Proton magnetometer
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.
They are used in land-based archaeology to map the positions of demolished walls and buildings and at sea to locate wrecked ships for recreational diving.
PPM's where once widely used in mineral explroation. They have largely been superceded by Overhauser Effect magnetometer and Alkali Vapour (Cesium Helium & Potassum) magnetometers which are faster samping and more sensitive.
The principle of operation is outlined in the magnetometer article. Other applications of PPM's are also outlined in this article.
Proton magnetometer in Archaeology
According to Christopher S. Peebles (Glenn A. Blaclc 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 from previous magnetometers for locating very large features, but it 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".
Further reading
- 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.