The proton magnetometer, also known as the 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, allowing ferrous objects on land and at sea to be detected.
PPMs were once widely used in mineral exploration. They have largely been superseded by Overhauser effect magnetometers and alkali vapour (cesium, rubidium, potassium) or helium magnetometers, which sample faster and are more sensitive.
Principles of operation
A direct current flowing in a solenoid creates a strong magnetic field around a hydrogen-rich fluid (kerosine, and decane is popular, and 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−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 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 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 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 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
According to Christopher S. Peebles (Glenn A. Black Laboratory of Archaeology, Indiana University, Bloomington, Geophysics at the Angel Site: Past, Present, and Future) in 1958 Black and Mr. Eli Lilly, following the work of M. J. Aitken and his associates at the Oxford University (UK) Archaeometric Laboratory, used proton magnetometers to locate and map buried archaeological features. Magnetometers measure minute differences in the magnetic field of the earth caused by ferrous concentrations (and iron objects) in the soil, thermo-remnant magnetism of fired clays, and differences in the magnetic susceptibility of disturbed soils. In 1959, with the support of the Indiana Historical Society, Black rented a general purpose magnetometer. It proved a bit better than previous magnetometers for locating very large features, but was not really suited to archaeological work. In 1959, with the support of the Society, Black purchased a magnetometer built specifically for archaeological work from the Oxford Archaeometric Laboratory".
"In 1961 and in 1963 the National Science Foundation awarded funds to Black and to the Indiana Historical Society to evaluate the magnetometer as a tool for archaeological research. During 1961-1963, more than 100,000 square feet (9,300 m2) of the Angel site were surveyed and more than 7,000 square feet (650 m2) were excavated, in order to match anomalous magnetometer readings with the archaeological features that produced those readings. For the most part the strategy was to locate and follow the course of the stockade trench, but other features, such as houses, fire basins, and pits, were sought and located as well. This work, which was the first systematic use of a proton magnetometer for archaeological research in North America, was reported by both Johnston and by Black".
- Black, G. A. and Johnston, R. B., "A Test of Magnetometry as an Aid to Archaeology," American Antiquity, Vol. 28, pp. 199-205, 1962.
- Black, G. A., Angel Site: An Archaeological Historical, and Ethnological Study, 2 vols., Indiana Historical Society, Indianapolis, 1967.
- Johnston, R. B., "Proton Magnetometry and its Application to Archaeology: An Evaluation at Angel Site," Indiana Historical Society, Prehistory Research Series, Vol. IV, No. II, 1962.
- Smekalova T. N., Voss O., Smekalov S. L. "Magnetic Surveying in Archaeology. More than 10 years of using the Overhauser GSM-19 gradiometer", Wormianum 2008.
- Dr. Ivan Hrvoic, Ph.D., P.Eng. "Requirements for obtaining high accuracy with proton magnetometers". GEM Systems Inc., 2010-01-11.