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Kent Archaeological Review extract

Science in Archaeology.
by Dr A R Cox, BSc, ARSM, AIM.

Ever since the discovery of the divining hazel twig archaeologists have looked with favour on devices that can be used for finding buried remains, so when a soil anomaly detector became available to the Sittingbourne and Upchurch groups, it was not surprising that they immediately utilised it in their search for Roman buildings at Boxted.

The detector measures the soil conductivity by transmitting a continuous radio signal through the ground to a receiver and gives a direct reading on a meter. The literature accompanying the instrument shows that walls, foundations and small metal objects can be detected and it is only its price of 185 which prevents it from becoming the travelling companion to every achaeologist's trowel.

The detector is supplied in an elegant case which prevents damage during transit, but once near the site the case is best discarded since its weight soon becomes noticeable. It takes two to three minutes to put the pieces together and a similar time to check the batteries and set the meter over undisturbed soil, although no advice is given on how you recognise undisturbed soil!

In use the receiver is held about 2 inches above the ground and it is essential that the angle the receiver makes with the ground is kept constant, for small changes produce large variations in the readings. On one particular day at Boxted, the wind was so strong that it deflected the receiver coil and little confidence could be placed in the readings.

GRAPHIC: Showing how the readings from the meter reflect the sub-surface composition. 

At Boxted the main effort was devoted to surveying two fields where surface finds had been made in the past. To cover the area with a 4-5 foot square grid was impossible in the time available and so surveys at 10ft. intervals were made across the field. The detector is especially suitable for this type of rapid plot, for the meter reads continuously and it is only necessary to stop walking and record when the meter deviates from the initial setting.

The first field contained apple trees at 30 foot intervals and the metal sheep guards around them affected the meter reading for a radius of 5 feet, and similar interference was found adjacent to the boundary fence: whilst for good measure, there was an overhead power cable 200 yards away. All these factors contribute to the general confusion and it was not surprising that the readings did not indicate any regular features. In contrast, a resistivity plot of the same area showed series of anomalies and trial holes confirmed the existence of Roman foundation; about 6 inches below the surface.

Somewhat dismayed by this apparent superiority of the resistivity meter, Mr J Williams and I turned our attention to the field known as Brick Earth Field and after several weekends' work produced conductivity plots showing anomalies of similar size and shape to the Roman building described by George Payne. At this stage we felt slightly "one up" over the resistivity meter, since it had shown no anomalies over the same area, but this evaporated once the first trial trench was dug, for the anomalies were due to a hard-core road about 13 inches below the surface and about 6 inches thick and constructed in the last 100 years. The conductivity plot and the respective soil section are shown in Figures 1 and 2.

However, the detector had identified an anomaly, all-be-it of little archaeological interest, and our brief acquaintance with the instrument has shown that it is easy to use. It is capable of rapid surveys over flat ground, but from our experience should be used in conjunction with another independent technique, for what one meter misses the other may find.

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