Archaeological Geophysics and OBs
Investigating buried structures using geophysical techniques has been increasingly common in recent years. It allows large areas to surveyed relatively quickly and uses differences in the responses generated by the buried archaeology and natural or fill materials to locate structures and identify their extent. If you have ever been confused by what it involves, Historic England have a straightforward explanation in their guide to research techniques.
The use of such techniques on Auxiliary Sites has been relatively limited. This may relate to the belief that the sites are identified and not recognising that elements of the structure may extend beyond the immediately visible OB, as we saw with the ventilation systems at Binnegar and Bromyards Downs in previous pages. Many structures are in woodland, where the application of the techniques are more difficult.
At Coleshill, a metal detecting survey and magnetometer was used to identify metallic targets for investigation. A ground penetrating radar was briefly trialed over the known demonstration Operational Base at Coleshill House and showed that it was detectable to the technique.
Subsequently a formal magnetometry survey was conducted by the short lived Churchill's Underground War Group, at Langrish House in Hampshire, in an attempt to identify a possible Operational Base in the grounds. Nothing was found though documentary evidence has subsequently suggested the OB was elsewhere.
The Buckland St Mary In-station was known to have been built at Castle Neroche. The Castle was an Iron Age Hillfort converted into a Motte and Bailey Castle during the Anarchy period. Sergeant Arthur Gabbitas who documented the role of his Royal Signals colleagues in the Auxiliary Units Special Duties Branch was stationed there for a while. He left a brief description of the site. Historian and former Royal Signals Officer, David Hunt, visited the site in 2008, obtaining a useful first hand account of the location of the Operational Base, a "Zero" station for the the Chirnside Network, from a local resident who remembered it. His account included the presence of ventilation pipes showing like rabbit holes in the side of pits in Castle grounds. No remains could be found at the time but the site was recorded on the Somerset Historic Environment Record. There are separate records for the above ground Met Hut, which David remembered as still present during his own childhood visiting the site, and the Operational Base.
In 2017, CART arranged a visit to the site, to see if any evidence remained. David's reports provided a focus for the search. The Met Hut base was measured and photographed. The location of the door could be identified. A search of the surrounding area found a small fragment of wire on the adjacent bank, twisted as if to connect to a terminal, perhaps for a battery. At the location identified for the Operational Base, fragments of glazed earthenware pipe were found, a feature often seen at such sites. A very careful search found a small hole near the edge of the bank with the top of two earthenware pipes just visible at its base, just a few centimetres below ground level. The earth had washed down into the pipes over the years, creating the hole. The pipes were almost certainly part of the ventilation system previously noted. Nearby an aerial tree was identified with fragments of the aerial feeder cable projecting from the bark. Because the entire site is a scheduled monument, no excavation of any sort was possible.
A report was prepared and submitted to the County Archaeologist who fortunately was interested enough to arrange for a further non-invasive investigation after obtaining the necessary permissions. Liz Caldwell from GeoFlo (www.geoflo.co.uk) was commissioned by the Southwest Heritage Trust, which was organised by Chris Webster, to survey the site. We think that this report represents the first time that an Operational Base was located by such techniques. With permission we include images from this report in an album of images form the site. The survey was conducted with a Fluxgate gradiometer (Bartington Grad 601-2 ) and Resistivity meter (TR/CIA Resistance Meter) processed with Geoscan Geoplot 3.00v software. The survey showed with a high degree of confidence the location of a structure underground, with a high magnetic signal, consistent with an elephant shelter structure. The ventilation pipes were not visible using these techniques.
On a repeat visit visit to the site, with a change in the conditions, a very shallow regular depression could be identified at the site identified by the non-invasive techniques. A further hole with a pipe junction was identified, in line with the first, supporting the interpretation of a ventilation system. Two aerial trees were identified. We do not believe there is a third extant tree at the site, though there is another tree with a lightning scar, an appearance that can look similar to an aerial scar. As both aerial trees and lightning strikes are located on high ground they can exist in close association.
Coming right up to date, a group from Keele and London South Bank Universities have published a pre-print paper utilising several techniques, namely, Ground Penetrating Radar (GPR), Resistivity, Conductivity and Metal detecting across three Operational Base sites, one intact, another collapsed and a third destroyed to test the modalities. Metal detecting proved valuable in locating the site of the structures, with GPR showing extant chambers and depths. The other modalities also produced useful information and allowed the construction materials to be identified. This is an interesting proof of concept study and suggests that the study of buried structures of the Auxiliary Units with non-invasive techniques is both useful and practicable. We look forward to further investigations of this nature.
Tomorrow we look at the Operational Base in 3D