Page "Geophysical survey (archaeology)" Paragraph 9

Magnetometers used in geophysical survey may use a single sensor to measure the total magnetic field strength, or may use two ( sometimes more ) spatially separated sensors to measure the gradient of the magnetic field ( the difference between the sensors ).

In most archaeological applications the latter ( gradiometer ) configuration is preferred because it provides better resolution of small, near-surface phenomena.

Magnetometers may also use a variety of different sensor types.

Proton precession magnetometers have largely been superseded by faster and more sensitive fluxgate and cesium instruments.