(af mag’) Audio-Frequency MAGnetic technique, the use of natural electromagnetic noise in the audio-frequency range to study lateral changes in earth resistivity. The quantities measured are the azimuth and inclination of the dip of the major axes of the ellipsoid of polarization. See polarization ellipse. Employs frequencies in the range of 1 to 1000 Hz, usually 100–500 Hz. Used in mineral prospecting and mapping faults and shear zones, especially in rugged terrain and heavy vegetation. See sferics and Ward et al. (1968).