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Geophysical Cave Prospecting

A short account of the possibilities of various methods written for the B. B.

by JOHN LETTEREN of M.N.R.C. & Wessex.


It has long been the dream of many cavers to construct a little black box to detect and locate caves. The great majority of known caves on Mendip occur on the Black Rock/Lower Limestone Shales boundary, and have been discovered by digging active or extinct swallet type entrances. A few large solutional cave systems, notably Lamb Leer and Pen Park have absolutely no surface features and were discovered (by mining operations) quite by chance.  The discovery of Pen Park in the early days was, in fact, facilitated by a natural opening in the roof of the main chamber, but there are probably a considerable number which, like Lamb Leer, retain their secrecy.  Other examples are Manor Farm Swallet whose roof collapsed in 1968; the larger caves of Fairy Cave quarry, which were broken into by quarrying and - if you believe in fairies - the gulf at Sandford Hill and Palmer's Chamber off Lamb Leer.


Until the physical principles of this method are understood it must be regarded as a black art.  No significant caves have been discovered by this method.


Various workers, notably the late Prof. Palmer, have measured earth resistance in an attempt to delineate caves.  The method is extremely slow and tedious, as it is necessary to traverse the area not once but many times with different probe spacings to work to different depths. One worker in the U.S.S.R. has taken 20,000 readings in one area alone.  Even then, there is a chance of detecting faults as the method is only capable of detecting surfaces, not volumes.  Although various people have claimed success; few, if any large caves have been discovered (i.e. entered) using this method.


I spent three years working on explosion seismology.  I received echoes from the region where Palmer’s Chamber should be, also from G.B. and some others.  This method is even more prone to detecting surfaces and was deliberately shelved by myself for that reason in favour of gravitational methods.

Microwave Thermometry

As the earth loses heat at night, cavities near the surface act as insulators and prevent the earth's heat from reaching the surface thus giving rise to cool areas.  However, the temperature differences are so small that a microwave thermometer is needed to measure the wavelength of the infra-red radiation.  The Americans fly such instruments at night to detect old mine workings under highways etc. and they claim to detect not only the tunnels but even the pit props! Unless one of these is hi - jacked, it would appear to be beyond the Scope of a club project.


The earths force (or acceleration, if you insist) of gravity diminishes slightly over a cavity.  The figure obtained from theory over Lamb Leer main chamber is a quarter of a part per million, or 0.25 milligal – a milligal being approximately a micro-g.  One can purchase an instrument having an accuracy of 0.01 milligal, but before putting this to your committee, I should mention that such instruments cost about £3,500.  Various types of gravimeter have been proposed.  The one referred to above is the Worden.  Others use variations of the Cavendish balance or clever overbalancing mass-spring systems (such as the von Thyssen).   Bristol University and others have realised that electronic timers are now fast and accurate enough to time a falling mass to one part in ten million at least, but there are problems in determining the start and finish times to the required accuracy.  I am myself working on a home made gravimeter but as it is just possible that the idea might be worth a patent, I won't discuss it here.

Other Methods

There are several other cave finding methods about, but I won't attempt to describe any more here. Ideas like putting down boreholes all over the place and lowering down miniature T.V. cameras I will leave for you to exploit if you so wish.


Is there a case for NOT using these methods?  Are they, like poisoning foxes, basically unsporting?  I do not think so.  Any method, however effective, will only detect caves in certain environments and even then, the problem of breaking into the detected cave arises.  The Palmer's Chamber dig has been going on steadily for generations, and had still a long way to go.  However, a really effective instrument could reward its designer by finding at least one new, big, shining, unspoilt cave and - a part from Rhino - it is a long time since anyone has done so on Mendip.

Editor's Note:     This is a subject which does not get heard of for long intervals in the B.B., and it is interesting to learn that workers are still busy in this field of enquiry.  Any comments from readers on either the scientific or ethical sides of this subject?