The Earth is a large magnet and, as such, has a magnetic field associated with it. Geomagnetism is the branch of Physics that deals with the study of this magnetic field, both its generation and its spatial and temporal variation. Its existence is very important since it protects us from space radiation because charged particles (protons, electrons...) are trapped in the magnetic field lines.

The observed and measured Earth's magnetic field has two different origins:

• The internal field, which represents 90%, comes from the movement of the external core, an internal layer rich in iron and nickel in a fluid state (geodynamo mode). This field has a lot of intensity, but it is not constant, it varies very slowly over time, it is called secular variation, and it is responsible for the drift of the magnetic poles over the years. 
   
• The external field comes mainly from solar activity, and more specifically, from the interaction of the layers of the atmosphere (magnetosphere) with the solar wind. Their intensity is much weaker but presents interesting consequences since the measurement and analysis of its disturbances on the Earth's crust allows scientists to locate and study some subsoil geological structures. Furthermore, this fact causes the phenomenon of the northern and southern lights, which we all know.

Imatge

Geoinformation from the Cartographic and Geological Institute of Catalonia (ICGC) subject to a Creative Commons Attribution 4.0 International license.     More information

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Aeromagnetism

Scale	Title	Edition	Year	Download    GeoPDF	e-Shop    printed	Miniature
1:250.000	Mapa aeromagnètic de Catalunya	1	1989	6,5 MB includes report (Catalan)	-	Imatge

The ICGC has acquired measurements of Earth’s magnetic field variations, called magnetic anomalies, which represent variations in the physical properties of rocks such as magnetic susceptibility and/or remaining magnetization, and have the ability to detect areas with ferromagnetic mineral deposits (magnetite, hematite and pyrrhotite) or to represent volcanic or intrusive rock structures. Data were initially acquired on 22,000 km of flight lines and were published in the Aeromagnetic map of Catalonia 1:250,000 in 1989. The distance between two measurement lines was set at one line for each centimeter of the projected map, giving rise to one line every 2,5 km. The resolved geological structures have, therefore, a minimum lateral dimension of 5 km.

In the Catalunya aeromagnetic flight, the ICGC used a precision proton magnetometer owned by the Institute of Geophysics of the Polytechnic School of Zurich (Switzerland). This instrument measures the total field intensity by precisely measuring the spin axis frequency of many ions in the vicinity of the direction of the Earth's magnetic field. One measurement was made per second, with a precision of 0.25nT which, considering the flight characteristics, became a measurement every 60 m. The flights altitude is divided into two zones: a first region, in the southern part of Catalunya, with a moderate topography, where the flight was carried out at 2,500 m above sea level; and a second region, in the Pyrenees, where the flight was carried out at 3,700 m above sea level.

The obtained data (more than 450,000) were interpolated into a regular 250x250 m mesh, from which the linear regional field was removed to obtain the residual magnetic anomalies map, truly valuable information to know the internal properties of our territory.