Progress Report, Autumn 2013 to Spring 2014
The long 2012-13 winter was followed by a warm dry summer, and optimism returned. On 2 August 2013 Met Geo Environmental was commissioned to survey the site using two other geophysical methods to achieve depth penetrations of 2-3m.
First, a rectangle some 90m x 30m orientated SSW-NNE along the ridge was laid out, and readings of electromagnetic (EM) ground conductivity were made along survey lines at 2m intervals and at depths of 0.5m, 1.0m and 1.8m. The results in Figure 1 show large high conductivity anomalies (yellow, orange and red) in one major and several minor areas some 15 to 25m east of the cairns. These are of uncertain origin but may be excavated features in the bedrock that have later been infilled with higher conductivity clay, peat deposits or burial materials. There is also a narrow but prominent low conductivity anomaly (dark green and blue) running parallel to the cairns on the east side some 2m wide and perhaps 20 to 25m long, which indicates an elongated void in this location, possibly a fissure, tunnel or adit. These features are present throughout each of the different depth-dependent datasets. This preliminary interpretation will be refined and expanded in the final report.
FIGURE ONE:
Secondly, 16 transects, with a total length of 1094m, were laid out across and along the ridge at pre-selected sites, and ground penetrating radar (GPR) with a mid-frequency antenna of 400MHz was used to examine the substrata to a depth of 2 to 3m. The results were compared with the EC data in Figure 1 and the topographic profile along each transect. In Figure 2, transect 8 shows results across the ridge on which the cairns stand, while in Figure 3 transect 12 shows the results along the ridge. The intense reflections across the stony cairn ridge indicate a very void-rich and mixed material, while along the ridge, much more uniform layers occur with fewer reflections, probably over bedrock. Some transects suggest subsurface platforms, channels and cavities in places.
FIGURE TWO:
FIGURE THREE:
Figure 4 shows an overlay of the EC and GPR data. While the GPR transects are too far apart to allow firm conclusions, the broken and disturbed material along the ridge is clearly different from – and appears to respect – the low and high EC anomalies east of the cairn ridge. Detailed analysis has been delayed by the poor radio signal at Nine Standards, and the consequent need for lengthy post-observation processing of the coordinate data; further work is particularly required to combine the above results with topographic information. However, it is already clear that both techniques have produced significant and worthwhile results, and in combination may justify either a limited programme of closely targeted excavations, or smaller blocks of very detailed GPR surveys in future.
FIGURE FOUR:
