The existence of natural magnetic fields in the audio and subaudio frequency range has been known for some time. The primary source of energy for these fields is usually considered to be distant and local thunderstorms. Because of this origin, the fields are quasi-random with both amplitudes and directions changing drastically over short periods of time. Hence, use of these fields in geophysical prospecting has been extremely limited. A new development, AFMAG, however, essentially eliminates the time variance in recording these fields without any sacrifice of the intelligence of their space variance. Since the space variance can be correlated with geologic features, AFMAG provides a new method of exploration with particular application to prospecting for conductive mineral deposits. Instrumentation of the AFMAG method currently is available for both ground and airborne operation; the tilt of the plane of polarization of the natural magnetic fields is recorded simultaneously at two frequencies. Examples drawn from airborne and ground surveys show that the method has a much greater depth of exploration than its conventional cousin, the induction electromagnetic method. Numerous other advantages, such as the possibility of choosing discrete operating frequencies over a broad band from 1 cps to 1,000 cps, are discussed. The chief disadvantage of the method lies in a sometimes restricted daily measuring period during which the fields are of an amplitude too low to permit measurement with current instrumentation; this is not a serious problem and is being minimized as the technology improves.

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