Abstract: An underground electromagnetic radiating source is detected by two coils (1, 2) one over the other. To avoid false readings due to air borne signals, their outputs are compared (13) and only if the lower coil output is a predetermined amount greater than that of the upper coil is an audio indicator (21) activated. Automatic gain control (9) is applied to both coil outputs, to limit the gain whenever either output threatens saturation.

Abstract: Apparatus for detecting underground conductors carrying AC, such as cables or pipelines, has two receiver coils with horizontal axes one above the other, and other coils. By making phase comparisons from a selection of these the apparatus is located directly above the conductor. Then using the two horizontal coils there is derived a signal corresponding to the depth of the conductor, which can be visually indicated. Also, this depth signal, which is continuously available as the conductor is followed, is used to modify an output from one of the coils so that as the depth varies that output remains constant provided there are no discontinuities or faults in the cable.