The earth is the source of, and is consequently enveloped in, a weak magnetic field which varies in intensity with time and location. The presence of ferrous metal disturbs this ambient magnetic field in a mathematically predictable manner, and measurement of this ambient magnetic field can be useful in locating and characterizing buried metallic objects. Near-surface magnetic objects, such as tanks, drums, and pipelines, result in high vertical gradients in the local magnetic field intensity.

Measurement of the vertical magnetic gradient, called magnetic gradiometry, is particularly useful for locating shallow metallic objects. The total magnetic field is measured for deeper applications such as iron ore deposits or groundwater basin studies.