Magnetometer

A magnetometer, pronounced mag-ne-TOM-e-ter, is a scientific instrument used to measure the strength and/or direction of the magnetic field, produced either in the laboratory or existing in nature. Some countries such as the USA, Canada and Australia classify the more sensitive magnetometers as military technology, and control their distribution.

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, which is very weak, is measured in nano Tesla (nT). 1nT = 1 Amp / meter. See International_System_of_Units. The terrestrial field varies from around 20,000nT (equator) to 80,000nT (poles). There is a daily variation of around 30nT at mid latitudes and hundreds at the poles. Geomagnetic storms can cause much larger diurnal variations.

Electrical engineers measuring large magnetic fields in Gauss, where 1 Gauss = 100,000nT. The most sensitive magnetometers can measure relative field strengths to 1 pT = 0.001nT. The most sensitive absolute instruments (Overhausers) can measure to 0.01nT. The field strengths at the poles of small magnets are of the order of 10,000,000 nT.

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. Magneotmeters are capable of detecting (large objects like cars at tens of meters, while a metal detector's range is unlikely to exceed 2m.

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

Uses

Magnetometers can measure the magnetic fields of planets.

Magnetometers have a very diverse range of applications from locating submarines and Spanish Galleons, positioning weapons systems, detecting unexploded ordnance, 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, electromagnetitics (EM), Induced Poliarisation (IP), Magneto-Tellurics (MT), Controlled Source Magneto-Tellurics (CSAMT), Sub-audio Magnetics (SAM), Mise-a-la-Masse, Resistivity, Self Potential (SP) and Very Low Frequencey (VLF). See Exploration geophysics

Archaeology

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

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

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

Auroras

Magnetometers can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the Earth's magnetic field, which is published on the K-index.

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 siderite - an impurity in some coal.

The best survey results are achieved on the ground in high-resolution surveys (10m line spacing 0.5m 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.

Surveys use integrated GPS magnetometers such as the GSM-19F Overhauser and are diurnally corrected using a quality Overhauuser base station to correct for the unwanted natural daily fluctuations in the earth's magnetic field.

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

Directional drilling

They are used in directional drilling for oil or gas to detect the azimuth of the drilling tools near the drill bit. They are most often paired up with accelerometers in drilling tools so that both the inclination and azimuth of the drill bit can be found.

Military

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

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

On the defensive side, navies use arrays of magnetometers laid across strategic sea floors (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 such 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 (eg. 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, however they 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

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, diamonds worth trillions of dollars. They have played a part in you being able to read this electronic document.

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 400m line spacing at 100m 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 80m x 80m 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 50m line spacing and 50m elevation (terrain permitting) - the image would be gridded on a 10 x 10m pixel offering 64 times the resolution.

Where targets are shallow (<200m), aeromag anomalies may be followed up with ground magnetic surveys on 10m to 50m line spacing with 1m station spacing to give the best detail (2m 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 400m 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 [3] is a leading magnetic (and gravity) interpretation package used extensively in the Australian exploration industry.

Magnetometers assist mineral explorers both directly (ie. gold mineralisation associated with magnetite, diamonds in kimberlite pipes) and more commonly by indirect means such as mapping geological structures conducive to mineralisation (ie. shear zones and alteration haloes around granites).

See Ultramag Geophysics Pty Ltd [4] for a range of the latest ground based magnetometer technologies and mineral exploration case studies as well as comparisons of airborne, helicopter and ground magnetics.

Mobile telephones

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

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.

Providing they are non-magnetic, mobile phones such as the Apple iPhone can also be used as sophisticated data loggers and controllers for magnetometers. The internal magnetometers are not sensitive enough for mineral and coal exploration work. Ultramag Geophysics Pty Ltd has developed software running on the iPhone that talks to Overhauser and Fluxgate magnetometers and GPS simultaneously via wireless Bluetooth. The data is then processed and sent over the Next-G network in real-time to our office computers and forwarded to clients. When the network is unavailable the phone simply stores the data then bursts data when back online. This provides an extra level of safety for field crews and allows for rapid location and detailing of anomalies, real-time diurnal corrections and real-time transmission to the stock exchange (only joking).

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 2 and Mariner 10 missions. A dual technique magnetometer is part of the Cassini-Huygens mission to explore Saturn. This system is composed of a vector helium and fluxgate magnetometers. Magnetometers are also a component instrument on the Mercury MESSENGER mission. A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of a planet's or moon's magnetic field.

Further information: [[:Spacecraft magnetometer]]

Types

Magnetometers, depending upon type, can cost around $US 50,000, plus a similar amount for good processing software.

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), and DC magnetometers are used for detecting mineralization and corresponding geological structures.

Vector magnetometers

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

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.

Rotating coil magnetometer

The magnetic field induces a sine wave in a rotating coil. The amplitude of the signal is proportional to the strength of the field, provided it is uniform, and to the sine of the angle between the rotation axis of the coil and the field lines. This type of magnetometer is obsolete.

Hall effect magnetometer

NMR pocket monitoring device containing three Hall-effect sensors.

The most common magnetic sensing devices are solid-state Hall effect sensors. These sensors produce a voltage proportional to the applied magnetic field and also sense polarity.

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

Proton precession magnetometer

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

A direct current flowing in a solenoid creates a strong magnetic field around a hydrogen-rich fluid (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 alternating magnetic field that is picked up by a (sometimes separate) inductor, amplified electronically, and fed to a digital frequency counter whose output is typically scaled and displayed directly as field strength or output as digital data.

The relationship between the frequency of the induced current and the strength of the magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 Hz/nT. The sensitivity 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 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 caried units, PPM sample rates are typically limited to 3 seconds, and require the sensor to be stationary. They work in field gradients up to 3,000nT/m which is adequate from most mineral explroraiton work. For higher gradient tolerance such as mapping banded iron formations and detecting large ferous objects Overhausers can handle 10,000nT/m and Cesium magnetometers can handle 30,000nT/m.

The two main sources of measurement errors are magnetic impurities in the sensor, errors in the measurement of the frequency and ferous materil on the operator and in the instruments. Portable instruments are also limited by sensor volume (weight) and power consumption.

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 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 much faster sampling as the electron-proton coupling can happen even as measurements are being taken.

Fluxgate magnetometer

A uniaxial fluxgate magnetometer.

A fluxgate compass/inclinometer.

Basic principles of a fluxgate magnetometer.

A fluxgate magnetometer consists of a small, magnetically susceptible, core wrapped by two coils of wire. An alternating electrical current is passed through one coil, driving the core through an alternating cycle of magnetic saturation; i.e., magnetised, unmagnetised, inversely magnetised, unmagnetised, magnetised, etc. This constantly changing field induces an electrical current in the second coil, and this output current is measured by a detector. In a magnetically neutral background, the input and output currents will match. However, when the core is exposed to a background field, it will be more easily saturated in alignment with that field and less easily saturated in opposition to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which this is the case will depend on the strength of the background magnetic field. Often, the current in the output coil is integrated, yielding an output analog voltage, proportional to the magnetic field.

Fluxgate magnetometers, paired in a gradiometer configuration, are commonly used for archaeological prospecting and UXO detection such as the German 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.

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.

Caesium vapor magnetometer

A basic example of the workings of a magnetometer may be given by discussing the common optically pumped caesium vapor magnetometer which is a highly sensitive (300 fT/Hz^0.5) and accurate device used in a wide range of applications. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.

The device broadly consists of a photon emitter containing a caesium light emitter or lamp, an absorption chamber containing caesium vapor and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.

Polarization

The basic principle that allows the device to operate is the fact that a caesium atom can exist in any of nine energy levels, which is the placement of electron atomic orbitals around the atomic nucleus. When a caesium atom within the chamber encounters a photon from the lamp, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The caesium atom is 'sensitive' to the photons from the lamp in three of its nine energy states, and therefore eventually, assuming a closed system, all the atoms will fall into a state in which all the photons from the lamp will pass through unhindered and be measured by the photon detector. At this point the sample (or population) is said to be polarized and ready for measurement to take place. This process is done continuously during operation.

Detection

Given that this theoretically perfect magnetometer is now functional, it can now begin to make measurements.

In the most common type of caesium magnetometer, a very small AC magnetic field is applied to the cell. Since the difference in the energy levels of the electrons is determined by the external magnetic field, there is a frequency at which this small AC field will cause the electrons to change states. In this new state, the electron will once again be able to absorb a photon of light. This causes a signal on a photo detector that measures the light passing through the cell. The associated electronics uses this fact to create a signal exactly at the frequency which corresponds to the external field.

Another type of caesium magnetometer modulates the light applied to the cell. This is referred to as a Bell-Bloom magnetometer after the two scientists who first investigated the effect. If the light is turned on and off at the frequency corresponding to the Earth's field, there is a change in the signal seen at the photo detector. Again, the associated electronics uses this to create a signal exactly at the frequency which corresponds to the external field.

Both methods lead to high performance magnetometers.

Applications

The caesium magnetometer is typically used where a higher performance magnetometer than the proton magnetometer is needed. In archaeology and geophysics, where the sensor is moved through an area and many accurate magnetic field measurements are needed, the caesium magnetometer has advantages over the proton magnetometer.

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

The lower noise of the caesium magnetometer allows those measurements to more accurately show the variations in the field with position.

Spin-exchange relaxation-free (SERF) atomic magnetometers

At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free (SERF) atomic magnetometers containing potassium, caesium or rubidium vapor operate similarly to the caesium magnetometers described above yet can reach sensitivities lower than 1 fT/Hz^0.5.

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/Hz^0.5 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.

SQUID magnetometer

SQUIDs, or superconducting quantum interference devices, measure extremely small magnetic fields; they are very sensitive vector magnetometers, with noise levels as low as 3 fT/Hz^0.5 in commercial instruments and 0.4 fT/Hz^0.5 in experimental devices. 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.

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

• Airborne Aeromagnetic survey
• Borehole
• Ground
• Marine

Gradiometer

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

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 sumiltaneously (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 distace 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 techologies suffer different kinds of noise problems. Heading errors are one group of noise. They comprise three sources :

• Sensor
• Console
• Operator

Some sensors give different readings depending on their orientation. Cesium systems for instance have a 1nT variation over 180 degrees and are thus not well suited to surveying in thick bush.

Console noise comes from magnetic components on or within the console. These incluse ferrite in torrids in power supplies, steel frames around LCD's, legs on IC chips and steel cases in disposable batteries.

Operators must take care to be magnetically clean and should check the 'magnetic hygiene' of all apparel and items carries durring 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 unnecesary noise in surveys. Pens, mobile phones and stainless steel implants can also be problematic.

The magnetic response (noise) from ferrous object on the opertor 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 bu the operator(especially in magneticaly 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 Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper on measurement of the Earth's magnetic field. It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer). It consisted of a permanent bar magnet suspended horizontally from a gold fibre. A magnetometer may also be called a gaussmeter.

See also

• Vibrating sample magnetometer
• Magnetic anomaly detector
• Magnetotellurics
• Nuclear Magnetic Resonance NMR
• Earth's field NMR (EFNMR)
• Magnetic immunoassay
• Intermagnet is a global network of observatories, monitoring the Earth's magnetic field.
• 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 seen by earth-based and satellite magnetometers and other sensors. Earthquake prediction, preseismic, and Earth Models are common applications.
• Magnetograms – are often displayed as images on the web, but usually the digital data are also available.

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.Korepanov, A. Marusenkov (Lviv Centre of Institute for Space Research, Lviv, Ukraine), J.Rasson (Royal Meteorological Institute, Dourbes, Belgium)