Magnetic gradiometer used in a geomagnetic mapping survey

Magnetic anomaly map on a waste dump site

Earth’s magnetic field is the magnetic field that extends from the Earth’s interior. It can be approximately described as the field of a magnetic dipole tilted at an angle of about 10 degrees with respect to Earth’s rotational axis. The Earth’s magnetic field changes over time because it is generated by the motion of molten iron alloys in its outer core. Many rocks and minerals are weakly magnetic or are magnetized by induction in the Earth’s field and cause spatial perturbations or “anomalies” in the Earth’s main field. Man-made objects containing iron or steel are often highly magnetized and locally can cause large anomalies up to several thousands of nT. Magnetic methods are generally used to map the location and size of ferrous objects. Usually, ground magnetic measurements are made with portable instruments at regular intervals along more or less straight and parallel lines that cover the survey area (anomaly mapping). Some other parameters (for example the anisotropy of susceptibility) can be used as a deformation proxy in cohesive soils.