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 the both the inclination and azimuth of the drill bit can be found.
Magnetometers are very sensitive, and 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.
The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined. Examples of vector magnetometers are fluxgates, superconducting quantum interference devices (SQUIDs), and the atomic SERF magnetometer. Some scalar magnetometers are discussed below.
A magnetograph is a special magnetometer that continuously records data.
One type of magnetometer is the proton precession magnetometer, also known as the proton magnetometer, which measures the resonance frequency of protons (hydrogen nuclei) in the magnetic field to be measured, due to Nuclear Magnetic Resonance (NMR).
A direct current flowing in an inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with that field. The current is then interrupted, and as protons are realigned with Earth's magnetic field they precess at a specific frequency. This produces a weak alternating magnetic field that is picked up by a (sometimes separate) inductor. The relationship between the frequency of the induced current and the strength of Earth's magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 hertz per nanotesla (Hz/nT).
Because the precession frequency depends only on atomic constants and the strength of the external magnetic field, the accuracy of this type of magnetometer is very good. Magnetic impurities in the sensor and errors in the measurement of the frequency are the two causes of errors in these magnetometers.
If several tens of watts are available to power the aligning process, these magnetometers can be moderately sensitive. Measuring once per second, standard deviations in the readings in the 0.01 nT to 0.1 nT range can be obtained.
The strength of the Earth's magnetic field varies with time and location, so that the frequency of Earth's field NMR (EFNMR) for protons varies between approximately 1.5 kHz near the equator to 2.5 kHz near the geomagnetic poles.
The measurement of the precession frequency of proton spins in a magnetic field can give the value of the field with high accuracy and is widely used for that purpose. In low fields, such as the Earth's magnetic field, the NMR signal is expected to be weak because the nuclear magnetization is small, but special devices can enhance the signal 100 or 1000 times. Incorporated in existing portable magnetometers, these devices make them capable of measuring fields to an absolute accuracy of about one part in 1,000,000 and detecting field variations of about 0.1nT. Apart from the direct measurement of the magnetic field on Earth or in space, these magnetometers prove to be useful whenever a phenomenon is linked with variations of magnetic field in space or in time, such as anomalies arising from submarines, skiers buried under snow, archaeological remains, and mineral deposits
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 (ie magnetised - unmagnetised - inversely magnetised - unmagnetised - magnetised). 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 prospection. In Britain the most common such instruments to be used are the Geoscan FM series of instruments and the Bartington GRAD601. Both are capable of resolving magnetic variations as weak as 0.1nT (roughly equivalent to one half-millionth of the earth's magnetic field strength).
The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This NMR effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field (i.e. generated from an RF source).
RF magnetic fields are ideal for use in magnetic devices because they are transparent to the Earth's DC magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. they do not contribute noise to the measuring system).
The unbound electrons in the special liquid transfer their excited state (i.e. energy) to the hydrogen nuclei (i.e. protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like a proton precession magnetometer – but with much less power and to much greater extent.
The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons. The constant of proportionality is known to a high degree of accuracy and is identical to the proton precession gyromagnetic constant. As with the proton magnetometer, magnetic impurities and inaccuracies in frequency measurement are two causes of error in the measurement. The Overhauser magnetometer may have an additional error because the frequency produced can be changed slightly by an interaction between the protons and the coil used to detect the magnetic field.
Overhauser magnetometers achieve some 0.01 nT/√Hz noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.
The device broadly consists of a photon emitter containing a caesium light emitter or lamp, an absorption chamber containing caesium vapour 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 photodetector 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 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 photodetector. 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 allow 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.
The SERF magnetometers only operate in small magnetic fields. The earth's field is about 0.5 Gauss. SERF magnetometers operate in fields less than 0.005 Gauss (5 miligauss)
As shown in large volume detectors have achieved 200 aT/√Hz 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 fiberoptic cables. 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·Hz−0.5 in commercial instruments and 0.4 fT·Hz−0.5 in experimental devices. Until the advent of SERF atomic magnetometers in 2002, this level of sensitivity was unreachable otherwise.
These 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 allow one to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively).
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