champ magnétique de fréquence audio

CONT: [...] la méthode AFMAG est basée sur les champs magnétiques de fréquence audio engendrés par des éclairs produits lors des orages électriques.

CONT: Among the natural field EM [electromagnetic] methods are the AFMAG (audio-frequency magnetic field), the AMT (audio-magnetotelluric), and the magnetotelluric methods, which make use of the electromagnetic energy in selected frequency ranges from natural sources such as lightning discharges and ionospheric oscillations.
CONT: The first ""passive transmitter"" AEM system, AFMAG was introduced in 1958. This system utilised a natural audiofrequency magnetic field arising from atmospheric discharges as the source of the primary field.