Sandberg was one of the first to start using Ground Penetrating Radar (GPR) for chimney flue location. I would go as far as to say that Sandberg is probably responsible for popularising the method as a non-destructive means of chimney flue location.

In this article, I look back at the early days of GPR flue location surveys and how things have changed to what they are today. A trip down memory lane.

The accepted way of locating flues before GPR was by drilling. If you hit a flue, you would usually use a borescope to work out the general direction of the flue. Alternatively, you could remove bricks. So, to use a survey method, which was non-intrusive was ground-breaking!

I remember undertaking the first chimney flue location surveys for Sandberg back in 1998, or maybe a little earlier. GPR was an archaeological survey method; it was extremely rare to use it for applications in the construction industry. This meant that equipment was big and geared to obtaining deep penetration depths.

GPR equipment

The first GPR control unit I used, was a GSSI SIR-2. It didn’t have an internal battery. On-site, power was supplied by a big car battery. This meant that the system was not very portable. Flexibility to scan was achieved by using a 10m or 20m cable from the control unit to the antenna.

GSSI SIR-2 GPR control unit
GSSI SIR-2 GPR control unit

The most common antennas we used then, had a central frequency of 900MHz or 400MHz. Big, heavy, cumbersome and low resolution compared to what we use for chimney flue location now. High-frequency antennas were just starting to be introduced.

GPR data

Software to process the GPR data was available (just!) but ran on DOS (not Windows). It was not user-friendly! You also needed a computer!

The generally accepted method of recording data at the time was to print the raw data on thermal paper using a special printer. This was done on site, – also powered by the car battery! Output was in black and white. The paper was on rolls about 125mm wide and many metres long. To record position, markers were added to the data by manually clicking a switch, typically at 1m intervals as the antenna was passed along the wall. The print-out was then interpreted either on or off-site, and the approximate position of any detected flues could be interpolated.

Spool of thermal paper with GPR data printed on it.
GPR data printed on thermal paper – note the dashed distance markers.

The present day

Now, GPR surveys are much easier, offer better resolution and higher positional accuracy. The control units are powered by internal batteries and the antennas are much smaller, utilising a higher frequency (typically 1.5GHz). This gives much better resolution.

The antennas incorporate a survey wheel which regulates the scan rate so that scans have a meaningful and constant proportion. The data is saved electronically for off-site processing. This typically includes surface position correction, gain adjustment and migration. The GPR data files are then visually inspected, analysed and detected flue positions plotted in CAD.

To find out more about chimney flue location surveys offered by Sandberg, please visit our web page about Chimney Flue Location. If you want to find out more about GPR in general, I suggest you start on our Ground Penetrating Radar home page.

CAD elevation drawing with detected flues marked-up.
Results from a modern flue location survey presented on a CAD elevation drawing with detected flues marked-up.
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