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In-pipe Radar(seeing behind the pipe wall) |
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Why use GPR The internal inspection of ducts, gravity pipes and culverts has been carried out for some 30 years or more using CCTV. However the camera is unable see beyond the wall of the conduit. Voids formed due to poor construction methods or repeated surcharging may exist but give no internal indication of their presence. A ground probing radar (GPR) survey is therefore a means by which the backfill can be assessed up to 1m or more above the crown of the pipe or duct. It is capable of working through a wide variety of materials such as plastic, concrete, brick and vitrified clay. It is not possible to survey through metal. |
 Inserting the antenna set up for inspection of a 90mm diameter BT duct - note the CCTV camera in the background that is used for presurvey inspection to ensure no obstructions exist in the duct |
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 The antenna mounted on a CCTV tractor unit set up for surveying a 450mm diameter pipe |
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How it works The radar system works by transmitting a pulse of radiowaves at a set central frequency into the material being examined. A receiver is set up to scan for returns from the transmitter. The receiver used in the illustrations is a 1.5GHz typically configured 40 scans per metre - these scans produce a scrolling picture as the antenna is moved along the pipe or duct. The success of GPR relies on the variability of different materials in the transmission of radiowaves. Contrasts in the dielectric constant of materials around the pipe result in variations in the amplitudes of the radiowaves returning to the receiver. These variations form anomalies within the mean level of ground response. A void whether, air or water filled, will normally result in a strong response being seen. Other features such as variations in bedding or backfill should also be detected. |
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Examples of the results of a survey A survey was carried out under controlled conditions where a known void had been constructed around a nest of nine BT underground ducts. The GPR survey detected the void, showing it to be some 3 metres in length and extending at least 450mm above the duct. |
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 Example of raw radar data fron within a section of BT duct showing a void from 19.5m to 22.5m. The antenna is looking vertically upwards into the crown so that the scale on the left, which is effectively inverted, shows the distance from the crown in metres |
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 The same section of duct but after the data has been processed to reduce noise and other characteristics associated with radar such as diffractions. |
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Stratascan are grateful for the help and permission of BT in the production of these images |
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