A proton magnetometer, also known as a proton precession magnetometer (PPM), uses the principle of Earth's field nuclear magnetic resonance (EFNMR) to measure very small variations in the

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...

, allowing ferrous objects on land and at

sea The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...

to be detected. It is used in land-based

archaeology Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landsc ...

to map the positions of demolished walls and buildings, and at sea to locate wrecked ships, sometimes for recreational diving. PPMs were once widely used in mineral exploration. They have largely been superseded by Overhauser effect magnetometers and alkali vapour ( cesium, rubidium, and

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

) or helium magnetometers, which sample faster and are more sensitive.

Principles of operation

A direct current flowing in a

solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whose ...

creates a strong magnetic field around a

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

-rich fluid ( kerosine and decane are popular; water can also be used), causing some of the protons to align with that field. The current is then interrupted, and as protons realign themselves with the 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⁻¹. Because the precession frequency depends only on atomic constants and the strength of the ambient magnetic field, the accuracy of this type of

magnetometer A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...

can reach 1 ppm. The frequency of Earth's field NMR for protons varies between approximately 900 Hz near the equator to 4.2 kHz near the

geomagnetic pole The geomagnetic poles are antipodal points where the axis of a best-fitting dipole intersects the surface of Earth. This ''theoretical'' dipole is equivalent to a powerful bar magnet at the center of Earth, and comes closer than any other po ...

s. These magnetometers can be moderately sensitive if several tens of watts are available to power the aligning process. If measurements are taken once per second, standard deviations in the readings is in the 0.01 nT to 0.1 nT range, and 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 approximately 10 meter increments. The 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 the instruments, as well as rotation of the sensor as a measurement is taken. Portable instruments are also limited by sensor volume (weight) and power consumption. PPMs work in field gradients up to 3,000 nT m⁻¹ 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 10,000 nT m⁻¹ and Caesium magnetometers can handle 30,000 nT m⁻¹.

Proton magnetometer in archaeology

In 1958 Glenn A. Black and Eli Lilly, following the work of

Martin Aitken Martin Jim Aitken FRS (11 March 1922 – 13 June 2017) was a British archaeometrist. Aitken was born in Stamford, Lincolnshire, and studied physics at Wadham College, Oxford. He was a fellow of Linacre College, Oxford. He was Professor of Arch ...

and his associates at the

Oxford University Oxford () is a city in England. It is the county town and only city of Oxfordshire. In 2020, its population was estimated at 151,584. It is north-west of London, south-east of Birmingham and north-east of Bristol. The city is home to the ...

(UK) Archaeometric Laboratory, used proton magnetometers to locate and map buried archaeological features, including iron objects in the soil,

thermoremanent magnetization When an igneous rock cools, it acquires a thermoremanent magnetization (TRM) from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature (see isothermal remanence). This remanence can also be ver ...

of fired clays, and differences in the magnetic susceptibility of disturbed soils. During 1961–1963, they surveyed more than of the Angel Mounds State Historic Site in

Indiana Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th ...

and excavated more than to match anomalous magnetometer readings with the archaeological features that produced them. This was the first systematic use of a proton magnetometer for archaeological research in North America.

Principles of operation

Proton magnetometer in archaeology

See also

References

Further reading