Overview
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 Proton precession magnetometers have largely been superseded by faster and more sensitive fluxgate and cesium instruments.
Every kind of material has unique magnetic properties, even those that we do not think of as being "magnetic". Different materials below the ground can cause local disturbances in the Earth’s magnetic field that are detectable with sensitive magnetometers. The chief limitation of magnetometer survey is that subtle features of interest may be obscured by highly magnetic geologic or modern materials.
Read more about this topic: Magnetic Survey (archaeology)