THE TECHNICAL PAGES
The induction-loop

A device that allows you to locate, on the surface, underground points in a very accurate way. It also allows a depth-estimation. (author: Jan Geboers).

APPLICATIONS

a) To check or improve cave surveys, or to position them on surface maps.

b) Determine the distance between 2 caves or 2 parts of the same cave, without having to survey the caves: a radiolocation in each cave or gallery is enough, afterwards you only have to measure the distance between the two points that you located on the surface.

c) To find the right spot on the surface, where to make additional (artificial) entrances.

HOW TO USE

In fact, there are two devices (transmitter & receiver), that both are made out of a small box and a loop-antenna. These loops are +/- 40 cm in diameter and made out of cupper-wire.

- the transmitter is put into the cave. The loop-antenna is put in a perfectly horizontal position. It will emit a strong low-frequency magnetic field.

- the receiver is held on the surface. It's loop-antenna is held vertically and will pick up the underground signals.

By rotating the surface-loop around it's vertical axis, one can determine the direction of the waves of the magnetic field. If the loop crosses the magnetic waves, one will pick up (and hear) the underground signal. However, if the loop is kept parallel to the waves, you'll hear nothing. So by rotating the loop, one will hear the signal fading out from maximum to nothing. The trick is to rotate the loop until the signal is at it's weakest. You'll then know the direction of the waves. Repeat this at a different spot, 20-30 metres away, and you'll find another direction that will intersect the first direction you found. The underground transmitter is below that intersection point! By repeating your measuring and by stepping closer and closer to the intersection point, you'll soon find "ground zero": the spot that is exactly above the underground transmitter!

The signal is an intermittend "bleep". The receiver will amplify it. Measuring the strength of the signal is one way of calculating the depth...

ALTERNATIVE WAY

There is also a way of putting both the transmitter and receiver underground, to determine the distance and direction of two parts of different caves (in case the caves are located to deep to permit measuring this on the surface). The technique is however a lot more difficult and less precise.

SPECIFICATIONS

- Depth-range: maximum 150 m
- Precision: +/- 30 cm at 25 m depth (if underground loop is put perfectly level!)
- Dimensions of the loops: 46 x 46 x 6 cm (a smaller version exists, more suitable to be taken along through narrow parts of the cave, but it is less powerful.)
- Weight: 9 kg (incl. 12 V dry lead-accu)
- Average time needed to make a surface-location: 20 minutes
- Autonomy per battery charge: 5 surface-locations

EXPERIENCES

This device was developed in Great-Britain and has been used numerous times, e.g. to realize the connection between Reyfad and Pollnacrom.

In Belgium also it was used a lot of times:

- verification and correction of the surveys of the Weron-Dellieux cave-system.
- exact surface-location of the place where a new entrance shaft (45 m deep) will be drilled in the famous show-caves of Hotton.
- exact surface-location, resulting in a second and artificial entrance, in the Grotte des Emotions(discovered in '95 by SC Avalon).
- surface-location of the sumps of Sainte-Anne Cave and Trou des Manants, in order to know the distance between the sumps of these 2 caves.
- surface-locations at the old mining-site of Jumet, ordered by Belgian police forces, and related to the Dutroux murder-investigation.

- surface-location in Chantoir de Grandchamps, in order to examine the possibility to make a dry (and safer) entrance.
- etc.

QUOTE

" It is a very useful device that has proven it's value over and over again!"

REMARK

It is clear that this device can be of interest to a lot of cavers. If you need it, don't hesitate to contact us or the builder of it ( Jan Geboers, tel. 014/81.68.07), and we will surely help you.

Laatste update/last update: 17 jan 2000  Paul De Bie  - Don't copy these pages without permission! Make a link instead!