KARST GEOMORPHOLOGICAL MAPPING PROGRAMME IN DEVON
updated 11 June 2006.
Geomorphological Mapping Programme.
Cave Location and Detection.
Apparent Resistivity...ρ.= 2π a R
At every opportunity, the Society locates and plots all karst geomorphological features that its Members encounter. The information is compiled on UK Ordnance Survey Plans, scale 1 : 2 500 and these form the basis of the Society's Primary Karst Evaluation Sheets, stored in the Society's Archive. Additionally, if of sufficient size and importance, such features are also surveyed
Examples of features plotted and surveyed are isolated and aligned dolines; karst sinks (ponors / swallets); cave entrances; open and buried karst shafts; buried karst doline fields; karst resurgence and exsurgence springs; karst scarps and escarpments; karst dry valleys; periodic ponor-overflow valleys; karst plateaux and terraces.
The Society also undertakes a Karst Rock-head Profiling Programme as a result of more specific projects. The Society's drilling rig is the "Minuteman" model produced by Mobile Drilling Company Inc. of the USA. The "Minuteman" Mobile Drill is a formidable research tool and was a gift to the Society from the Materials Laboratory of Devon County Council, several years ago.
The Mobile Drill can be used as a "power auger" for penetration of up to 11 metres (36 feet) of sediment or unconsolidated material. Its rotary "rock drill" capability can be used down to a depth of 16 metres (50 feet) in solid rock. For reasons of both need and cost, the Society only uses the "power auger" capability to reach and profile the general rock-head contours of karst geology, often concealed in Devon by varying thicknesses of sediment drape.
Of more specific use, it is ideal when applied to determining the physical extent of a karst shaft that has been buried by sediment drape. The "Minuteman" Mobile Drill has been adapted by the Society to be able to "power auger" through sediments to a depth of 16 metres (50 feet).
The application of the Mobile Drilling Rig is a destructive method of ground surveying and is particularly suited to Karst Rock-head Profiling. However, when the karst rock-head itself has to be penetrated in the search for underground caves, the Society has often turned to using the possibly more desirable methods of ground penetration via the non-destructive testing routes. Non-destructive testing procedures include Geo-electric Surveying Methods such as Ground Resistivity Surveying and Ground Penetrating Radar, both of which the Society has also used over a period of 30 years. Proton Magnetometry has also been employed in the search for caves.
In the UK., more recent innovations in the accurate positioning and plotting of fully accessible cave passages in relation to the surface features above, can be obtained by the use of equipment such as the "Molephone" and its successors.
depth..=..√ α ao
Bristow's Single Electrode Probe Technique
KARST ROCK-HEAD PROFILING PROGRAMME :
Our very limited resources have prevented us from doing all that we would wish to do with the "Minuteman" Mobile Drill. It is one thing to own a capital resource and quite another to afford to maintain it when used on a regular basis! Such are the problems of a severely under-funded voluntary sector.
The use of the "Minuteman" Mobile Drill has been very successful in locating the whereabouts, at karst rock-head, of buried and previously unknown karst shafts and other speleological objects. It has also helped decide the physical dimensions of such sites which have catastrophically collapsed or subsided as a result of naturally evolving processes.
Perhaps the best use of this academic resource has been in defining the rock-head profile contours of a whole sections of Buried Karst Doline Fields. In South Devon, these large-scale karst geomorphological features always occur at the karst margins and are best described as maturely developed, hydrologically-active karst ponor zones. They are usually found at the Shallow Karst Hydrological Zone and the Society has found many of them in South Devon. Further details about these large-scale karst features can be found via the Link relating to Karst Morphogenesis and Speleogenesis (Devon) on the ACTIVITIES UK Home Page.
The Link below will take you to a page detailing an individual Case Study within the Karst Rock-head Profiling Programme of the Society's Activities (UK).
Resolution of the Karst Rock-head Profile beneath Down Town Orchard (Buried
Karst Doline Field BKDF.01.), Denbury, S. Devon.
The reliability and acquisition
of data using this methodology varies both with the prevailing ground conditions
and the experience of the operator. An inherent understanding of the limitations
and correct use of the machine and of its response to the ground through
which it is penetrating, are as important as the observation and interpretation
of the retrieved sediment "returns" brought to suface through the borehole
by the flights of the rotating auger. Hence, the machine can not only offer
the position of the karst rock-head below the sediment cover but can also
offer spatial details of the sediment composition in vertical sections
(borehole profile) and horizontally via interpolation of many vertical
sections. The resultant boreholes also have the possibility to be used
for gaining an insight into the conditions and characteristics of the local
groundwater regime. A successful interpretation of the ground being investigated
thus relies on maximizing the opportunities for data acquisition during
the survey. The recognition of such opportunities, let alone using them
to maximum advantage, is a skill borne out of an understanding of the karst
3. CAVE DETECTION AND LOCATION.
3.1. GEO-ELECTRIC GROUND RESISTIVITY SURVEYING :
The detection and specific location of obscured Air- or Water-filled cavities beneath the surface in Limestone areas has been a particularly successful aspect of the Society's work. This has been achieved by borrowing Ground Resistivity Surveying equipment and operating it ourselves. Again, the cost of acquiring this type of equipment has been prohibitive, even though the cost of maintaining and operating it is far lower than that of the Mobile Drilling Rig and certainly well within the Society's budgetary contstraints to be able to do so.
The equipment itself is fairly simple and consists essentially of an integral electrical generator and resistance-measuring unit; ground electrodes and linking conductor wires. This particular procedure is more reliant on the application of appropriate interpretative methods of the acquired data than is the case with the Mobile Drilling Rig. We suggest that reference is made to the Bibliography Section below for detailed information.
Indeed, the acquisition of the data itself is a skill in its own right and relies on an understanding of physical and mathematical theories.
To obtain the raw data, a particular "electrode array or configuration" has to be chosen which, when set out on the ground, best suits the type of ground and the ground conditions under investigation to optimize equipment performance and data acquisition.
Location of the Rift Cave System extending out beneath the floor of Higher
Kiln Quarry, Buckfastleigh, S. Devon, 1977.
GEO-ELECTRIC GROUND-PENETRATING RADAR SURVEYING :
PROTON MAGNETOMETRY GROUND SURVEYING :
[more to follow]
KARST GEOMORPHOMETRY - SURVEYING OF MORPHOLOGICAL FEATURES :
(Text in preparation)
[more to follow]
BIBLIOGRAPHY AND FURTHER READING :
(.*denotes that the Bibliographic reference is a Stock item in the Society's Reference Library.)
New Graphical Resistivity Technique for Detecting Air-filled Cavities."
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