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Magnetometer

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A magnetometer, pronounced mag-ne-TOM-e-ter, is a scientific instrument used to measure the strength and/or direction of the magnetic field, produced either in the laboratory or existing in nature. The Earth's magnetic field (the magnetosphere) varies temporally and spatially, for various reasons including inhomogeneity of rocks and interaction between charged particles from the Sun and the magnetosphere.

Further information: [[:Earth's magnetic field]]

Uses

Magnetometers can measure the magnetic fields of planets.

Magnetometers have a very diverse range of applications from locating submarines and Spanish Galleons, positioning weapons systems, detecing unexploded ordnance, locating toxic waste drums, heart beat monitors, sensors in anti-locking breaks, weather prediction via solar cycles, depths of steel pilons, drill guidance systems, locating hazards for tunnel boring machines, archaeology, Plate Tectonics, finding a wide range of mineral deposits and geological structures, hazards in coal mines, to radiowave propogation and planetry exploration. And there are many more applications.

Depending on the application, magnetometers can be deployed in spacecraft, aeroplanes (fixed wing), helicopters (stinger and bird), on the ground (backpack), towed at a distance behind quad bikes (sled or trailer), lowered into boreholes (tool, probe or sonde) and towed behind boats (tow fish).

Magnetometers applied to the study the earth are called geophysical surveys - a term that also embraces a wide range of other techniques.

Archaeology

Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects. Fluxgate gradiometers are popular due to low cost. Gradiometers enhance shallow features and negate the need for a base station. That said, fast sampling Overhauser and alkalai vapour magnetometers are also very effective when used as gradiometers and with base stations.

The TV program 'Time Team' popularised 'geophizz' including magnemtics for archaeological work. Targets include fire hearths, walls of baked bricks, magnetic stones (basalts, granites). Walking tracks and roadways can sometimes be mapped with differential compaction in magnetic soils and/or disturbances in clays such as on the Great Hundarian Plain. Ploughed fields can provide magnetic noise sources to such surveys.

For the budding scholar, "Seeing Beneth The Soil" by Anthony Clark is a good textbook on magnetics applied to archaeology if not somewhat dated. Other textbooks include the classic Geometrics 'Applications Manual for Portable Magnetometers' by Sheldon Breiner.

Auroras

Magnetometers 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.[1]

Coal Exploration

At a regional scale, magnetometers can be used to help map basin shape.

Magnetometers are used to map hazards to coal mining including basaltic intrusions (dykes), sills and Volcanic_plugs) that replace coal resources, faults and burn zones (ignited by lightning). Magnetometers can also map siderite - an impurity in some coal.

The best surveys results are achieved on the ground and in boreholes due to the subtle nature of the smaller anomalies. Surveys are dirnally corrected using a base station to correct for the unwanted natural daily fluctuations in the earth's magnetic field.

Examples of high-resolution ground magetic surveys from the Australian coal industry conducted by Ultramag Geophysics Pty Ltd can be found at [2]

Directional Drlling

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.

Military

Magnetometers are a classified technology in countries such as Australia Canada and USA. In short they can be used as strategic weopans.

It is thus no suprise that the US military is one of the largest owners of magnetometers. Like all tools they can be used for good or bad purposes - purposes that vary with your political outlook.

On the defensive side, Navies use arrays of magnetometers laid across strategic seafloors (such as near ports) to monitor submarine activity. The Russian 'Goldfish' (titanium submarines) were designed and build at great expense to thwart such systems (pure titanium is non-magnetic).

As part of the cat and mouse games people play, miliraty submarines get degaussed by driving into large underwater loops at regular intervals in a bid to escape detection by such seafloor monitoring systems,Magnetic anomaly detector and mines that trigger on magnetic anomalies. Submarines are never completely demagnetided. By measring their magentic field - one can tell how deep that have been diving because the pressure presssure distoprs the steel and changes its magentic field. Heating can also change the magnetization of steel.

Submarines tow long sonar arrays to listen for ships - they can even recognise different propeller noises. The sonar arrays need to be accurately positioned so they can triangulate direction to targets (eg. ships). Now the arrays so not tow in a nice straight line so fluxgate magnetometers are used to orient each sonar node in the towed array.

Fluxgates can also be used in weopans navigation systems, however they ate often superceded by GPS and ring laser gyros.

Magnetotmeters such as the German Forster are used to locate ferrous ordnance. Cesium and Overhauser magnetometers are used to locate and help clean up old bombing/test ranges.

UAV payloads also include magnetometers for a range of defensive and agressive tasks.

Mineral Exploration

This is one of the major commercial applications of magnetometers because that are used to locate world class deposits of gold, silver, copper, iron, tin, platinum, diamonds and even some rubies and opal.

First world countries such as Australia, Canada and USA invest heavily in systematic airborne magnetic surveys of their respective continents (and surrounding oceans) to help map geology and leverage the discovery of mineral deposits. They use airplanes like the Shrike Commander.[2] 

Such surveys may be followed up with ground magentic surveys (and other geophysical surveys including gravity EM and IP) to give more detail of the shallower structures prior to drilling.

Magneotmeters assist mineral explorers both directly (ie. gold mineralisation associated with magnetite) and by indirect means such as mapping geological structures (ie. shear zones more conducive to mineralisation).

Mobile phones

Magnetometers are appearing in mobile phones. The HTC HD2, Nokia C7-00, HTC Dream, HTC Desire HD, HTC Evo, HTC Wildfire, Apple iPhone 3GS, iPhone 4 and iPad, Motorola Droid,[3] Motorola Quench,Motorola Atrix 4g Nokia N97, Nokia E72, Nokia N8, Nokia E5, Xperia X10, Samsung i8910, Samsung Wave S8500, Blackberry Torch,Samsung Galaxy S, Nexus One, Nexus S, and the HTC Hero all have a magnetometer and come with compass apps for showing direction.[4][5]

Researchers at Deutsche Telekom have used magnetometers embedded in mobile devices to permit touchless 3-D interaction. Their interaction framework, called MagiTact, tracks changes to the magnetic field around a cellphone to identify different gestures made by a hand holding or wearing a magnet.[6]

Oil Exploration

Magnetometers are a secondary techniques used with seismics for oil epxloration. They are used for basin shape, and faults.

Oil deposits can leak hydrocarbons. The hydrocarbins find their way up fractures in the ground are eaten by bugs at or near the surface. The bugs can precipitate magnetite from haematite producing subtle magnetic anomalies.

Spacecraft

A three-axis fluxgate magnetometer was part of the Mariner 2 and Mariner 10 missions.[7] A dual technique magnetometer is part of the Cassini-Huygens mission to explore Saturn.[8] This system is composed of a vector helium and fluxgate magnetometers.[9] 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.

Further information: [[:Spacecraft magnetometer]]

Magnetometers vs Metal Detectors

Magnetomers should not be confused with metal detectors which measure conductivity. Ferrous targets such as a 44 gallon drum are both conductive and magnetic (channeling the earths natural magnetic field),whereas a brass bullet shell is non-magnetic but conductive. In average ground, a sensitive magnetometer can detect a 44 gallon drum buried to 8m depth, a 2 ton car buried 30m depth, while the best metal detectors are capable of 1-2 metres.

A good pair of magnetometers can cost up to $US 50,000 + similar price for good processing software; a good metal detector is around $US 4,000.

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, relative to the spatial orientation of the device.

Magnetometers can also be divided into AC that measure time varying fields and DC systems that measure very slow changing fields. Whilst AC magnetometers find use in electromagnetic systems (such as magnetotellurics DC magnetometers predominate in detecting mineralization and corresponding geological structures.

Vector Magnetometers

Vectors have both magnitide and direction. The earth magnetic field at a given point can be visualised as a compass needle pointing in a given direction (azimuth or bearing) from north. If you look closely at the needle, it will also be tilting from the horizintal plane (providing you are not on the equator and the pivot mechanisim is not too ridgid). We call the later measurmeent dip.

Now imagine placing a cardboad box over this compass. Imagine a flat sensor on the bottom of the box and sine a light from the top of the box. The shhaddow you will see on the boddom of the box form the compass needle will be the vertical component of the magnetic field. It will always be smaller or equal to the needle length.

A vector magnetometer measures such a component of the total magnnetic field. AN example of sucha sa decvice is a variometer used in magnetic observatores for monitoring the ionosphere.

Now lets add another sensor to the front of the box and to the rigth hand side opf the box - now we have three sensors and they are orthogonal.

Vector magnetometers electromicaly measure one or more componets of the magnetic field. Using three orthogonal magnetometyer we can measure both aizumth and dip. By taking the square root of the sum of the squares of the components Pythagorus, the total magnetic field strength (TMF) or total magntic intensity (TMI) can be caluclated.

Examples of vector magnetometers are fluxgates, superconducting quantum interference devices (SQUIDs), and the atomic SERF magnetometer. Fluxgaes come in the following 'flavors' : ring core, ractrack, rod and Vacquier depending on the geometry of the ferrite cores.

Traditionaly they suffer drift from temprature and the dimensional stability of the ferrite cores. They also require leveling to obtain component information - a tedious and more expensive proposition to total field instruments. For this reason they are no longer used for mineral exploration.

Scalar Magnetometers

Scalar magnetometers measure the total magnetic field strength but not the direction. This can be visualised as the force pulling a compass needle. These include Proton Precession, Overhauser, and a range of Alkali vapour instruments including Cesium, Helium and Potassium.

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

NMR pocket monitoring device containing three Hall-effect sensors.

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.

They are used in applications where the magnetic field strength is relative large - for example in Anti-lock_braking_system in cars to sense wheel rotation speed via slots in the wheel disks.

Proton precession magnetometer

Proton precession magnetometers, also known as proton magnetometers, PPM's or simply mags, measure the resonance frequency of protons (hydrogen nuclei) in the magnetic field to be measured, due to nuclear magnetic resonance (NMR). Because the precession frequency depends only on atomic constants and the strength of the ambient magnetic field, the accuracy of this type of magnetometer is very good.

A direct current flowing in a solenoid creates a strong magnetic field around a hydrogen-rich fluid, causing some of the protons to align themselves with that field. The current is then interrupted, and as protons realign themselves with ambient magnetic field, they precess at a frequency that is directly proportional to the magnetic field. This produces a weak alternating magnetic field that is picked up by a (sometimes separate) inductor, amplified electronically, and fed to a digital frequency counter whose output is typically scaled and displayed directly as field strength or output as digital data.

The relationship between the frequency of the induced current and the strength of the magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 Hz/nT.

These magnetometers can be moderately sensitive if several tens of watts are available to power the aligning process. Measuring once per second, standard deviations in the readings in the 0.01 nT to 0.1 nT range can be obtained. Variations of about 0.1 nT can be detected.

The two main sources of measurement errors are magnetic impurities in the sensor and errors in the measurement of the frequency.

The Earth's magnetic field varies with time, geographical location, and local magnetic anomalies. The frequency of Earth's field NMR (EFNMR) for protons varies between approximately 900 Hz near the equator to 4.2 kHz near the geomagnetic poles. Typical short-term magnetic field variations at a particular location during Earth's daily rotation is about 25 nT (i.e., about 1 part in 2,000), with variations over a few seconds of typically around 1nT (i.e., about 1 part in 50,000).

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 (often referred to as magnetic anomalies), caused by submarines, skiers buried under snow, archaeological remains, and mineral deposits.

They are relatively inexpensive (< $US 8,000) and once widely used eg Geometrics G-856, Envi Smartmag, Scintex and GEM Systems GSM-19T. Proton samle rated are typicialy limited to 3 seconds and require the user to be statioary while a reading is undwerway. For mineral explroation they have been supercede by Overhauser and Cesium instruments which are both fast cycling - the operator does not need to stop thereby increasing production.

Overhauser effect magnetometer

The Overhauser effect magnetometer also known as an Overhauser magnetometer measures the same fundamental effect as the Proton precession magnetometer. By adding free radicals to the measurement fluid the Nuclear Overhauser effect can be exploited to improve upon the functioning of the Proton precession magnetometer. Rather than aligning the protons using a solenoid, a low power RF field is used to align the electron spin of the free radicals which then couples to the protons via the Overhauser effect. This has two main advantages, driving the RF field takes a fraction of the energy, and much faster sampling as the electron-proton coupling can happen even as measurements are being taken.

Gradiometer

Magnetic gradiometers are pairs of magnetometers with their sensors separated by a fixed distance, usually horizontally. The readings are subtracted in order to measure the difference between the sensed magnetic fields, which measures the field gradients caused by magnetic anomalies. This is one way of compensating both for the variability in time of the Earth's magnetic field and for other sources of electromagnetic interference, allowing more sensitive detection of anomalies. Because nearly equal values are being subtracted, the noise performance requirements for the magnetometers is more extreme. For this reason, high performance magnetometers are the rule in this type of system.

Fluxgate magnetometer

A uniaxial fluxgate magnetometer.
A fluxgate compass/inclinometer.
Basic principles of a fluxgate magnetometer.

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, etc. 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 prospecting and UXO detection such as the German militarys popular Forster.

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 and compact. This, plus their typically low power consumption makes them ideal for a variety of sensing applications.

The typical fluxgate magnetometer consists of a "sense" (secondary) coil surrounding an inner "drive" (primary) coil that is wound around permeable core material. 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.[10]

Caesium vapor magnetometer

A basic example of the workings of a magnetometer may be given by discussing the common optically pumped caesium vapor magnetometer which is a highly sensitive (300 fT/Hz0.5) 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 caesium light emitter or lamp, an absorption chamber containing caesium 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 caesium atom can exist in any of nine energy levels, which is the placement of electron atomic orbitals around the atomic nucleus. When a caesium 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 caesium 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 caesium 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 caesium magnetometer modulates the light applied to the cell. This is referred to as 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 caesium 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 an area and many accurate magnetic field measurements are needed, the caesium magnetometer has advantages over the proton magnetometer.

The caesium magnetometer's faster measurement rate allows the sensor to be moved through the area more quickly for a given number of data points.

The lower noise of the caesium 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, caesium or rubidium vapor operate similarly to the caesium magnetometers described above yet can reach sensitivities lower than 1 fT/Hz0.5.

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/Hz0.5 sensitivity.[11] This technology has greater sensitivity per unit volume than SQUID detectors.[12]

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/Hz0.5 in commercial instruments and 0.4 fT/Hz0.5 in experimental devices. Many liquid-helium-cooled commercial SQUIDs achieve a flat noise spectrum from near DC (less than 1 Hz) to tens of kiloHertz, making such devices ideal for time-domain biomagnetic signal measurements. SERF atomic magnetometer demonstrated in a laboratory so far reaches competitive noise floor but in relatively small frequency ranges.

SQUID 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 are most commonly used to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively). Geophysical surveys use SQUIDS from time to time, but the logistics is much more complicated than coil-based magnetometers.

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.[13] It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer).[citation needed] It consisted of a permanent bar magnet suspended horizontally from a gold fibre.[14] A magnetometer may also be called a gaussmeter.

See also

References

  1. ^ "The K-index". Space Weather Prediction Center. 1 October 2007. Retrieved 2009-10-21.
  2. ^ David Eyre (1985). "Picture of the North American Rockwell 500U Shrike Commander aircraft". Airliners.net. Retrieved 2009-10-21.
  3. ^ MOTODEV > Products > DROID by Motorola, A855. Developer.motorola.com (2011-02-13). Retrieved on 2011-03-23.
  4. ^ Nokia E72. Forumnokia.mobi (2009-06-15). Retrieved on 2011-03-23.
  5. ^ [1][dead link]
  6. ^ MagiTact. Portal.acm.org. Retrieved on 2011-03-23.
  7. ^ Coleman Jr., P.J; Davis Jr., L; Smith, E.J.; Sonett, C.P. (1962). "The Mission of Mariner II: Preliminary Observations – Interplanetary Magnetic Fields". Science. 138 (3545): 1099–1100. Bibcode:1962Sci...138.1099C. doi:10.1126/science.138.3545.1099. JSTOR 1709490.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ "Cassini Orbiter Instruments – MAG". JPL/NASA.
  9. ^ Dougherty M.K., Kellock S., Southwood D.J.; et al. (2004). "The Cassini magnetic field investigation". Space Science Reviews. 114: 331–383. Bibcode:2004SSRv..114..331D. doi:10.1007/s11214-004-1432-2. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  10. ^ Thomas H. Maugh II (24 January 2009). "Victor Vacquier Sr. dies at 101; geophysicist was a master of magnetics". The Los Angeles Times.
  11. ^ Kominis, I.K.; Kornack, T.W.; Allred, J.C.; Romalis, M.V. (4 February 2003). "A subfemtotesla multichannel atomic magnetometer". Bibcode:2003Natur.422..596K. doi:10.1038/nature01484. {{cite journal}}: Cite journal requires |journal= (help)
  12. ^ Budker, D.; Romalis, M.V. (2006). "Optical Magnetometry". arXiv:physics/0611246. {{cite arXiv}}: |class= ignored (help)
  13. ^ Gauss, C.F (1832). "The Intensity of the Earth's Magnetic Force Reduced to Absolute Measurement" (PDF). Retrieved 2009-10-21.
  14. ^ "Magnetometer: The History". CT Systems. Retrieved 2009-10-21.
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