Geophysics was used by Phase to locate the extent of underground structures associated with a World War II defence station in Scotland. The presence of sub-surface structures was known from above ground remains and historical records but the extent of the buried structures and whether they were connected was not known.
A geophysical survey comprising several different techniques would normally be the best survey strategy to ensure the most reliable results. In this case however, the target features were known to be comprised of reinforced concrete and located within 2 m of the ground surface and so it was believed that utilising one technique, a magnetic survey, would provide the required information.
It was known in advance that the site conditions would be challenging as the presence of spoil heaps, dense vegetation and rubble would restrict the survey area. Despite this the site, which measured approximately 12 5m by 40 m, was surveyed in only half a day using a Caesium Vapour gradiometer.
The data was collected on a grid system and stored electronically for office based processing. The position of the survey grid was tied-in using a Global Positioning System so that any features identified could be easily referenced and targeted at a later date.
The survey identified two very strong magnetic anomalies believed to be caused by the reinforcement within sub-surface bunkers. There was no evidence of a structure linking the two bunkers, which was of great interest to the client. Information obtained from the geophysical survey has enabled the client to assess which areas to target for future investigation before redeveloping the site. This survey demonstrates one of the rapid and cost effective services offered by Phase Site Investigations.