The magnetic method, or magnetometry, uses the Earth's magnetic field, and is therefore simple and inexpensive to use. However, there are limitations on the usefulness of the method. Clearly, it is impossible to modify the primary field, as is often is done with other measuring methods, e.g., the electromagnetic or seismic methods.
In general terms, the Earth's magnetic field functions like N-S oriented bar-magnet within the innermost core of the planet. This normal magnetic field changes slowly. Today the magnetic north pole is a point in northern Canada, but may be moving gradually towards Siberia.
The compass, one of the earliest geophysical instruments, was first used in China about 900 years ago, reaching Europe about a century later. Quite simply, a compass consists of a small piece of magnetic rock or mineral hanging by a thin thread, so 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 1269 manuscript entitled "Epistola de magnete" (Letter on the magnet), the author and Italian military engineer Petrus Peregrini examines the theory of magnetism and associated experiments, as well as important related themes such as magnetic poles and their forces of attraction and repulsion. Peregrini suggested 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 the conclusion of a book written in London in 1600, entitled "De magnete" (On the magnet), the Englishman William Gilbert writes – 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 irregularities, or anomalies, to the normal gravitational field. The magnetic method is based on the measurement and interpretation of the departure from the norm. Since the method is cheap and simple it is used frequently, especially in prospecting. In the early years it was used almost exclusively for identifying highly magnetic iron ore deposits.
When it became possible to carry out magnetic measurements from aircraft, the method became more useful in regional bedrock mapping and in prospecting for ore deposits and petroleum.
During the first airborne magnetic mapping of the Norwegian continental shelf in the 1960s and 1970s, geophysicists were interested principally in depth to magnetic bedrock and the magnetic behaviour of the igneous 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.
Although geophysical measurements of the gravitation field can provide useful information about the Earth’s structure, scientists are generally more interested in the gravity anomalies. The gravity anomaly describes the deviation from the normal gravity that is caused by different magnetisation in the subsurface geology.
Magnetic anomalies display 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 of the bedrock. Magnetic susceptibility is a key physical parameter which point to 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.