How does GPR technology actually work?
Tuesday, 31 January, 2017
Ground-penetrating radar (GPR), also known as ground-probing radar, ground radar or georadar, uses radar pulses to provide images of subsurface locations. It is a reliable, non-destructive means to locate subsurface targets such as cables and rock, and helps to make what could be an extremely difficult job a relatively simple one.
GPR works by using a radio signal with a frequency that is high, usually in the range of 10 MHz to 2 GHz, which is pulsed underground using an antenna. This signal is intended to encounter a variation below the surface, such as a geological structure or a buried object, like a cable. These underground variations can be sought specifically as targets or a general area can be scanned speculatively.
When the signal strikes a target, it is reflected back to the surface. A receiving antenna picks up the returning signal and then records the variations. These are then digitally stored on a surface computer and the time taken for the pulse to get to the target and return to the antenna is measured by a computer. The system interprets the signals that have been reflected back, and this interpretation is then displayed for the individuals conducting the survey on the screen of the unit. This provides an image of the subsurface to the individuals using the system.
GPR technology can be used to provide accurate pictures of specific subsurface objects as well as to map out whole subsurface areas.
GPR waves can travel through a lot of distinct materials. Various kinds of soils, concrete, fill dirt and fluctuating levels of saturation in water all differ in their properties and these affect the radar waves produced. This in turn influences interpretation of the data produced, which will differ depending on the materials encountered by the waves. When the LCD monitor displays the data after scans are conducted, it needs someone with the proper experience and training to interpret them and give a clear explanation of the findings.
This technology is also applied to concrete scanning. To locate objects such as cables, rebar mats and conduits when they are embedded in concrete, a higher frequency system of around 1.6 GHz is used. This penetrates the concrete more effectively and gives a more accurate picture when using GPR technology for concrete scanning. The data can be collected in either simple line scans to determine the thickness of concrete or in a grid format which will produce a map of any targets located in the concrete. Using this method, virtual slices are produced in the picture to determine the depth of the objects and create a 3D map of the image, which further enhances the detail provided. This provides a non-destructive, accurate means to locate targets within concrete structures prior to drilling, cutting or coring.
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