GPR case study: How we used GPR survey methods to locate voids present within the church stone wall construction, which was up to 1m thick, comprising a stone inner and outer leaf, with a rubble fill in-between.
The church of St Helen dates back to the twelfth century. It was one of only a few churches in the City to survive both the Great Fire of London of 1666 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.
Objectives
The purpose of the survey was to establish any voids present within the stone wall construction of St Helen’s church, Broadgate London. 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
Ground Penetrating Radar was used to scan the walls; horizontal scan lines were collected at approximately 500 mm centres. Equipment used:
- 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 detecting anomalies within the wall. Results were plotted on elevation drawings categorised as strong anomalies, anomalies or possible anomalies. Several breakouts were undertaken by others 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.