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.

Abstract: In seismic prospecting a local response is achieved by striking a hammer on the ground, and the recording apparatus is arranged to be triggered by the hammer to run from a short time before the impulse to a predetermined time after it. The impulse is recorded into one channel and the reflections into another, and the latter are slowed down by successive recordings to a speed where they can be mechanically plotted. The impulse signal in the other channel is used as a timing and control signal for the re-recording and playbacks.