Local anomalies in the Earth’s magnetic field are caused by magnetic minerals. However, most magnetic surveying is used to locate ferrous man-made objects. Ferrous objects are inherently strongly magnetised and consequently produce large anomalies easily detectable with magnetometers, but less strongly magnetised objects such as brick also produce detectable anomalies.

High-resolution magnetic surveys are increasingly used in engineering applications for the detection of shallow man-made objects and structures. Mine shafts are commonly sensed using magnetic surveying as the brick lining and metal staging produces a significant anomaly.