Developments in Geophysical Cave Location

Methods of cave discovery have in the past relied on probing physical evidence such as sink holes or other surface indicators with shovels and picks. Various people, including some Croydon members, and more recently the Green Sites project, have in the last few years tried using established geophysical methods such as resistivity to try to find caves. Unfortunately, these have generally failed to distinguish non-cave features such as the rock/soil interface or the ground water profile from real voids. In desperation, others have even resorted to magical methods and have been seen wandering across the surface or whizzing about in 4x4 vehicles waving wands which are often the residue of the dry cleaning industry. The author can now reveal that he has been directly instrumental in the development of a totally novel geophysical technology that he believes to be at least as successful as the other above mentioned techniques.

The new technique is called Cave Resonance Audio Processing and has somewhat unfortunately become known as the CRAP method for reasons that will become clear later. This method involves beaming audio frequency (AF) vibrations into the ground at varying frequencies such that any voids encountered resonate in a similar way to the cavity magnetron of a wartime radar transmitter. The resonated energy is then received by a surface transducer, relayed to our non fashion statement 4x4 vehicle, and beamed by electro magnetic radiation to Godre Pentre where it is processed and analysed by a giant non-parallel processing Compaq 086 computer (so now you know what the huge aerial is for).

There is, however, a major problem with this method. As even a moron knows, AF is attenuated to the ninth power according to Randolp's Law. Hence the amount of power required to achieve a worthwhile penetration is really quite enormous. Where could all this energy come from, as it was quite beyond the capacity of even the most audiophilic member of our team? The problem was solved by setting up the so called Rave Sites project with the object of attracting raves to sites of speleological interest. This project has been very successful, and initial trials were carried out at the rave in Cwm Porth car park last year. Unfortunately, several additional problems were encountered which we are currently trying to remedy.

The first of these concerned the frequency spectrum of modern rave music. As we had calibrated our equipment on 70's rock music, we found that the monotonous rhythm of the rave music wasn't giving the required spectral width. Unfortunately when one of the team sneaked a copy of "I can't get no satisfaction" on, there was a bit of aggro from the ravers. Another problem occurred because we set up our recording station down wind of the site, became somewhat affected by the "bonfire smoke" drifting across, and had some difficulty concentrating on the task. The transducer used for receiving the signals was also a bit problematical as it was essential to achieve a good acoustic coupling with the ground. After extensive testing it was discovered that the best solution was to place an Elsan bucket firmly on the ground and allow the contents to slowly leak away through a hole in the bottom.


After many hours of computer image enhancement, we came up with a number of print-outs indicating big extensions to Ogof Fynnon only a short distance from the rave site. Unfortunately we then discovered another disadvantage of this method as the acoustic resonances had weakened the roof of the existing cave and there was a large boulder fall resulting in a rescue call-out so we have been unable to fully verify the accuracy of our results. We do, however, believe that our CRAP method is at least as good as anyone else's.

Geophysical Correspondent