Repeat stations of the magnetic field are sites which are occupied temporarily and where the intensity and direction of the Earth’s magnetic field are measured at regular intervals. These stations are also known as a magnetic repeat network.
Such networks were installed in France as early as the end of the nineteenth century with two aims in mind. The first aim was to map the spatial distribution of the magnetic field, mainly resulting from the magnetic properties of the Earth’s crust. The second objective was to record the variations over time of the magnetic field’s intensity, as well as its direction. Both types of variations are linked to the way the dynamo process in the Earth outer core operates over very long periods of time. Between 1921 and 1927, the aim of achieving better spatial resolution led researchers to measure coordinates in 1,328 stations, each station being at the centre of an average size of a square with sides 20 km long. Once the spatial resolution had been improved, it was possible to decrease the number of stations, and researchers are now able to follow changes in magnetic variation over a very long time scale.
This magnetic cartography, carried out under the supervision of the Institut de Physique du Globe de Paris, has been considerably improved by the general magnetic survey of France and of the continental shelf. Between 1964 and 1965 measurements were taken using aircraft over mainland France, in order to conduct the magnetic survey at a stable altitude of 3,000 m (9,000ft). This did not include the Alps which were flown over at an altitude of 5,000 m (15,000 ft). The trajectory lines were 10 km apart, and crossed by perpendicular lines 100 km apart (Le Borgne, Le Mouel, 1966). Although researchers have been carrying out higher resolution regional surveys since 1998, the map resulting from the 1964 survey is the only one that can be used at the scale of the whole mainland territory.
Although magnetic anomalies have a spatial distribution which you might see as a geophysical invariant at the scale of human time, iso-intensity maps showing the difference between true north and magnetic north (declination) and the vertical intensity and direction of Earth’s magnetic field (inclination) have to be regularly updated. Up until 2007, this update was carried out by re-occupying the network of 31 stations constructed between 1947 and 1977, at intervals of five years.
The new repetition network of magnetic stations.
This network became increasingly difficult to replicate for the following reasons:
For these reasons, it is decided for measurement campaign of 2012 to establish a new network of magnetic repeat stations with a double objective: to operate a reduced number of stations, and to maintain a stable, robust, network over a long time-period.
Modeling work showed that it is possible to follow the variation of the magnetic field over a very long time scale with a precision of approximately 1nT/year in mainland France and Corsica with only one observatory and 11 regularly distributed stations.
In order to ensure the durability of the stations, researchers decided to establish them in airports. All the selected aerodromes belong to categories B or C, and a possible closure would require reasonable advance notice.
This operation was supported by users of updated magnetic declination data: the French Civil Aviation Authority (Direction générale de l’Aviation civile or DGAC), and more particularly the Air Navigation Engineering Service (Service Technique de la Navigation Aérienne or STNA) as well as the airport operators.
During summer 2012, 11 stations were established and a series of measures carried out on the airports of Biarritz-Bayonne-Anglet, Brive - Dordogne valley, Cannes - Mandelieu, Caen - Carpiquet, La Roche sur Yon - Les Ajoncs, Nancy - Essay, Figari - South Corsica, Lyon - Bron, Morlaix - Plougean, Perpignan - Rivesaltes and Valenciennes - Denain.
A first series of measurements was carried out in 2012, then repeat measurements took place in 2013, 2015 and 2017.
Map of the repeat station network for metropolitan France