Proton magnetometer

It is used in land-based archaeology to map the positions of demolished walls and buildings, and at sea to locate wrecked ships, sometimes for recreational diving.

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.

[1] 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.

Black and Eli Lilly, following the work of Martin Aitken and his associates at the Oxford 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.