Editing Magnetometer (section)

==Uses==
{{refimprove section |date=September 2024}}

[[File:Magnetometers Can Measure the Magnetic Fields of Planets.ogv|thumb|Magnetometers can measure the magnetic fields of planets.]]

Magnetometers have a very diverse range of applications, including locating objects such as submarines, sunken ships, hazards affecting [[tunnel boring machine]]s, coal mine hazards, unexploded ordnance, toxic waste drums, as well as a wide range of mineral deposits and geological structures. They also have applications in heart beat monitors, concealed weapons detection,<ref>{{Cite news |last=Javaid |first=Maham |date=2022-06-29 |title=What are magnetometers, or mags? |url=https://www.nytimes.com/2022/06/28/us/what-are-magnetometers-mags.html |access-date=2024-08-07 |work=The New York Times |issn=0362-4331}}</ref> military weapon systems positioning, sensors in anti-locking brakes, weather prediction (via solar cycles), steel pylons, drill guidance systems, archaeology, plate tectonics, radio wave propagation, and planetary exploration. Laboratory magnetometers determine the magnetic dipole moment of a magnetic sample, typically as a function of [[temperature]], [[magnetic field]], or other parameter. This helps to reveal its magnetic properties such as [[ferromagnetism]], [[antiferromagnetism]], [[superconductivity]], or other properties that affect [[magnetism]].

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

===Mechanical stress measurement===
Magnetometers are used to measure or monitor mechanical stress in ferromagnetic materials. Mechanical stress will improve alignment of magnetic domains in microscopic scale that will raise the magnetic field measured close to the material by magnetometers. There are different hypothesis about stress-magnetisation relationship. However the effect of mechanical stress on measured magnetic field near the specimen is claimed to be proven in many scientific publications. There have been efforts to solve the inverse problem of magnetisation-stress resolution in order to quantify the stress based on measured magnetic field.<ref>{{Cite journal|last1=Staples|first1=S. G. H.|last2=Vo|first2=C.|last3=Cowell|first3=D. M. J.|last4=Freear|first4=S.|last5=Ives|first5=C.|last6=Varcoe|first6=B. T. H.|date=2013-04-07|title=Solving the inverse problem of magnetisation–stress resolution|journal=Journal of Applied Physics|volume=113|issue=13|pages=133905–133905–6|doi=10.1063/1.4799049|issn=0021-8979|bibcode=2013JAP...113m3905S|url=http://eprints.whiterose.ac.uk/78162/7/BarFieldExptJan2013_with_coversheet.pdf}}</ref><ref>{{Cite journal|last1=Wilson|first1=John W.|last2=Tian|first2=Gui Yun|last3=Barrans|first3=Simon|date=April 2007|title=Residual magnetic field sensing for stress measurement|journal=Sensors and Actuators A: Physical|volume=135|issue=2|pages=381–387|doi=10.1016/j.sna.2006.08.010|bibcode=2007SeAcA.135..381W }}</ref>

===Accelerator physics===
[[File:Aust.-Synchrotron,-Quadrupole-Magnets-of-Linac,-14.06.2007.jpg|thumb|Aust.-Synchrotron,-Quadrupole-Magnets-of-Linac,-14.06.2007]]
Magnetometers are used extensively in experimental particle physics to measure the magnetic field of pivotal components such as the concentration or focusing beam-magnets.

===Archaeology===
{{Main|Magnetic survey (archaeology)}}
Magnetometers are also used to detect [[archaeological site]]s, [[shipwreck]]s, and other buried or submerged objects. Fluxgate [[gradiometer]]s 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 magnetic techniques used in archaeological work to detect fire hearths, walls of baked bricks and magnetic stones such as basalt and granite. Walking tracks and roadways can sometimes be mapped with differential compaction in magnetic soils or with 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 auroral activity before the [[light]] from the [[aurora]] becomes visible. A grid of magnetometers around the world constantly measures the effect of the solar wind on the Earth's magnetic field, which is then published on the [[K-index]].<ref>{{cite web |date=1 October 2007 |title=The K-index |publisher=[[Space Weather Prediction Center]] |url=http://www.swpc.noaa.gov/info/Kindex.html |access-date=2009-10-21 |url-status=dead |archive-url=https://web.archive.org/web/20131022183358/http://www.swpc.noaa.gov/info/Kindex.html |archive-date=22 October 2013 }}</ref>

===Coal exploration===
While magnetometers can be used to help map basin shape at a regional scale, they are more commonly used to map hazards to coal mining, such as basaltic intrusions ([[dike (geology)|dykes]], [[sill (geology)|sills]], and [[volcanic plug]]) that destroy resources and are dangerous to longwall mining equipment. Magnetometers can also locate zones ignited by lightning and map [[siderite]] (an impurity in coal).

The best survey results are achieved on the ground in high-resolution surveys (with approximately 10&nbsp;m line spacing and 0.5 m station spacing). Bore-hole magnetometers using a Ferret{{clarify|What is a Ferret in this context and how is it used?|date=January 2023}}can also assist when coal seams are deep, by using multiple sills or looking beneath surface basalt flows.{{citation needed|date=June 2011}}

Modern surveys generally use magnetometers with [[Global Positioning System|GPS]] technology to automatically record the magnetic field and their location. The data set is then corrected with data from a second magnetometer (the base station) that is left stationary and records the change in the Earth's magnetic field during the survey.<ref>{{cite report |author=Abraham, Jared D. |display-authors=etal |date=April 2008 |title=Aeromagnetic Survey in Afghanistan: A Website for Distribution of Data |url=http://pubs.usgs.gov/of/2007/1247/html/afghan_dataproc.html |publisher=United States Geological Survey |docket=OF 07-1247 |access-date=25 August 2011 |archive-date=26 October 2011 |archive-url=https://web.archive.org/web/20111026023808/http://pubs.usgs.gov/of/2007/1247/html/afghan_dataproc.html |url-status=dead }}</ref>

===Directional drilling===
Magnetometers are used in [[directional drilling]] for oil or gas to detect the [[azimuth]] of the drilling tools near the drill.<ref name=":0">{{Cite web |title=GMW Associates - Oil & Gas |url=https://gmw.com/industry-application/geophysics-oil-gas/ |access-date=2022-03-16 |website=GMW Associates |language=en-US}}</ref> They are most often paired with [[accelerometer]]s in drilling tools so that both the [[inclination]] and azimuth of the drill can be found.<ref name=":0" />

===Military===
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 [[Alfa-class submarine|Alfa-class]] titanium submarines were designed and built at great expense to thwart such systems (as pure titanium is non-magnetic).<ref>{{cite news|title=The application of titanium Navy|url=http://www.free-press-release.com/news-the-application-of-titanium-navy-1284608253.html|access-date=9 December 2013|newspaper=Free press release|date=15 September 2010}}</ref>

Military submarines are [[degaussing|degaussed]]—by passing through large underwater loops at regular intervals—to help them escape detection by sea-floor monitoring systems, [[magnetic anomaly detector]]s, and magnetically-triggered mines. However, submarines are never completely de-magnetised. It is possible to tell the depth at which a submarine has been by measuring its magnetic field, which is distorted as the pressure distorts the hull and hence the field. Heating can also change the magnetization of steel.{{clarify|date=June 2011}}

Submarines tow long sonar arrays to detect ships, and 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 Foerster are used to locate ferrous ordnance. Caesium and Overhauser magnetometers are used to locate and help clean up old bombing and test ranges.

UAV payloads also include magnetometers for a range of defensive and offensive tasks.{{Example needed|date=June 2011}}

===Mineral exploration===
{{Main|Exploration geophysics}}
[[File:VHFNV.JPG|thumb|A [[Diamond DA42]] [[light aircraft]], modified for aerial survey with a nose-mounted boom containing a magnetometer at its tip]]
Magnetometric surveys can be useful in defining magnetic anomalies which represent ore (direct detection), or in some cases gangue minerals associated with ore deposits (indirect or inferential detection). This includes [[iron ore]], [[magnetite]], [[hematite]], and often [[pyrrhotite]].

Developed countries such as Australia, Canada and USA invest heavily in systematic airborne magnetic surveys of their respective continents and surrounding oceans, to assist with map geology and in the discovery of mineral deposits. Such aeromag surveys are typically undertaken with 400&nbsp;m line spacing at 100&nbsp;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 then averaged. Such data is gridded to an 80&nbsp;m × 80&nbsp;m pixel size and 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&nbsp;m line spacing and 50&nbsp;m elevation (terrain permitting). Such an image is gridded on a 10 x 10&nbsp;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&nbsp;m to 50&nbsp;m line spacing with 1&nbsp;m station spacing to provide the best detail (2 to 10&nbsp;m pixel grid) (or 25 times the resolution prior to drilling).

Magnetic fields from magnetic bodies of ore 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 a distance of 400&nbsp;m one sees one glowing haze, but as it approaches, two headlights, and then the left blinker, are visible.

There are many challenges interpreting magnetic data for mineral exploration. Multiple targets mix together like multiple heat sources and, unlike light, there is no magnetic telescope to focus fields. The combination of multiple sources is measured at the surface. The geometry, depth, or magnetisation direction (remanence) of the targets are also generally not known, and so multiple models can explain the data.

Potent by Geophysical Software Solutions [https://web.archive.org/web/20131213095908/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 pipe]]s) and, more commonly, indirectly, such as by mapping geological structures conducive to mineralisation (i.e., shear zones and alteration haloes around granites).

Airborne Magnetometers detect the change in the Earth's magnetic field using sensors attached to the aircraft in the form of a "stinger" or by towing a magnetometer on the end of a cable. The magnetometer on a cable is often referred to as a "bomb" because of its shape. Others call it a "bird".

Because hills and valleys under the aircraft make the magnetic readings rise and fall, a radar altimeter keeps track of the transducer's deviation from the nominal altitude above ground. There may also be a camera that takes photos of the ground. The location of the measurement is determined by also recording a GPS.

===Mobile phones===
[[File:Motorola Xoom - AKM Semiconductor AKM8975-1693.jpg|thumb|Tri-axis Electronic Magnetometer by [[AKM Semiconductor, Inc.|AKM Semiconductor]], inside [[Motorola Xoom]]]]

Many smartphones contain miniaturized [[microelectromechanical systems]] (MEMS) magnetometers which are used to detect magnetic field strength and are used as [[compass]]es. The iPhone 3GS has a magnetometer, a magnetoresistive permalloy sensor, the AN-203 produced by Honeywell.<ref>{{cite book|last=Allan|first=Alasdair|chapter=5. Using the magnetometer |title=Basic sensors in iOS|date=2011|publisher=O'Reilly|location=Sebastopol, CA|isbn=978-1-4493-1542-9|pages=57–70|edition=1st}}</ref> In 2009, the price of three-axis magnetometers dipped below US$1 per device and dropped rapidly. The use of a three-axis device means that it is not sensitive to the way it is held in orientation or elevation. Hall effect devices are also popular.<ref>{{citation|title=A Compass in Every Smartphone|url=https://spectrum.ieee.org/a-compass-in-every-smartphone|journal=IEEE Spectrum |author=Willie D. Jones|date=Feb 2010|access-date=21 October 2012}}</ref>

Researchers at [[Deutsche Telekom]] have used magnetometers embedded in mobile devices to permit touchless [[3D 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.<ref>[http://portal.acm.org/citation.cfm?id=1720048 MagiTact]. Portal.acm.org. Retrieved on 23 March 2011.</ref>

===Oil exploration===
[[Seismic]] methods are preferred to magnetometers as the primary survey method for oil exploration although magnetic methods can give additional information about the underlying geology and in some environments evidence of leakage from traps.<ref>{{cite web|url=http://www.paper.edu.cn/scholar/downpaper/liuqingsheng-6 |archive-url=https://web.archive.org/web/20180911045038/http://www.paper.edu.cn/scholar/downpaper/liuqingsheng-6 |url-status=dead |archive-date=2018-09-11 |title=中国科技论文在线 }}</ref> Magnetometers are also used in oil exploration to show locations of geologic features that make drilling impractical, and other features that give geophysicists a more complete picture of [[stratigraphy]].

===Spacecraft===
{{Main|Spacecraft magnetometer}}
A three-axis fluxgate magnetometer was part of the ''[[Mariner 2]]'' and ''[[Mariner 10]]'' missions.<ref name="Coleman et al.">{{cite journal |author1=Coleman Jr., P.J. |author2=Davis Jr., L. |author3=Smith, E.J. |author4=Sonett, C.P. |date=1962 |title=The Mission of Mariner II: Preliminary Observations&nbsp;– Interplanetary Magnetic Fields |journal=[[Science (journal)|Science]] |volume=138 |issue=3545 |pages=1099–1100 |jstor=1709490|bibcode=1962Sci...138.1099C |doi=10.1126/science.138.3545.1099 |pmid=17772967|s2cid=19708490 }}</ref> A dual technique magnetometer is part of the ''[[Cassini–Huygens]]'' mission to explore Saturn.<ref>{{cite web |url=http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instrumentscassinimag/ |title=Cassini Orbiter Instruments&nbsp;– MAG |publisher=[[Jet Propulsion Laboratory|JPL]]/[[NASA]] |url-status=dead |archive-url=https://web.archive.org/web/20140408045815/http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instrumentscassinimag/ |archive-date=8 April 2014 }}</ref> This system is composed of a vector helium and fluxgate magnetometers.<ref>{{cite journal |author1=Dougherty M.K. |author2=Kellock S. |author3=Southwood D.J. |display-authors=etal |date=2004 |title=The Cassini magnetic field investigation |journal=[[Space Science Reviews]] |volume=114 |issue=1–4 |pages=331–383 |doi=10.1007/s11214-004-1432-2 |bibcode=2004SSRv..114..331D |s2cid=3035894 |url=http://lasp.colorado.edu/~horanyi/graduate_seminar/Magnetometer.pdf |access-date=1 November 2017 |archive-date=10 August 2017 |archive-url=https://web.archive.org/web/20170810142403/http://lasp.colorado.edu/~horanyi/graduate_seminar/Magnetometer.pdf |url-status=dead }}</ref> Magnetometers were also a component instrument on the Mercury ''[[MESSENGER]]'' mission. A magnetometer can also be used by satellites like [[Geostationary Operational Environmental Satellite|GOES]] to measure both the [[magnitude (mathematics)|magnitude]] and [[Direction (geometry, geography)|direction]] of the magnetic field of a planet or moon.

===Magnetic surveys===
[[File:Ground surveying in Surprise Valley, California.jpg|thumb|right|Ground surveying in Surprise Valley, Cedarville, California]]
Systematic surveys can be used to in searching for mineral deposits or locating lost objects. Such surveys are divided into:
*[[Aeromagnetic survey]]
*Borehole
*Ground
*Marine

Aeromag datasets for Australia can be downloaded from the [https://web.archive.org/web/20110601172707/http://www.geoscience.gov.au/bin/mapserv36?map=%2Fpublic%2Fhttp%2Fwww%2Fgeoportal%2Fgadds%2Fgadds.map&mode=browse GADDS database].

Data can be divided in point located and image data, the latter of which is in ERMapper format.

====Magnetovision====
On the base of space measured distribution of magnetic field parameters (e.g. amplitude or direction), the [[magnetovision]] images may be generated. Such presentation of magnetic data is very useful for further analyse and [[data fusion]].

====Gradiometer====
Magnetic [[gradiometer]]s are pairs of magnetometers with their sensors separated, usually horizontally, by a fixed distance. The readings are subtracted to measure the difference between the sensed magnetic fields, which gives 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, thus allowing for more sensitive detection of anomalies. Because nearly equal values are being subtracted, the noise performance requirements for the magnetometers is more extreme.

Gradiometers enhance shallow magnetic anomalies and are thus good for archaeological and site investigation work. They are also good for real-time work such as [[unexploded ordnance]] (UXO) location. 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 as Siledus.

Non-magnetic electronic hipchain triggers were developed to trigger magnetometers. They 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 from noise coming from a range of sources. Different magnetometer technologies suffer different kinds of noise problems.

Heading errors are one group of noise. They can come from three sources:
*Sensor
*Console
*Operator

Some total field sensors give different readings depending on their orientation. Magnetic materials in the sensor itself 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 LCDs, 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 carried during a survey. [[Akubra]] hats are very popular in Australia, but their steel rims must be removed before use on magnetic surveys. Steel rings on notepads, steel capped boots and 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 pattern]]s in survey images. Alternate lines can also be corrugated.

====Image processing of magnetic data====
Recording data and image processing is 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.

===Aircraft navigation===
The Magnetometer Navigation (MAGNAV) algorithm was initially running as a flight experiment in 2004.<ref>{{cite book |author1=Julie Thienel |author2=Rick Harman |author3=Itzhack Bar-Itzhack |title=AIAA/AAS Astrodynamics Specialist Conference and Exhibit |chapter=Results of the Magnetometer Navigation (MAGNAV) Inflight Experiment |publisher=Research Gate |year=2004 |doi=10.2514/6.2004-4749 |isbn=978-1-62410-075-8 }}</ref> Later on, diamond magnetometers were developed by the [[United States Air Force Research Laboratory]] (AFRL) as a better method of navigation which cannot be jammed by the enemy.<ref>{{cite news |url=https://www.economist.com/science-and-technology/2020/07/18/magnetometers-based-on-diamonds-will-make-navigation-easier |title=Magnetometers based on diamonds will make navigation easier |newspaper=The Economist |date=18 July 2020 }}</ref>