In a Magnetics survey, the Earth's magnetic field and the magnetic responses due to magnetic minerals are measured. Naturally magnetic minerals such as magnetite occur in rocks and in varying percentages. Other minerals have a high magnetic susceptibility resulting in induced fields. It is both the remnant and induced magnetic responses that are used to map an exploration area and calculate the susceptibility of rock types. Because of its speed, the ease of the physical measurement and its economy, magnetics is the most widely used and popular geophysical exploration method. From a detailed study of an anomaly, it is possible to calculate magnetic susceptibility, length, width, depth, dip, and the remnant magnetism of the causative body.

Ground magnetics is used for detailed work, occasionally for the location of airborne anomalies, and in areas where there is no suitably accurate and detailed airborne data, and where the area of interest is too small to justify mobilising an airborne crew. However, the availability of cheap and accurate GPS systems has allowed grids or cut lines to be avoided in many areas.

For ground magnetics, Fugro Ground Geophysics offers proton magnetometers (Envimag, G856) or caesium vapour magnetometers (SmartMag, G858). Each has their advantages and disadvantages, and which is applied depends on the local conditions and the objective of the survey.

With the proton magnetometer readings are usually taken every 5 or 10m along lines that are 50 or 100m apart. The sensor is mounted on a staff that is usually 3m above the ground to reduce magnetic noise due to laterite rubble near or on the surface. By putting the sensor on an extended pole the noise is decreased because its effect decreases with the inverse cube of distance.

With the caesium vapour magnetometer, the sensor is either about waist height or shoulder height and the caesium magnetometer samples continuously as the operator walks along the line.

Both magnetometers measure the total magnetic field and are operated with a base station magnetometer to enable accurate diurnal corrections.