When a ferrous material is placed within the earth's magnetic field, it develops an induced magnetic field. The induced field is superimposed on the earth's field at that location, creating a magnetic anomaly.
Using the state of the art geomagnetic instruments, we measure the total magnetic field, the vertical gradient of the 3 components of the magnetic field vector. These measurements can be done from the surface and in boreholes down to 2000m. The visualisation and interpretation is done in 2D and 3D.