Aproton magnetometer, also known as aproton precession magnetometer (PPM), uses the principle ofEarth's field nuclear magnetic resonance (EFNMR) to measure very small variations in theEarth's magnetic field, allowingferrous objects on land and atsea to be detected.
It is used in land-basedarchaeology to map the positions of demolished walls and buildings, and at sea to locate wrecked ships, sometimes forrecreational diving.
PPMs were once widely used in mineral exploration. They have largely been superseded byOverhauser effect magnetometers and alkali vapour (caesium, rubidium, andpotassium) or helium magnetometers, which sample faster and are more sensitive.
A direct current flowing in asolenoid creates a strong magnetic field around ahydrogen-rich fluid (kerosine anddecane 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 theambient magnetic field, theyprecess 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 theproton 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 ofmagnetometer can reach 1ppm.[1]
The frequency of Earth's field NMR for protons varies between approximately 900 Hz near the equator to 4.2 kHz near thegeomagnetic poles. 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 mappingbanded 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.
In 1958 Glenn A. Black and Eli Lilly, following the work ofMartin Aitken and his associates at theOxford University (UK) Archaeometric Laboratory, used proton magnetometers to locate and map buried archaeological features, including iron objects in the soil,thermoremanent magnetization of fired clays, and differences in the magnetic susceptibility of disturbed soils. During 1961–1963, they surveyed more than 100,000 square feet (9,300 m2) of theAngel Mounds State Historic Site inIndiana and excavated more than 7,000 square feet (650 m2) 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.[2]