The magnetic method, or magnetometry, utilises the Earth's magnetic field, and is therefore simple and inexpensive to use. These properties, on the other hand, do put certain limitations on the usefulness of the method. There is no possibility, for example, to modify the primary field such as one can do with other measuring methods, e.g., the electromagnetic or seismic methods.
In general terms, the Earth's magnetic field can be described as a N-S oriented bar-magnet within the innermost core of the planet. This normal magnetic field changes slowly with time and today (2009) the magnetic north pole is situated in Canada. Recent research has shown that this magnetic north pole is gradually moving across to Siberia.
The compass
The compass, one of the earliest geophysical instruments, was first used in China about 900 years ago and reached Europe about a century later. A compass consists quite simply of a small piece of magnetic rock or mineral hanging by a thin thread, such that it can rotate about a vertical axis. The magnetic rock will then orient itself north-south and the compass can then be used for navigation.
In a manuscript which is usually entitled "Epistola de magnete" (Letter on the magnet), written in 1269 by the Italian military engineer Petrus Peregrini, the author discussed the theory of magnetism and associated experiments, as well as important related terms such as magnetic poles and their forces of attraction and repulsion. Peregrini considered that the compass needle pointed towards the polar star whereas the general belief at the time was that there must be a mountain of magnetite at the north pole. In a book published in London in 1600, entitled "De magnete" (On the magnet) (but the original title is much longer), and written by the Englishman William Gilbert, the main conclusion states – the Earth is one big magnet!
The magnetic method
Rocks and ores are magnetised to a greater or lesser degree by the Earth's magnetic field, and thus show disturbances, or anomalies, in this field. The magnetic method is based on the measurement and interpretation of these departures from the norm. Since the method is so cheap and simple it is much used, especially in prospecting. In the early years it was used almost exclusively in connection with prospecting for strongly magnetic iron ore deposits.
After it became possible to carry out magnetic measurements from aeroplane and helicopter, the method has become an effective and useful aid in regional bedrock mapping and for prospecting for ore deposits and petroleum plays.
During the first airborne magnetic mapping of the Norwegian continental shelf in the 1960s and 1970s, geophysicists were interested principally in depths to magnetic basement and the occurrence of volcanic rocks. By the 1990s, the sensitivity of the magnetometers and precision of navigation were considerably improved and NGU embarked on a programme of aeromagnetic remapping of the continental shelf. It was then possible to detect and map structures in the various sedimentary basins such as, e.g., faults, sand channels and shallow salt diapirs.
Aeromagnetic acquisition with a towed system.
Departures from the normal magnetic field
The normal magnetic field can provide useful information on the Earth as a whole, but for geoscientific interpretation we are particularly interested in the anomaly field. The anomaly field records and describes the departures from the normal field caused by variations in magnetisation in the Earth's crust.
Magnetic anomalies record variations in the magnetic field principally from two sources: (1) induced field and (2) remanent field. The induced magnetic field is a product of the intensity of the geomagnetic field and the magnetic susceptibility in the bedrock. Magnetic susceptibility is a physical parameter which reflects the material properties of the different magnetic minerals (e.g., magnetite and pyrrhotite). Remanent magnetisation is a property which produces a magnetic field without a geomagnetic (extra-) field.