GPR case study: How we used GPR survey methods to locate voids in stone walls. The church stone wall construction comprised a stone inner and outer leaf, with a rubble fill in between. Total wall thickness was up to a metre thick.

Brief history

St Helen’s church in Bishopsgate, London, dates back to the twelfth century. It was one of only a few churches in the City to survive both the 1666 Great Fire of London and The Blitz during World War II. In 1992 and 1993, however, St Helen’s was badly damaged by two IRA bombs that were set off nearby. The roof of the building was lifted, and one of the city’s largest medieval stained glass windows was shattered. The church has since been fully restored, although many of the older monuments within it were entirely destroyed. It was designated a Grade I listed building on 4 January 1950.

Survey objectives

The purpose of the survey was to locate any voids present within the stone walls of the church. The North wall and part of the East wall were nominated for the survey. The wall construction comprised a stone inner and outer leaf, with rubble fill in between; the total wall construction is approximately 900 mm to 1 m thick.

Survey methods to locate voids in stone walls

Ground Penetrating Radar was used to scan the walls; horizontal scan lines were collected at approximately 500 mm centres.

We used the following GPR equipment for the survey:

  • GSSI SIR3000 control unit
  • 900 MHz antenna
  • Custom survey wheel to measure distance and regulate scan rate

Survey findings

Ground Penetrating Radar was successful in locating voids in the stone walls. GPR detected anomalies within the walls of varying magnitude. Results were plotted on elevation drawings and categorised as strong anomalies, anomalies, or possible anomalies.

Others undertook several breakouts to verify our results. The strong anomalies were found to be voids; the weaker anomalies were often due to a concentration of a different material, typically chalk.

You may also be interested in our non-intrusive surveys to detect embedded metal in masonry.