After spending endless hours of fun tramping across the countryside trying to work out exactly where underground workings were in relation to the surface, the Club's electronics expert developed his through-rock locator beacon system, to help pin-point the cavities.
Based on an induction loop transmitter (placed underground) and a receiver (moved about on the surface), working at a frequency of 3kHz, the system can fairly accurately relate underground positions to the surface by exploring for 'nulls' in the signal.
The latest version uses a bicycle wheel wound with 500 turns of wire for the receiver aerial, which is moved about on the surface to detect the 'nulls'. Once the spot on the surface vertically above the transmitter is located, by moving some distance away and noting the angle to the tranmitter signal the depth can be estimated (with a cunning bit of maths).
Underground 'the Wand' (the transmitter in the form of a sealed cylinder, with a wire-wound ferrite core inside) is hung from a light-weight tripod to ensure that it is vertical, then switched on. The operator on the surface can then start walking around to detect the signal - it is possible to detect it through several hundred metres of rock.
The receiver and loop aerial in use on the surface
The 'Wand' being set-up underground