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Ground Penetrating Radar Survey at St. John’s Church, Bemerton

Wessex Archaeology recently conducted a geophysical survey at St. John’s Church in Bemerton, a mile or so west of Salisbury, Wiltshire. Residents from the Bemerton Community Group are in the process of converting part of the church into a community centre, whilst maintaining part of the building as a worship space, and need to be connected to mains utilities. We were asked to survey the routes the utilities would take through the churchyard, from the road to the church building itself.

214 St. John’s Church, Bemerton, near Salisbury, Wiltshire

The construction of St. John’s Church began in 1859 and dedicated in 1861, so the team expected to find archaeology associated with the church, along with graves, up to 150 years old. Out of the techniques we commonly use, ground penetrating radar (GPR) was considered the most useful for this project as we can look at the results in three dimensions. The GPR sends pulses of energy into the ground and measures the time taken for them to bounce off buried features; it works in exactly the same way as radar used to find aircraft, only we point ours downwards!

215 A radargram showing a buried reflector, with time increasing downwards

The site was surveyed on a closely spaced grid, with each data profile showing the distance along the line and the time it took the GPR signal to travel through the ground, reflect off any buried features and return to the antenna. The data are processed so that we can peel layers away that took the same length of time to return to the GPR. We can work out the speed the GPR signal travels through the ground, so work out how deep the responses are.

216 A time slice showing the approximate depth of buried archaeology

The GPR is pushed around in a wheeled cart and the data appear on a screen in front of the surveyor as they push the GPR forwards. A group of children from Bemerton St John C of E Primary School were able to join us in the afternoon to help collect some of the data. It’s always a pleasure to meet the possible archaeologists and geophysicists of the future!
 
The results successfully showed the locations of a number of unmarked graves, as well as modern services and possible elements of the church foundations. The information provided by the survey will help with more detailed planning of the construction of the new community centre.
 
For more information about the work of the Bemerton Community Group, you can follow the progress of the project on their blog.
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