Magnetometer
Encyclopedia
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field
either produced in the laboratory or existing in nature. Some countries such as the USA, Canada and Australia classify the more sensitive magnetometers as military technology, and control their distribution.
The International System of Units
unit of measure for the strength of a magnetic field is the tesla
. As this is a very large unit, workers in the earth sciences commonly use the nanotesla (nT) as their working unit of measure. Engineers often measure magnetic fields in Gauss
. 1 Gauss = 100,000 nT or 1 Gauss = 100,000 gamma.
The Earth's magnetic field
(the magnetosphere
) is a potential
field. It varies both temporally and spatially for various reasons, including inhomogeneity of rocks and interaction between charged particles from the Sun
and the magnetosphere.
The earth's magnetic field is relatively weak. A simple magnet that may be purchased in a hardware store produces a field many hundreds of times stronger than the earth's field. The earth's magnetic field varies from around 20,000 nT near the equator to 80,000 nT near the poles. It also varies with time. There is a daily variation of around 30 nT at mid latitudes and hundreds of nT at the poles. Geomagnetic storms can cause much larger variations.
Magnetometers, which measure magnetic fields, are distinct from metal detectors, which detect hidden metals by their conductivity
. When used for detecting metals, a magnetometer can detect only magnetic (ferrous
) metals, but can detect such metals buried much deeper than a metal detector. Magnetometers are capable of detecting large objects like cars at tens of meters, while a metal detector's range is unlikely to exceed 2 meters.
s, archaeology, Plate Tectonics, finding a wide range of mineral deposits and geological structures, hazards in coal mines, to radio wave propagation and planetary 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 survey
s — a term that also embraces a wide range of other geophysical techniques including gravity, seismic refraction, seismic reflection, electromagnetics (EM), Induced Polarisation (IP), Magneto-Tellurics (MT), Controlled Source Magneto-Tellurics (CSAMT), Sub-audio Magnetics (SAM), Mise-a-la-Masse, Resistivity, Self Potential (SP) and Very Low Frequency (VLF). See Exploration geophysics
s, shipwreck
s and other buried or submerged objects. Fluxgate gradiometers are popular due to their compact configuration and relatively low cost. Gradiometers enhance shallow features and negate the need for a base station. Caesium and Overhauser magnetometers are also very effective when used as gradiometers or as single-sensor systems with base stations.
The TV program 'Time Team' popularised 'geophys' including magnetics 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 Hungarian Plain.
Ploughed fields behave as sources of magnetic noise in such surveys.
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
.
– an impurity in some coal.
The best survey results are achieved on the ground in high-resolution surveys (10 m line spacing, 0.5 m station spacing). Borehole magnetometers such as the Ferret2 can also assist when coal seams are deep; with multiple sills and/or looking beneath surface basalt flows.
Modern surveys generally use magnetometers with GPS to record the magnetic field and locations automatically. The data are corrected using data from a second magnetometer that is left stationary during the survey. This second magnetometer (called the base station) records the change in the earths magnetic field during the time of the survey.
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.
For defensive purposes, navies use arrays of magnetometers laid across sea floors in strategic locations (i.e. around ports) to monitor submarine activity. The Russian 'Goldfish' (titanium submarines) were designed and built at great expense to thwart such systems (pure titanium is non-magnetic).
Military submarines are degaussed by passing through large underwater loops at regular intervals in a bid to escape detection by sea-floor monitoring systems, magnetic anomaly detector
s, and mines that trigger on magnetic anomalies. Submarines are never completely de-magnetised. It is possible to tell how deep a submarine has been diving by measuring its magnetic field, because the pressure distorts the steel and changes the 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 (e.g. ships). The arrays do not tow in a straight line, so fluxgate magnetometers are used to orient each sonar node in the array.
Fluxgates can also be used in weapons navigation systems, but have been largely superseded by GPS and ring laser gyroscope
s.
Magnetometers 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 aggressive tasks.
Quarry/Gemstone applications include mapping 'Blue Metal' for concrete aggregate and roadbase as well as sapphires, rubies and opal bearing structures.
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 such as the Shrike Commander.
Such aeromag surveys are typically undertaken on 400 m line spacing at 100 m elevation with readings every 10 meters or more. To overcome the asymmetry in the data density, data is interpolated between lines (usually 5 times) and data along the line is averaged. Such data would be gridded to a 80 m x 80 m pixel size then image processed using a program like ERMapper. At an exploration lease scale, the survey may be followed by a more detailed helimag or crop duster style fixed wing at 50 m line spacing and 50 m elevation (terrain permitting) – the image would be gridded on a 10 x 10 m pixel offering 64 times the resolution.
Where targets are shallow (<200 m), aeromag anomalies may be followed up with ground magnetic surveys on 10 m to 50 m line spacing with 1 m station spacing to give the best detail (2 m to 10 pixel grid) or 25 times the resolution prior to drilling.
Magnetic fields from magnetic orebodies fall off with the inverse distance cubed (dipole
target) or at best inverse distance squared (magnetic monopole
target). One analogy to the resolution-with-distance is a car driving at night with lights on. At 400 m one sees one glowing haze — as one gets closer one sees two headlights then the left blinker.
There are many challenges interpreting magnetic data for mineral exploration. Multiple targets mix together like multiple heat sources. Unlike light, there is no magnetic telescope to focus fields. We measure the combination of multiple sources at the surface. We also do not know the geometry, depth or magnetisation direction (remanence) of the targets. We can produce multiple models the explain the data — the classic ambiguity problem.
Potent by Geophysical Software Solutions http://www.geoss.com.au is a leading magnetic (and gravity) interpretation package used extensively in the Australian exploration industry.
Magnetometers assist mineral explorers both directly (i.e. gold mineralisation associated with magnetite
, diamonds in kimberlite pipes) and more commonly by indirect means such as mapping geological structures conducive to mineralisation (i.e. shear zones and alteration haloes around granites).
apps
which show direction.:
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.
Oil deposits can leak hydrocarbons which find their way up fractures in the ground to be eaten by bugs at or near the surface. The bugs can precipitate magnetite from haematite producing subtle magnetic anomalies. Such anomalies are best mapped by ground based magnetometers.
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.
Magnetometers can also be classified as "AC" types that measure fields that vary relatively rapidly in time, and "DC" types that measure fields that vary only slowly, if at all (quasi-static). AC magnetometers find use in electromagnetic systems (such as magnetotellurics
), and DC magnetometers are used for detecting mineralization and corresponding geological structures.
is a mathematical entity with both magnitude and direction. The earth's magnetic field at a given point is a vector; it is not just a numerical value, but also points in a specific direction. The direction is three-dimensional, not just north-south but also an inclination from the horizontal. A magnetic compass is designed to give a horizontal bearing direction; a vector magnetometer measures the magnitude and direction of the total magnetic field. An example of such a device is a Variometer
used in magnetic observatories for monitoring the ionosphere. Three orthogonal sensors are required to measure the components of the magnetic field in all three dimensions.
Vector magnetometers electronically measure one or more components of the magnetic field. Using three orthogonal magnetometers, both azimuth and dip (inclination) can be measured. By taking the square root of the sum of the squares of the components the total magnetic field strength (also called total magnetic intensity, TMI) can be calculated by Pythagoras's theorem.
Examples of vector magnetometers are fluxgates, superconducting quantum interference devices (SQUIDs), and the atomic SERF
magnetometer.
Fluxgates come in the following 'flavors': ring core, ractrack, rod and Vacquier depending on the geometry of the ferrite
cores.
They are subject to temperature drift and the dimensional instability of the ferrite cores. They also require leveling to obtain component information, unlike total field (scalar) instruments. For these reasons they are no longer used for mineral exploration.
magnetometers measure the total magnetic field strength but not its direction.
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.
. 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
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 relatively large — for example in Anti-lock braking system
in cars to sense wheel rotation speed via slots in the wheel disks.
s, PPM's or simply mags, measure the resonance frequency of proton
s (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[Please quantify].
A direct current flowing in a solenoid
creates a strong magnetic field around a hydrogen
-rich fluid (kerosine, and decane is popular — even water can be used), 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 rotating 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. The accuracy of PPM's is this limited by the accuracy of this constant.
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 pole
s. 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.
For hand/backpack carried units, PPM sample rates are typically limited to less than one sample per second. Measurements are typically taken with the sensor held at fixed locations at perhaps 10 meter increments.
The two main sources of measurement errors are magnetic impurities in the sensor, errors in the measurement of the frequency and ferrous material on the operator and in the instruments. If the sensor is rotated as the measurement is made, an additional error is generated.
Portable instruments are also limited by sensor volume (weight) and power consumption. PPMs work in field gradients up to 3,000nT/m which is adequate from most mineral exploration work. For higher gradient tolerance such as mapping banded iron formations and detecting large ferrous objects Overhauser magnetometers can handle 10,000nT/m and Cesium magnetometers can handle 30,000nT/m.
They are relatively inexpensive (< $US 8,000) and once widely used in mineral exploration. Three manufacturers dominate the market : GEM Systems, Geometrics and Scintrex. Popular models include G-856, Smartmag and GSM-18 and GSM-19T.
For mineral exploration they have been superseded by Overhauser and Cesium instruments which are both fast-cycling; the operator does not need to pause between readings, thereby increasing production.
can be exploited to significantly improve upon the proton precession magnetometer. Rather than aligning the proton
s using a solenoid, a low power radio-frequency field is used to align (polarise) 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 (allowing lighter-weight batteries for portable units), and faster sampling as the electron-proton coupling can happen even as measurements are being taken. An Overhauser magnetometer produce readings with a 0.01nT to 0.02nT standard deviation while sampling once per second.
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.
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 military's 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.
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 photon
s 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 level
s, which is the placement of electron
atomic orbital
s 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.
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. Caesium magnetometers are insensitive to rotation of the sensor while the measurement is being made.
The lower noise of the caesium magnetometer allows those measurements to more accurately show the variations in the field with position.
) 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. 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 fiber-optic cables. This would allow the magnetic measurement to be made in places where high electrical voltages exist.
s, 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.
Aeromag datasets for Australia can be downloaded from GADDS database for free.
http://www.geoscience.gov.au/bin/mapserv36?map=/public/http/www/geoportal/gadds/gadds.map&mode=browse
There is point located and image data. Image data is in ERMapper format.
See also Magnetic Surveying in Archaeology (book)
.
s 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.
Gradiometers enhance shallow magnetic anomalies and are thus good for archaeological and some site investigation work. They are also good for real-time work such as Unexploded ordnance
location. In the commercial world, it is twice as efficient to run a base station and use two (or more) mobile sensors to read parallel lines simultaneously (assuming data is stored and post-processed). In this manner both along-line and cross-line gradients can be calculated.
Non-magnetic electronic hipchain triggers were developed to trigger magnetometers. There used rotary shaft encoders to measure distance along disposable cotton reels.
Modern explorers use a range of low-magnetic signature GPS units including Real-Time Kinematic GPS.
Heading errors are one group of noise. They comprise three sources :
Some total field sensors give different readings depending on their orientation. Magnetic materials in the sensor its self are the primary cause of this error. In some magnetometers such as the vapor magnetometers (caesium potassium etc.) there are sources of heading error in the physics that contribute small amounts to the total heading error.
Console noise comes from magnetic components on or within the console. These include ferrite in cores in inductors and transformers, steel frames around LCD's, legs on IC chips and steel cases in disposable batteries. Some popular MIL spec connectors also have steel springs.
Operators must take care to be magnetically clean and should check the 'magnetic hygiene' of all apparel and items carries during a survey. Acubra hats are very popular in Australia, however their steel rims must be removed before use on magnetic surveys. Steel rings on notepads, steel capped boots, steel springs in overall eyelets can all cause unnecessary noise in surveys. Pens, mobile phones and stainless steel implants can also be problematic.
The magnetic response (noise) from ferrous object on the operator and console can change with heading direction because of induction and remanence. Aeromagnetic survey aircraft and quad bike systems can use special compensators to correct for heading error noise.
Heading errors look like herringbone patterns in survey images. Alternate lines can also be corrugated.
, 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 may also be called a gaussmeter.
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
either produced in the laboratory or existing in nature. Some countries such as the USA, Canada and Australia classify the more sensitive magnetometers as military technology, and control their distribution.
The International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
unit of measure for the strength of a magnetic field is the tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
. As this is a very large unit, workers in the earth sciences commonly use the nanotesla (nT) as their working unit of measure. Engineers often measure magnetic fields in Gauss
Gauss (unit)
The gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...
. 1 Gauss = 100,000 nT or 1 Gauss = 100,000 gamma.
The Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
(the magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
) is a potential
Potential
*In linguistics, the potential mood*The mathematical study of potentials is known as potential theory; it is the study of harmonic functions on manifolds...
field. It varies both temporally and spatially for various reasons, including inhomogeneity of rocks and interaction between charged particles from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and the magnetosphere.
The earth's magnetic field is relatively weak. A simple magnet that may be purchased in a hardware store produces a field many hundreds of times stronger than the earth's field. The earth's magnetic field varies from around 20,000 nT near the equator to 80,000 nT near the poles. It also varies with time. There is a daily variation of around 30 nT at mid latitudes and hundreds of nT at the poles. Geomagnetic storms can cause much larger variations.
Magnetometers, which measure magnetic fields, are distinct from metal detectors, which detect hidden metals by their conductivity
Conductivity
Conductivity may refer to:*Electrical conductivity, a measure of a material's ability to conduct an electric current*Conductivity , also the specific conductance, is a measurement of the electrical conductance per unit distance in an electrolytic or aqueous solution*Ionic conductivity, a measure of...
. When used for detecting metals, a magnetometer can detect only magnetic (ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
) metals, but can detect such metals buried much deeper than a metal detector. Magnetometers are capable of detecting large objects like cars at tens of meters, while a metal detector's range is unlikely to exceed 2 meters.
Uses
Magnetometers have a very diverse range of applications from locating submarines and Spanish Galleons, positioning weapons systems, detecting unexploded ordenance, locating toxic waste drums, heart beat monitors, sensors in anti-locking brakes, weather prediction (via solar cycles), depths of steel pylons, drill guidance systems, locating hazards for tunnel boring machineTunnel boring machine
A tunnel boring machine also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre to almost 16 metres to date...
s, archaeology, Plate Tectonics, finding a wide range of mineral deposits and geological structures, hazards in coal mines, to radio wave propagation and planetary 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 survey
Geophysical survey
Geophysical survey is the systematic collection of geophysical data for spatial studies. Geophysical surveys may use a great variety of sensing instruments, and data may be collected from above or below the Earth's surface or from aerial or marine platforms. Geophysical surveys have many...
s — a term that also embraces a wide range of other geophysical techniques including gravity, seismic refraction, seismic reflection, electromagnetics (EM), Induced Polarisation (IP), Magneto-Tellurics (MT), Controlled Source Magneto-Tellurics (CSAMT), Sub-audio Magnetics (SAM), Mise-a-la-Masse, Resistivity, Self Potential (SP) and Very Low Frequency (VLF). See Exploration geophysics
Exploration geophysics
Exploration geophysics is the applied branch of geophysics which uses surface methods to measure the physical properties of the subsurface Earth, in order to detect or infer the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, groundwater reservoirs, and other geological...
Archaeology
Magnetometers are also used to detect archaeological siteArchaeological site
An archaeological site is a place in which evidence of past activity is preserved , and which has been, or may be, investigated using the discipline of archaeology and represents a part of the archaeological record.Beyond this, the definition and geographical extent of a 'site' can vary widely,...
s, shipwreck
Shipwreck
A shipwreck is what remains of a ship that has wrecked, either sunk or beached. Whatever the cause, a sunken ship or a wrecked ship is a physical example of the event: this explains why the two concepts are often overlapping in English....
s and other buried or submerged objects. Fluxgate gradiometers are popular due to their compact configuration and relatively low cost. Gradiometers enhance shallow features and negate the need for a base station. Caesium and Overhauser magnetometers are also very effective when used as gradiometers or as single-sensor systems with base stations.
The TV program 'Time Team' popularised 'geophys' including magnetics 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 Hungarian Plain.
Ploughed fields behave as sources of magnetic noise in such surveys.
Auroras
Magnetometers can give an indication of possible auroral activity before one can see the lightLight
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
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
K-index
The K-index quantifies disturbances in the horizontal component of earth's magnetic field with an integer in the range 0-9 with 1 being calm and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a...
.
Coal exploration
Whilst magnetometers can be used to help map basin shape at a regional scale, they are more commonly used to map hazards to coal mining including basaltic intrusions (dykes, sills and volcanic plugs) that destroy resources and wreak havoc with longwall mining equipment. Magnetometers can also locate faults and burn zones (ignited by lightning). and map sideriteSiderite
Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word σίδηρος sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus...
– an impurity in some coal.
The best survey results are achieved on the ground in high-resolution surveys (10 m line spacing, 0.5 m station spacing). Borehole magnetometers such as the Ferret2 can also assist when coal seams are deep; with multiple sills and/or looking beneath surface basalt flows.
Modern surveys generally use magnetometers with GPS to record the magnetic field and locations automatically. The data are corrected using data from a second magnetometer that is left stationary during the survey. This second magnetometer (called the base station) records the change in the earths magnetic field during the time of the survey.
Directional drilling
They are used in directional drillingDirectional drilling
Directional drilling is the practice of drilling non-vertical wells. It can be broken down into three main groups: Oilfield Directional Drilling, Utility Installation Directional Drilling Directional drilling (or slant drilling) is the practice of drilling non-vertical wells. It can be broken down...
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
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
and azimuth of the drill bit can be found.
Military
Because a magnetometer can be used to detect a submarine, magnetometers are a classified technology in countries such as Australia, Canada, and the USA.For defensive purposes, navies use arrays of magnetometers laid across sea floors in strategic locations (i.e. around ports) to monitor submarine activity. The Russian 'Goldfish' (titanium submarines) were designed and built at great expense to thwart such systems (pure titanium is non-magnetic).
Military submarines are degaussed by passing through large underwater loops at regular intervals in a bid to escape detection by sea-floor monitoring systems, magnetic anomaly detector
Magnetic anomaly detector
A magnetic anomaly detector is an instrument used to detect minute variations in the Earth's magnetic field. The term refers specifically to magnetometers used by military forces to detect submarines ; the military MAD gear is a descendent of geomagnetic survey instruments used to search for...
s, and mines that trigger on magnetic anomalies. Submarines are never completely de-magnetised. It is possible to tell how deep a submarine has been diving by measuring its magnetic field, because the pressure distorts the steel and changes the 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 (e.g. ships). The arrays do not tow in a straight line, so fluxgate magnetometers are used to orient each sonar node in the array.
Fluxgates can also be used in weapons navigation systems, but have been largely superseded by GPS and ring laser gyroscope
Ring laser gyroscope
A ring laser gyroscope consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation...
s.
Magnetometers 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 aggressive tasks.
Mineral exploration
Mineral exploration is one of the major commercial drivers and users of magnetometers. Magnetometers are one of the prime tools used to locate world class deposits of gold, silver, copper, iron, tin, platinum and diamonds.Quarry/Gemstone applications include mapping 'Blue Metal' for concrete aggregate and roadbase as well as sapphires, rubies and opal bearing structures.
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 such as the Shrike Commander.
Such aeromag surveys are typically undertaken on 400 m line spacing at 100 m elevation with readings every 10 meters or more. To overcome the asymmetry in the data density, data is interpolated between lines (usually 5 times) and data along the line is averaged. Such data would be gridded to a 80 m x 80 m pixel size then image processed using a program like ERMapper. At an exploration lease scale, the survey may be followed by a more detailed helimag or crop duster style fixed wing at 50 m line spacing and 50 m elevation (terrain permitting) – the image would be gridded on a 10 x 10 m pixel offering 64 times the resolution.
Where targets are shallow (<200 m), aeromag anomalies may be followed up with ground magnetic surveys on 10 m to 50 m line spacing with 1 m station spacing to give the best detail (2 m to 10 pixel grid) or 25 times the resolution prior to drilling.
Magnetic fields from magnetic orebodies fall off with the inverse distance cubed (dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
target) or at best inverse distance squared (magnetic monopole
Magnetic monopole
A magnetic monopole is a hypothetical particle in particle physics that is a magnet with only one magnetic pole . In more technical terms, a magnetic monopole would have a net "magnetic charge". Modern interest in the concept stems from particle theories, notably the grand unified and superstring...
target). One analogy to the resolution-with-distance is a car driving at night with lights on. At 400 m one sees one glowing haze — as one gets closer one sees two headlights then the left blinker.
There are many challenges interpreting magnetic data for mineral exploration. Multiple targets mix together like multiple heat sources. Unlike light, there is no magnetic telescope to focus fields. We measure the combination of multiple sources at the surface. We also do not know the geometry, depth or magnetisation direction (remanence) of the targets. We can produce multiple models the explain the data — the classic ambiguity problem.
Potent by Geophysical Software Solutions http://www.geoss.com.au is a leading magnetic (and gravity) interpretation package used extensively in the Australian exploration industry.
Magnetometers assist mineral explorers both directly (i.e. gold mineralisation associated with magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, diamonds in kimberlite pipes) and more commonly by indirect means such as mapping geological structures conducive to mineralisation (i.e. shear zones and alteration haloes around granites).
Mobile telephones
Many smartphones contain magnetometers. There are compassCompass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
apps
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...
which show direction.:
Researchers at Deutsche Telekom
Deutsche Telekom
Deutsche Telekom AG is a telecommunications company headquartered in Bonn, Germany. It is the largest telecommunications company in Europe....
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.
Oil exploration
Seismic methods are preferred to magnetometers for oil exploration. Aeromag surveys can be used for basin shape, and locating faults.Oil deposits can leak hydrocarbons which find their way up fractures in the ground to be eaten by bugs at or near the surface. The bugs can precipitate magnetite from haematite producing subtle magnetic anomalies. Such anomalies are best mapped by ground based magnetometers.
Spacecraft
A three-axis fluxgate magnetometer was part of the Mariner 2Mariner 2
Mariner 2 , an American space probe to Venus, was the first space probe to conduct a successful planetary encounter . The first successful spacecraft in the NASA Mariner program, it was a simplified version of the Block I spacecraft of the Ranger program and an exact copy of Mariner 1...
and Mariner 10
Mariner 10
Mariner 10 was an American robotic space probe launched by NASA on November 3, 1973, to fly by the planets Mercury and Venus. It was launched approximately two years after Mariner 9 and was the last spacecraft in the Mariner program...
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
MESSENGER
The MErcury Surface, Space ENvironment, GEochemistry and Ranging space probe is a robotic NASA spacecraft in orbit around the planet Mercury. The spacecraft was launched aboard a Delta II rocket in August 2004 to study the chemical composition, geology, and magnetic field of Mercury...
mission. A magnetometer can also be used by satellites like GOES
Goes
Goes is a municipality and a city in the southwestern Netherlands in Zuid-Beveland, in the province Zeeland. The city of Goes has approximately 27,000 residents.-History of Goes:...
to measure both the magnitude
Magnitude (mathematics)
The magnitude of an object in mathematics is its size: a property by which it can be compared as larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which it belongs....
and direction
Direction (geometry, geography)
Direction is the information contained in the relative position of one point with respect to another point without the distance information. Directions may be either relative to some indicated reference , or absolute according to some previously agreed upon frame of reference Direction is the...
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, relative to the spatial orientation of the device.
Magnetometers can also be classified as "AC" types that measure fields that vary relatively rapidly in time, and "DC" types that measure fields that vary only slowly, if at all (quasi-static). AC magnetometers find use in electromagnetic systems (such as magnetotellurics
Magnetotellurics
Magnetotellurics is an electromagnetic geophysical method of imaging the earth's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with...
), and DC magnetometers are used for detecting mineralization and corresponding geological structures.
Vector magnetometers
A vectorVector
Vector, a Latin word meaning "carrier", may refer in English to:-In computer science:*A one-dimensional array**Vector , a data type in the C++ Standard Template Library...
is a mathematical entity with both magnitude and direction. The earth's magnetic field at a given point is a vector; it is not just a numerical value, but also points in a specific direction. The direction is three-dimensional, not just north-south but also an inclination from the horizontal. A magnetic compass is designed to give a horizontal bearing direction; a vector magnetometer measures the magnitude and direction of the total magnetic field. An example of such a device is a Variometer
Variometer
The term variometer also refers to a type of variable transformer or an instrument for measuring the magnitude and direction of a Magnetic field....
used in magnetic observatories for monitoring the ionosphere. Three orthogonal sensors are required to measure the components of the magnetic field in all three dimensions.
Vector magnetometers electronically measure one or more components of the magnetic field. Using three orthogonal magnetometers, both azimuth and dip (inclination) can be measured. By taking the square root of the sum of the squares of the components the total magnetic field strength (also called total magnetic intensity, TMI) can be calculated by Pythagoras's theorem.
Examples of vector magnetometers are fluxgates, superconducting quantum interference devices (SQUIDs), and the atomic SERF
SERF
A spin exchange relaxation-free magnetometer is a type of magnetometer developed at Princeton University in the early 2000s. SERF magnetometers measure magnetic fields by using lasers to detect the interaction between alkali metal atoms in a vapor and the magnetic field.The name for the technique...
magnetometer.
Fluxgates come in the following 'flavors': ring core, ractrack, rod and Vacquier depending on the geometry of the ferrite
Ferrite
Ferrite may refer to:* Ferrite , iron or iron alloys with a body centred cubic crystal structure.* Ferrite , ferrimagnetic ceramic materials used in magnetic applications....
cores.
They are subject to temperature drift and the dimensional instability of the ferrite cores. They also require leveling to obtain component information, unlike total field (scalar) instruments. For these reasons they are no longer used for mineral exploration.
Scalar magnetometers
ScalarScalar
Scalar may refer to:*Scalar , a quantity used to multiply vectors in the context of vector spaces*Scalar , a quantity which is independent of specific classes of coordinate systems...
magnetometers measure the total magnetic field strength but not its direction.
These include Proton Precession, Overhauser, and a range of Alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
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 coilCoil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
. 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-stateSolid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
Hall effect
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...
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 relatively large — for example in Anti-lock braking system
Anti-lock braking system
An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...
in cars to sense wheel rotation speed via slots in the wheel disks.
Proton precession magnetometer
Proton precession magnetometers, also known as proton magnetometerProton magnetometer
The proton magnetometer, also known as the proton precession magnetometer , uses the principle of Earth's field nuclear magnetic resonance to measure very small variations in the Earth's magnetic field, allowing ferrous objects on land and at sea to be detected.It is used in land-based archaeology...
s, PPM's or simply mags, measure the resonance frequency of proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s (hydrogen nuclei) in the magnetic field to be measured, due to nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(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[Please quantify].
A direct current flowing in a solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
creates a strong magnetic field around a hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
-rich fluid (kerosine, and decane is popular — even water can be used), 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
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
at a frequency that is directly proportional to the magnetic field. This produces a weak rotating magnetic field that is picked up by a (sometimes separate) inductor, amplified
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
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. The accuracy of PPM's is this limited by the accuracy of this constant.
The frequency of Earth's field NMR
Earth's field NMR
Nuclear magnetic resonance in the geomagnetic field is conventionally referred to as Earth's field NMR . EFNMR is a special case of low field NMR....
(EFNMR) for protons varies between approximately 900 Hz near the equator to 4.2 kHz near the geomagnetic pole
Geomagnetic pole
The geomagnetic poles are antipodal points where the axis of a theoretical dipole intersects the Earth's surface. This dipole is equivalent to a powerful bar magnet at the center of the Earth, and it is the dipole that comes closest to accounting for the magnetic field observed at the Earth's...
s. 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.
For hand/backpack carried units, PPM sample rates are typically limited to less than one sample per second. Measurements are typically taken with the sensor held at fixed locations at perhaps 10 meter increments.
The two main sources of measurement errors are magnetic impurities in the sensor, errors in the measurement of the frequency and ferrous material on the operator and in the instruments. If the sensor is rotated as the measurement is made, an additional error is generated.
Portable instruments are also limited by sensor volume (weight) and power consumption. PPMs work in field gradients up to 3,000nT/m which is adequate from most mineral exploration work. For higher gradient tolerance such as mapping banded iron formations and detecting large ferrous objects Overhauser magnetometers can handle 10,000nT/m and Cesium magnetometers can handle 30,000nT/m.
They are relatively inexpensive (< $US 8,000) and once widely used in mineral exploration. Three manufacturers dominate the market : GEM Systems, Geometrics and Scintrex. Popular models include G-856, Smartmag and GSM-18 and GSM-19T.
For mineral exploration they have been superseded by Overhauser and Cesium instruments which are both fast-cycling; the operator does not need to pause between readings, thereby increasing production.
Overhauser effect magnetometer
The Overhauser effect magnetometer or Overhauser magnetometer measures the same fundamental effect as the proton precession magnetometer. By adding free radicals to the measurement fluid the nuclear Overhauser effectNuclear Overhauser effect
The Nuclear Overhauser Effect is the transfer of nuclear spin polarization from one nuclear spin population to another via cross-relaxation. It is a common phenomenon observed by nuclear magnetic resonance spectroscopy. The theoretical basis for the NOE was described and experimentally verified...
can be exploited to significantly improve upon the proton precession magnetometer. Rather than aligning the proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s using a solenoid, a low power radio-frequency field is used to align (polarise) 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 (allowing lighter-weight batteries for portable units), and faster sampling as the electron-proton coupling can happen even as measurements are being taken. An Overhauser magnetometer produce readings with a 0.01nT to 0.02nT standard deviation while sampling once per second.
Fluxgate magnetometer
Victor Vacquier
Victor Vacquier, Sr. was a professor of geophysics at the Scripps Institution of Oceanography at the University of California, San Diego.Vacquier was born in St. Petersburg, Russia...
at Gulf Research Laboratories; Vacquier applied them during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
as an instrument for detecting submarines, and after the war confirmed the theory of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
by using them to measure shifts in the magnetic patterns on the sea floor.
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 military's 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.
Caesium vapor magnetometer
A basic example of the workings of a magnetometer may be given by discussing the common optically pumped caesiumCaesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
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
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
to operate, its basic principles are easily explained.
The device broadly consists of a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
emitter containing a caesium light emitter or lamp, an absorption chamber containing caesium vapor and a "buffer gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
" through which the emitted photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s 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 level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
s, which is the placement of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
atomic orbital
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
s around the atomic nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
. 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. Caesium magnetometers are insensitive to rotation of the sensor while the measurement is being made.
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 (SERFSERF
A spin exchange relaxation-free magnetometer is a type of magnetometer developed at Princeton University in the early 2000s. SERF magnetometers measure magnetic fields by using lasers to detect the interaction between alkali metal atoms in a vapor and the magnetic field.The name for the technique...
) atomic magnetometers containing potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
or rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
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. This technology has greater sensitivity per unit volume than SQUID
SQUID
A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions....
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 fiber-optic cables. This would allow the magnetic measurement to be made in places where high electrical voltages exist.
SQUID magnetometer
SQUIDSQUID
A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions....
s, 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
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
(4.2 K) or liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
(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
Magnetoencephalography
Magnetoencephalography is a technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using arrays of SQUIDs...
and magnetocardiography
Magnetocardiography
Magnetocardiography is a technique to measure the magnetic fields produced by electrical activity in the heart using extremely sensitive devices such as the Superconducting Quantum Interference Device...
, respectively). Geophysical surveys use SQUIDS from time to time, but the logistics is much more complicated than coil-based magnetometers.
Magnetic Surveys
Systematic surveys may be used to cover areas of interest such as exploring for mineral deposits or locating lost objects. Such surveys can be divided into- Aeromagnetic surveyAeromagnetic surveyAn aeromagnetic survey is a common type of geophysical survey carried out using a magnetometer aboard or towed behind an aircraft. The principle is similar to a magnetic survey carried out with a hand-held magnetometer, but allows much larger areas of the Earth's surface to be covered quickly for...
- Borehole
- Ground
- Marine
Aeromag datasets for Australia can be downloaded from GADDS database for free.
http://www.geoscience.gov.au/bin/mapserv36?map=/public/http/www/geoportal/gadds/gadds.map&mode=browse
There is point located and image data. Image data is in ERMapper format.
See also Magnetic Surveying in Archaeology (book)
Magnetic Surveying in Archaeology (book)
Magnetic Surveying in Archaeology is a book written by Russian archaeologist T. N. Smekalova together with O. Voss and S. L. Smekalov. In the book researches collected information about magnetic prospecting of archaeological sites...
.
Gradiometer
Magnetic gradiometerGradiometer
A gradiometer measures the gradient of a physical quantity, such as a magnetic field or gravity.-Types of gradiometer:There are at least two types of gradiometer to measure magnetic fields:...
s 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.
Gradiometers enhance shallow magnetic anomalies and are thus good for archaeological and some site investigation work. They are also good for real-time work such as Unexploded ordnance
Unexploded ordnance
Unexploded ordnance are explosive weapons that did not explode when they were employed and still pose a risk of detonation, potentially many decades after they were used or discarded.While "UXO" is widely and informally used, munitions and explosives of...
location. In the commercial world, it is twice as efficient to run a base station and use two (or more) mobile sensors to read parallel lines simultaneously (assuming data is stored and post-processed). In this manner both along-line and cross-line gradients can be calculated.
Position Control of Magnetic Surveys
In traditional mineral exploration and archaeological work, grid pegs placed by theodolite and tape measure were used to define the survey area. Some UXO surveys used ropes to define the lanes. Airborne surveys used radio triangulation beacons such a Siledus (sp?).Non-magnetic electronic hipchain triggers were developed to trigger magnetometers. There used rotary shaft encoders to measure distance along disposable cotton reels.
Modern explorers use a range of low-magnetic signature GPS units including Real-Time Kinematic GPS.
Heading Errors in Magnetic Surveys
Magnetic surveys can suffer noise from a range of sources. Different magnetometer technologies suffer different kinds of noise problems.Heading errors are one group of noise. They comprise three sources :
- Sensor
- Console
- Operator
Some total field sensors give different readings depending on their orientation. Magnetic materials in the sensor its self are the primary cause of this error. In some magnetometers such as the vapor magnetometers (caesium potassium etc.) there are sources of heading error in the physics that contribute small amounts to the total heading error.
Console noise comes from magnetic components on or within the console. These include ferrite in cores in inductors and transformers, steel frames around LCD's, legs on IC chips and steel cases in disposable batteries. Some popular MIL spec connectors also have steel springs.
Operators must take care to be magnetically clean and should check the 'magnetic hygiene' of all apparel and items carries during a survey. Acubra hats are very popular in Australia, however their steel rims must be removed before use on magnetic surveys. Steel rings on notepads, steel capped boots, steel springs in overall eyelets can all cause unnecessary noise in surveys. Pens, mobile phones and stainless steel implants can also be problematic.
The magnetic response (noise) from ferrous object on the operator and console can change with heading direction because of induction and remanence. Aeromagnetic survey aircraft and quad bike systems can use special compensators to correct for heading error noise.
Heading errors look like herringbone patterns in survey images. Alternate lines can also be corrugated.
Image Procesing of Magnetic Data
Recording data and image processing is a superior to real time work because subtle anomalies often missed by the operator(especially in magnetically noisy areas) can be correlated between lines, shapes and clusters better defined. A range of sophisticated enhancement techniques can also be used. There is also a hard copy and need for systematic coverage.Early magnetometers
In 1833, Carl Friedrich GaussCarl Friedrich Gauss
Johann Carl Friedrich Gauss was a German mathematician and scientist who contributed significantly to many fields, including number theory, statistics, analysis, differential geometry, geodesy, geophysics, electrostatics, astronomy and optics.Sometimes referred to as the Princeps mathematicorum...
, 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
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
fibre. A magnetometer may also be called a gaussmeter.
See also
- Vibrating sample magnetometerVibrating sample magnetometerA vibrating sample magnetometer or VSM is a scientific instrument that measures magnetic properties, invented in 1955 by Simon Foner at Lincoln Laboratory MIT. The paper about his work was published shortly afterward in 1959 A sample is placed inside a uniform magnetic field to magnetize the sample...
- Magnetic anomaly detectorMagnetic anomaly detectorA magnetic anomaly detector is an instrument used to detect minute variations in the Earth's magnetic field. The term refers specifically to magnetometers used by military forces to detect submarines ; the military MAD gear is a descendent of geomagnetic survey instruments used to search for...
- MagnetotelluricsMagnetotelluricsMagnetotellurics is an electromagnetic geophysical method of imaging the earth's subsurface by measuring natural variations of electrical and magnetic fields at the Earth's surface. Investigation depth ranges from 300m below ground by recording higher frequencies down to 10,000m or deeper with...
- Nuclear Magnetic ResonanceNuclear magnetic resonanceNuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
NMRNMRNMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR... - Earth's field NMREarth's field NMRNuclear magnetic resonance in the geomagnetic field is conventionally referred to as Earth's field NMR . EFNMR is a special case of low field NMR....
(EFNMR) - Magnetic immunoassayMagnetic immunoassayMagnetic immunoassay is a novel type of diagnostic immunoassay using magnetic beads as labels in lieu of conventional enzymes , radioisotopes or fluorescent moieties . This assay involves the specific binding of an antibody to its antigen, where a magnetic label is conjugated to one element of...
- IntermagnetIntermagnetIntermagnet is a global network of observatories, monitoring the Earth's magnetic field.- Participating Countries :Intermagnet now has Geomagnetic Information Nodes in 5 continents....
is a global network of observatories, monitoring the Earth's magnetic fieldEarth's magnetic fieldEarth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
. - Search Coil Magnetometer – sensitive broadband magnetometers, ULF applications, Space Applications
- Magnetometer Arrays – Existing Arrays, How Arrays are used for imaging
- Magnetometer Networks – Linking the arrays and instruments together, data fusion, applications, goals
- Magnetometer Data – Where is the data? What models exist? What software and algorithms are commonly applied?
- Transionospheric – Geomagnetic phenomena are visible from orbiting and geostationary satellites
- Lithospheric-Ionospheric – Seismo-electromagnetic data can be detected by earth-based and satellite magnetometers and other sensors. Earthquake predictionEarthquake predictionAn earthquake prediction is a prediction that an earthquake of a specific magnitude will occur in a particular place at a particular time . Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month...
, preseismic, and Earth Models are common applications. - MagnetogramMagnetogramThe term magnetogram has two meanings, used separately in the contexts of magnetic fields of the Sun and the Earth.In the context of the magnetic field of the Sun, the term magnetogram refers to a pictorial representation of the spatial variations in strength of the solar magnetic field...
s – are often displayed as images on the web, but usually the digital data are also available. - EMF measurementsEMF measurementsEMF measurements are measurements of ambient electromagnetic fields that are taken with particular sensors or probes, such as EMF meters. These probes can be generally considered as antennas although with different characteristics...
- MEMS magnetometers - as seen in many hand-held devices such as smartphones.
External links
- Earthquake forecasting techniques and more research on the study of electromagnetic fields
- Dan's Homegrown Proton Precession Magnetometer Page
- USGS Geomagnetism Program
- A home built PPM that actually works
- Earth's Field NMR (EFNMR)
- Magnetic archaeological prospection (Swedish National Heritage Board)
- Magnetometer – Measuring the magnetic field of Earth
- Space-based magnetometers
- Practical guidelines for building a magnetometer by hobbyists – Part 1 Introduction
- Practical guidelines for building a magnetometer by hobbyists – Part 2 Building
- The new INTERMAGNET 1-second standard fluxgate magnetometer. V.KorepanovValery KorepanovValery Korepanov is a Ukrainian scientist was born on 1 July 1943 in the Soviet Union. Since 1996 he has been the Scientific Director of the Lviv Centre of Institute for Space Research of National Academy of Sciences of Ukraine and National Space Agency of Ukraine.-Biography:Graduated from Lviv...
, A. Marusenkov (Lviv Centre of Institute for Space ResearchLviv Centre of Institute for Space ResearchThe Lviv Centre of Institute of Space Research of the National Academy of Sciences of Ukraine and National Space Agency of Ukraine was formed in 1996 at the basis of Special Design Bureau of Karpenko Institute for Physics and Mechanics of NASU as a scientific experimental division of Institute...
, LvivLvivLviv is a city in western Ukraine. The city is regarded as one of the main cultural centres of today's Ukraine and historically has also been a major Polish and Jewish cultural center, as Poles and Jews were the two main ethnicities of the city until the outbreak of World War II and the following...
, UkraineUkraineUkraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...
), J.Rasson (Royal Meteorological InstituteRoyal Meteorological InstituteThe Royal Meteorological Institute of Belgium is a Belgian federal institute of the Belgian Federal Science Policy Office . It is located in Uccle and in Dourbes...
, Dourbes, BelgiumBelgiumBelgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
)