A desk study undertaken prior to the construction of new school buildings at a site in Lincolnshire had identified the presence of an infilled Victorian reservoir within part of the site. However, the position of the reservoir as shown on several different historic maps varied by more than ten metres.
Intrusive work could have located the edges of the reservoir but a more cost effective, and less destructive, option was to carry out a geophysical survey.
A geophysical survey utilising electromagnetic (EM), magnetic and ground penetrating radar (GPR) covered the entire playing field in just a couple of days. Utilising several different techniques meant that a wider range of sub-surface features could potentially be identified. The survey was able to accurately locate the edges of the reservoir, as well as identifying several other potential sub-surface features.
The survey determined that the sides of the reservoir sloped. The magnetic survey located the bottom of the slope, and suggested that there may be a brick retaining structure in place. The EM survey identified the top and bottom of the slope and the steepness and profile of the upper part of the slope was determined by GPR. Each of the techniques also identified possible variations in the fill material and / or the presence of other buried features and objects.
Combining the interpretations of the different techniques provided a more comprehensive assessment of the sub-surface site conditions and intrusive investigation can now be targeted to investigate specific features.