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.

The principle of operation is outlined in the magnetometer article.

Archaeologists, treasure hunters and skin divers have a common problem, where to dig or dive to discover the loot. A buried metal locator would come in handy, but most are either expensive or Have too limited a range. The one described here is a proton magnetometer using discoveries made in nuclear magnetic resonance first published in 1946. Some care is required in putting together the high gain amplifier, but otherwise construction is straightforward, using standard Components, and the exotic nuclear material is distilled water. Essentially, the magnetometer measures the intensity of the earth’s magnetic field at two nearby points. A difference in the
intensities produces an output from the device, which can be either an audio signal or a meter reading. The earth’s field is normally uniform. but will be disturbed by local concentrations of magnetic materials, such as iron ore or just iron junk. Hence the magnetometer can only be used to search for ferrous materials or compounds. For this purpose it is extremely sensitive with considerable range. It will, under ideal conditions, detect a one pound mass of iron at about four or five feet below one of the bottles, and larger masses at much greater distances. Typical of the latter is a twelve foot length of three inch diameter iron water pipe at twelve feet. It is difficult to give performance figures, since much depends upon the size, density and attitude of the object disturbing the field, and experiment provides the best answers. This high sensitivity to field variations means that the magnetometer may only be used remote from known earth field disturbers, such as buildings and power lines.