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.
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 machines, 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 surveys — 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
Magnetometers are also used to detect archaeological sites, shipwrecks 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.
Magnetometers can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the Earth's magnetic field, which is published on the K-index.[1]
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 siderite – 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.[2]
They are used in directional drilling for oil or gas to detect the azimuth of the drilling tools near the drill bit. They are most often paired up with accelerometers in drilling tools so that both the inclination and azimuth of the drill bit can be found.
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 detectors, 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 gyroscopes.
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 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.[3]
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 [1] 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).
Many smartphones contain magnetometers. There are compass apps which show direction.[4]:
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.[5]
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.
A three-axis fluxgate magnetometer was part of the Mariner 2 and Mariner 10 missions.[6] A dual technique magnetometer is part of the Cassini–Huygens mission to explore Saturn.[7] This system is composed of a vector helium and fluxgate magnetometers.[8] 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 be divided into two basic types:
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.
A vector 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.
Scalar 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 magnetic field induces a sine wave in a rotating coil. The amplitude of the signal is proportional to the strength of the field, provided it is uniform, and to the sine of the angle between the rotation axis of the coil and the field lines. This type of magnetometer is obsolete.
The most common magnetic sensing devices are solid-state Hall effect sensors. These sensors produce a voltage proportional to the applied magnetic field and also sense polarity.
They are used in applications where the magnetic field strength is relatively large — for example in Anti-lock braking system in cars to sense wheel rotation speed via slots in the wheel disks.
Proton precession magnetometers, also known as proton magnetometers, PPM's or simply mags, measure the resonance frequency of protons (hydrogen nuclei) in the magnetic field to be measured, due to nuclear magnetic resonance (NMR). Because the precession frequency depends only on atomic constants and the strength of the ambient magnetic field, the accuracy of this type of magnetometer is very good[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 thus 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 poles. 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.
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 effect can be exploited to significantly improve upon the proton precession magnetometer. Rather than aligning the protons 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.[9]
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.
A basic example of the workings of a magnetometer may be given by discussing the common optically pumped caesium vapor magnetometer which is a highly sensitive (300 fT/Hz0.5) and accurate device used in a wide range of applications. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.
The device broadly consists of a photon emitter containing a caesium light emitter or lamp, an absorption chamber containing caesium vapor and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.
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 use 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 use 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 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.
At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free (SERF) atomic magnetometers containing potassium, caesium or rubidium vapor operate similarly to the caesium magnetometers described above, yet can reach sensitivities lower than 1 fT/Hz0.5. The SERF magnetometers only operate in small magnetic fields. The Earth's field is about 50 µT; SERF magnetometers operate in fields less than 0.5 µT.
Large volume detectors have achieved a sensitivity of 200 aT/Hz0.5.[10] This technology has greater sensitivity per unit volume than SQUID detectors.[11] 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.[12] This would allow the magnetic measurement to be made in places where high electrical voltages exist.
SQUIDs, or superconducting quantum interference devices, measure extremely small magnetic fields; they are very sensitive vector magnetometers, with noise levels as low as 3 fT/Hz0.5 in commercial instruments and 0.4 fT/Hz0.5 in experimental devices. Many liquid-helium-cooled commercial SQUIDs achieve a flat noise spectrum from near DC (less than 1 Hz) to tens of kilohertz, making such devices ideal for time-domain biomagnetic signal measurements. SERF atomic magnetometer demonstrated in a laboratory so far reaches competitive noise floor but in relatively small frequency ranges.
SQUID magnetometers require cooling with liquid helium (4.2 K) or liquid nitrogen (77 K) to operate, hence the packaging requirements to use them are rather stringent both from a thermal-mechanical as well as magnetic standpoint. SQUID magnetometers are most commonly used to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively). Geophysical surveys use SQUIDS from time to time, but the logistics is much more complicated than coil-based magnetometers.
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
Aeromag datasets for Australia can be downloaded from GADDS database for free.
There is point located and image data. Image data is in ERMapper format.
See also Magnetic Surveying in Archaeology (book).
Magnetic gradiometers are pairs of magnetometers with their sensors separated by a fixed distance, usually horizontally. The readings are subtracted in order to measure the difference between the sensed magnetic fields, which measures the field gradients caused by magnetic anomalies. This is one way of compensating both for the variability in time of the Earth's magnetic field and for other sources of electromagnetic interference, allowing more sensitive detection of anomalies. Because nearly equal values are being subtracted, the noise performance requirements for the magnetometers is more extreme. For this reason, high performance magnetometers are the rule in this type of system.
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.
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.
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 :
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.
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.
In 1833, Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the Earth's magnetic field.[13] It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer). It consisted of a permanent bar magnet suspended horizontally from a gold fibre.[14] A magnetometer may also be called a gaussmeter.
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