They read the magnetic field directions recorded in rocks and sediments !

Luigi Jovane , university of Sao Paolo Brazil and Luca Lanci, university of Urbino Italy, are  paleomagnetists.

field can change strength and location
periodically. The strength and the direction are recorded  by magnetic
particules in the sediments and rocks at the time that they have been
deposited or formed. The magnetometer allows to “read” the magnetism and
give informations on the magnetic orientation  of the core’s layers.

Pour les élèves de 1ère S tout cela doit maintenant vous parler. Car oui ! ce sont eux qui produisent et déchiffrent les fameuses “peaux de zèbre”!! les code-barres qui vous semblent si obscures.