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Hon. Sec: A.R. Thomas, Westhaven School, Uphill, Weston s Mare, Somerset.
Hut Warden: P.Townsend, 154 Syvlia Avenue, Bristol 3.
EDITOR:  D.J. Irwin. 23 Camden Road, Bristol 3.

Letter To The Editor

Dear Sir,

The recent article by ‘Prew’ (Speech Communication Underground, November B.B. No. 248) raises a point of some interest. A grade 6 survey of St. Cuthbert’s implies that the position of the end of the cave (Gour Rift) in relation to the entrance contains a possible error of ±40ft. in the easting and northing.  If, by means of magnetic induction method, the position could be located to an accuracy of say ±10ft. or even ±20ft., the survey could be closed onto this position thus gaining considerable accuracy throughout the surveyed network.  It would therefore be interesting to know the accuracy of this method and also the likely effects of local magnetic disturbances due to be buried bedsteads and the like.  Clearly any method of improving the results of the magnetic survey of the cave is of considerable importance.

Mike Luckwill
Sedgeley, 9-1-69.

‘Prew’ has sent in the following:

Dear Sir,

I have read with interest Mike Luckwill’s letter referring to the use of the Magnetic Induction System I described in the November B.B. No.248, for checking the accuracy of the St. Cuthbert’s survey.

Firstly, if anyone is interested in the use of magnetic induction as an instrument of surveying I could recommend they obtain the article by Dr. H. Lord in the Proceedings of the B.S.A.  No.1, August 1963.  Dr. Lord describes in his articles the “pros” and “cons” of using Inductive System for pinpointing parts of a cave system on the surface.

Secondly, my own feelings on the subject are as follows bearing in mind the limitations of the system that I have produced.  At present the maximum range of the device when used with speech is only 300ft., however, if a continuous tone were used this could possibly be increased to 400ft. In order to obtain the degree of accuracy suggested then the range must be considerable reduced, say halved, as it would be impossible to pinpoint the underground transmitter accurately if the received cone were only just detectable.  Unfortunately Gour Rift is, almost certainly, outside the range of the present equipment.  There is, however, an improved version being designed at present and it is hoped that this will give considerably greater range of operation.

A second snag with the Magnetic Induction System is the use of rather large aerial coils, at least 10ft. in diameter for a range of 300ft.  If an accurate position is to be obtained then it is essential that the aerial coils (underground and surface) be rigid and that all the wires in the coil lie in the same place.  This presents quite a problem underground.  It is also important that the underground aerial is positioned accurately in the horizontal plane.

I think from the above remarks you will realise that at present accurate pinpointing of the parts of the cave can only be done, at present, where the areas of interest lie within 200ft. of the surface.