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.

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.

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 They have largely been superseded by Overhauser Effect magnetometers and alkali vapour (cesium, rubidium, potassium) or helium magnetometers, which sample faster and are more sensitive.