Sandberg was one of the first to start using Ground Penetrating Radar (GPR) for chimney flue location. 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 method 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. Consequently, this meant that equipment was big and geared to obtaining deep penetration depths.
The first GPR control unit I used, was a GSSI SIR-2. Things I remember the most: Firstly, it didn’t have an internal battery. It had to be plugged into the mains, or use an external battery. On-site, power was supplied by a big car battery. Secondly, the car battery meant that the system was not very portable. Flexibility to scan was achieved by using a 10 m or 20 m cable from the control unit to the antenna. Thirdly, it ran on DOS. No Windows operating system!
The most common antennas we used then, had a central frequency of 900 MHz or 400 MHz. 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.
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 125 mm wide and many metres long. To record position, markers were added to the data by manually clicking a switch, typically at 1 m 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.
The present-day chimney flue surveys
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 1.5 GHz). This gives a 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 how GPR works, see our How does Ground Penetrating Radar work?