light will vary, i.e., the intensity of light passing through the sample of spins will be modulated in synchronization with the preceding magnetization. We can detect this modulation, amplify it and measure its frequency and compute the value of the applied magnetic field from it.
In reality, the situation is somewhat different. Due to magnetic properties of the nucleus of the alkali metals, there is a whole subspectrum of spectral lines instead of a single one.
Broad line versus narrow line spectra: potassium and rubidium have six spectral lines of various intensities, cesium-133 14 and helium-4 just one but very wide. Width of the spectral line depends on many parameters such as the size of cells, collision of the atoms with the walls of the cells, collision with buffer gas, spin exchange, etc.
Contemporary Cs and Rb magnetometers have wide overlapping spectral lines. A composite spectral line is not symmetrical but the position of its peak depends on the geometry of the sensor and the applied magnetic field. There is a large shift in precession frequency when we change the orientation of the sensor in steady magnetic field. This weakness is largely corrected by applying a split beam technique that makes the shape of the strong but wide spectral single line symmetrical.
Advantaged of the strong single line are:
- Very high tolerance to gradients of magnetic field.
- Simplicity, since the cesium magnetometer can self-oscillate its amplified signal is used to create a rotating magnetic field around the sensor, causing self-oscillations.
Weaknesses are:
- Reduced sensitivity
- Poor absolute accuracy
- Pronounced tilt or heading error.
Potassium spectral lines can be made very narrow and completely separated from each other. Self-oscillation is now not suitable, as it would result in a beating of several individual frequencies. Instead an auxiliary oscillator is used to create rotating field around the sensor for one spectral line only. Signal generated from that operation is then used to frequency lock the auxiliary oscillator’s frequency. Technically this is more complex than self-oscillations.
Advantages are:
- A maximum of the resolution
- Very high absolute accuracy
- “Heading error” due to varying geometry between the sensor axis and the magnetic field is very much reduced.
The disadvantage is also a limited tolerance to gradients, as gradients widen the spectral lines.
The sensors of potassium magnetometers need to be larger size than in cesium magnetometers in order to achieve narrow spectral lines. In practice, we use 70 mm diameter cells to achieve about 1 nT line width and 120 mm cells will give about 0.15 nT.
Standard and super-resolution K-sensors and systems
We have built the observatory like testing site at Georgina Island in the Lake Simcoe, Ontario to test our “supergradiometer”. Latest results show about 0.1 pTpp noise gradient mode and one-second measuring interval. This is somewhat more than 10 fT RMS per channel. Our standard gradiometers are about one-fifth as sensitive.
Geometrical restrictions of potassium are very similar to those of cesium: right angles and collinear orientation related to the magnetic field directions are forbidden. Whether one defines operating angles from 2° to 88° or 1° to 80° is irrelevant; the physics of it stays the same.
Future directions
Current research is aimed at reducing the sensor size thereby reducing sensitivity to gradients while maintaining a relatively high sensitivity in comparison with other commercial instrumentation.
GEM Systems continues to advance its research and development, which is leading to the next generation of gradient-tolerant ground systems, using new sensors, as well as high sensitivity airborne systems, using existing sensors, as well as configurations of multi-sensor airborne gradiometers.
Cesium vapor magnetometer
A basic example of the workings of a magnetometer may be given by discussing the common "optically pumped cesium vapor magnetometer" which is a highly sensitive (0.004 nT/vHz) and accurate device used in a wide range of applications. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.
The device broadly consists of a photon emitter containing a cesium light emitter or lamp, an absorption chamber containing cesium vapor and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.
Polarization
The basic principle that allows the device to operate is the fact that a cesium atom can exist in any of nine energy levels, which is the placement of electron atomic orbitals around the atomic nucleus. When a cesium atom within the chamber encounters a photon from the lamp, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The cesium atom is 'sensitive' to the photons from the lamp in three of its nine energy states, and therefore eventually, assuming a closed system, all the atoms will fall into a state in which all the photons from the lamp will pass through unhindered and be measured by the photon detector. At this point the sample (or population) is said to be polarized and ready for measurement to take place. This process is done continuously during operation.
Detection
Given that this theoretically perfect magnetometer is now functional, it can now begin to make measurements.
In the most common type of cesium magnetometer, a very small AC magnetic field is applied to the cell. Since the difference in the energy levels of the electrons is determined by the external magnetic field, there is a frequency at which this small AC field will cause the electrons to change states. In this new state, the electron will once again be able to absorb a photon of light. This causes a signal on a photo detector that measures the light passing through the cell. The associated electronics uses this fact to create a signal exactly at the frequency which corresponds to the external field.
Another type of cesium magnetometer modulates the light applied to the cell. This is referred a Bell–Bloom magnetometer after the two scientists who first investigated the effect. If the light is turned on and off at the frequency corresponding to the Earth's field, there is a change in the signal seen at the photo detector. Again, the associated electronics uses this to create a signal exactly at the frequency which corresponds to the external field.
Both methods lead to high performance magnetometers.
Applications
The cesium magnetometer is typically used where a higher performance magnetometer than the proton magnetometer is needed. In archaeology and geophysics, where the sensor is moved through and area and many accurate magnetic field measurements are needed, the cesium magnetometer has advantages over the proton magnetometer.
The cesium magnetometer's faster measurement rate allow the sensor to be moved through the area more quickly for a given number of data points.
The lower noise of the cesium magnetometer allows those measurements to more accurately show the variations in the field with position.
Spin-exchange-relaxation-free (SERF) atomic magnetometers
At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free (SERF) atomic magnetometers containing potassium, cesium or rubidium vapor operate similarly to the cesium magnetometers described above yet can reach sensitivities lower than 1 fT/vHz.
The SERF magnetometers only operate in small magnetic fields. The Earth's field is about 50 µT. SERF magnetometers operate in fields less than 0.5 µT.
As shown in large volume detectors have achieved 200 aT/vHz sensitivity. This technology has greater sensitivity per unit volume than SQUID detectors.
The technology can also produce very small magnetometers that may in the future replace coils for detecting changing magnetic fields.
Rapid developments are ongoing in this area. This technology may produce a magnetic sensor that has all of its input and output signals in the form of light on fiberoptic cables. This would allow the magnetic measurement to be made in places where high electrical voltages exist.
SQUID magnetometer
SQUIDs, or superconducting quantum interference devices, measure extremely small magnetic fields; they are very sensitive vector magnetometers, with noise levels as low as 3 fT·Hz−0.5 in commercial instruments and 0.4 fT·Hz−0.5 in experimental devices. Until the advent of SERF atomic magnetometers in 2002, this level of sensitivity was unreachable otherwise.
These magnetometers require cooling with liquid helium (4.2 K) or liquid nitrogen (77 K) to operate, hence the packaging requirements to use them are rather stringent both from a thermal-mechanical as well as magnetic standpoint. SQUID magnetometers allow one to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively).
Early magnetometers
In 1833 Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the Earth's magnetic field.
It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer) . It consisted of a permanent bar magnet suspended horizontally from a gold fibre . A magnetometer is also called a gaussmeter.
See also
External links
|
| |
|
|
