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Kent Archaeological Review extract

Geophysical Surveying in Archaeology --
Some Recent Kentish Developments.
by Michael Ocock.

The use of various forms of geophysical prospecting is becoming increasingly common in archaeology and as techniques improve the information which the surveys provide is likely to become more than just a guide to the most profitable areas to excavate. One big difficulty which faces local societies with only limited funds is how to obtain the very expensive instruments which are needed. It is often these local societies and groups who, when involved in emergency excavations or extensive field surveys in advance of building development, have the greatest need for a rapid means of assessing unexplored sites.

The systematic investigation of a locality in order to discover and record new archaeological remains is one of the most important tasks required of them. Only by this means can new discoveries be brought to the notice of university and other research teams and the excavation of sites threatened with destruction, carried out well in advance of the bulldozer.

However the situation in Kent improved considerably earlier this year when, as announced in the May issue of the Review, the Kent Archaeological Research Groups' Council (KARGC) purchased, with the aid of a Council for British Archaeology grant, a differential proton magnetometer, which is now available on loan to any member group throughout the county. The proton magnetometer is an instrument which indicates even very small changes in the intensity of the earth's magnetic field. Such instruments which reveal changes in geological structures are widely used for geological and geophysical surveys, especially for mineral and oil prospecting. The differential proton magnetometer does not measure the absolute intensity but only the difference in the magnetic intensities experienced by two detector bottles, one, four or five feet above the other. The bottom detector bottle is affected by local disturbances such as an archaeological feature. The strength of the anomaly is recorded and provides the means of detecting such features. The KARGC equipment consists of the electronic instrument, two detector bottle, each surrounded by a coil and a 12 volt battery.


Most surveys are carried out using a grid which is marked out with a 5 feet or 2 metre mesh. This means that more than 400 readings are taken in every 100 foot square. The number of readings can be halved without too much loss of accuracy by working on a triangular mesh with readings at 10 feet intervals. The instrument is first set up and adjusted on a piece of ground which as far as is known contains no archaeological or other features; this then becomes the control point. Readings, taken at other points will then show whether or not there is a difference between them and the control point. The strength of the anomaly gives some indication of the nature of the feature. One member of the team holds the pole in a vertical position at each point on the grid with the bottles aligned east-west, another reads the instrument and records, whilst, in ideal circumstances, a third person moves both the instrument and the heavy battery!

Interpretation and other hazards!

Burnt features such as hearths, ovens, furnaces and pottery kilns produce strong magnetic disturbances and are easily detected. Ditches, pits and walls can also be located and iron objects produce such a strong reaction in the instrument that the presence of stray nails and marking-out skewers can create havoc; even the operator with the pole must ensure that he is not carrying any ferrous metal.

PHOTO: Magnetometer in use at Reculver.

Photo Caption: The differential proton Magnetometer at Reculver.
Photo by Ralph Mills.

Other problems are caused by overhead power lines, wire fences and the close proximity of buildings. The combination of these and other factors rules out the possibility of using proton magnetometers in towns and other built up areas. On one site recently surveyed, overhead power cables about 35 feet above the ground caused a disturbance extending at least 50 feet on either side of them and on the same site a 4 foot-high wire fence affected readings as much as 20 feet away.

When the readings have been recorded it is necessary to interpret them and as an aid "dot-density" or other diagrams are drawn. In the "dot-density" diagram dots are scattered at random around each point on the grid, the number of dots depending on the strength of the reading at that point. In each case the density of the dots shows the relative strength of the anomaly. Interpretation can be very difficult and although linear features such as ditches may be recognisable other anomalies often appear indistinct. Geophysical surveying is still only a valuable aid to the excavator and should not be adopted as an alternative to excavation.

A very full description of proton magnetometers and the techniques involved is given in Chapters 2 and 3 of Physics and Archaeology by M J Aitken.

Results so far.

The KARGC instrument has already been used with some success on several sites including those at Cooling near Rochester, Warbank Keston, Reculver, Radfield near Sittingbourne (a recent emergency dig) and elsewhere in East and West Kent. Plans are in hand for an extensive survey at Cooling where pottery kilns have been found and for further work on other sites. In the meantime a small number of people from each group are learning the techniques involved so that on any operation there will be someone available who can successfully use the equipment. It was hoped to include in this article the results of some recent work at Lenham, where a survey was carried out over an area of land at the side of the Pilgrim's Way above the village. A burial was accidentally discovered there in 1946 and it is possible that more exist. However overhead power lines were unfortunately successful in preventing the instrument from giving useful indications in all but a small part of the field. The only anomaly recorded was later found to have been caused by a 9 inch length of fence wire buried about 10 inches deep!

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