The majority of chimney flue location surveys we undertake are in Victorian houses. These have uniformly built, not overly thick, brick walls, which makes detection and location of flues fairly straight forward. Ground Penetrating Radar detects changes in the subsurface. The flue voids offer a big contrast to the masonry; therefore, they appear as a strong reflection in the GPR radargram.
Occasionally we are set a more challenging task. This was the case with a recent survey of an Elizabethan house in Plymouth. The walls were of stone construction and very thick. Wall thicknesses of 1.1m were regularly recorded, but in places they were thicker (we couldn’t detect the back face). As well as flues, we were asked to detect and plot any other voids detected within the wall construction. The survey posed a number of challenges, most of which were solved satisfactorily by our survey team:
- The thick walls meant we had to scan from both sides to ensure we penetrated the full thickness.
- To provide best resolution and adequate depth penetration, all walls were scanned using two antenna frequencies: 1.5GHz and 900MHz.
- The walls were very rough and uneven, so surface coupling of the antenna was poor.
- Also, due to the rough surface, tracking of the survey wheel was variable. To correct for this, distance markers were added manually to the data and corrections were applied in post processing.
- The walls were damp, resulting in signal attenuation. Scanning from both sides helped alleviate this problem.
- The wall construction was very variable, possibly with a rubble infill. This caused GPR signal scatter making data interpretation more difficult.
All GPR data was saved electronically and processed off site. All detected chimney flues and voids were then plotted on CAD elevation drawings.
We were successful in detecting and tracking the majority of the flues; however, due to poor data, a few appeared to terminate unexpectedly and could not be fully traced. It is possible that these were blocked.
For further information about our flue location surveys, please visit our chimney flue location web page.