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Ground Penetrating Radar explained. Discover how we use GPR as an NDT method to investigate building construction detail, identify defects and locate buried objects.

Ground Penetrating Radar, commonly abbreviated to GPR, uses high-frequency radio waves to detect changes and objects in the subsurface. It is a geophysical technique, initially developed for geological mapping.

It is now widely recognised as a highly cost-effective non-intrusive testing technique. It offers a rapid means of obtaining subsurface information from not only soil but a variety of materials, including concrete, brick, masonry, wood and asphalt.

Out of this world!

GPR owes much of its rapid development due to its use on high budget and high technology projects. The Americans used it on missions to the Moon and Mars. During the 1970's Apollo missions, they used GPR to explore the Moon's near-surface geological structure. They also used it to locate potential resources for future human outposts on the Moon. More recently, a pulse radar mounted on a Mars orbiter, found evidence of liquid water, or water-rich sediments, under the southern polar ice cap.

The advantages of Ground Penetrating Radar

GPR detects changes in the sub-surface; however, it is not limited to the detection of just metallic objects. It will also differentiate between most materials such as plastics, clay pipes, wood, disturbed ground, layer interfaces and voids. The main requirement is that the target material has sufficiently different electrical properties to the material surrounding it.

GPR will also provide estimates of depth and layer thickness. All done remotely. Sandberg use it to determine the depth and extent of objects, the width of walls, and also the thickness of slabs, asphalt and screed.

Compared to other non-destructive techniques, such as infrared thermography, ultrasonic or microwave, GPR offers more penetrating power and so can detect concrete defects or deteriorations at greater depths (Dong and Ansari, 2011).

How does it work?

GPR detects fires electromagnetic pulses, usually in the 10 MHz to 4 GHz range into the subsurface. The equipment includes a control unit, with an integral power supply, and an antenna. The antenna (typically a combined transmitter and receiver) transmits the electromagnetic energy into the sub-surface. When this energy encounters changes, notably different permittivities, some of the power is reflected and returned to the antenna. By recording the time taken and variations in the return signal, the system can interpret the information and provide details about the sub-surface.

Several factors affect Ground Penetrating Radar data quality:

  1. Conductive materials attenuate the GPR signal and; therefore, limit the penetration depth.
  2. Higher frequencies do not penetrate as far as lower frequencies.
  3. But, higher frequencies provide better resolution.

In site surveys, there is always a trade-off between resolution and the penetration depth. It is, therefore, desirable to use the highest frequency possible, while still meeting the objectives of the survey.

Find out more about How Ground Penetrating Radar works.

Ground Penetrating Radar radargram showing concrete floor detail

GPR radargram showing reinforced concrete floor with services in screed layer.

NDT

GPR can determine sub-surface detail, rebar distribution, voids, etc. non-intrusively without damage to the surrounding structure.

Speed and Efficiency

GPR scanning is quick and efficient and is suitable for scanning large or small areas. Preliminary results are available in real-time.

Limit Risk

GPR can help limit exposure and control risk by locating potential problems and confirming compliance with the specification.

GPR in the Construction Industry

Due to recent progress in the development of GPR equipment, particularly at the high-frequency end of the spectrum, its use is increasingly common within the Civil Engineering and Construction industries.
Sandberg were amongst the first to recognise the potential of GPR as an investigative survey technique in these areas. As we specialise in the testing and inspection of construction materials and buildings, it was the perfect NDT survey method for us. Consequently, over the years Sandberg have developed techniques and considerable expertise in the application and interpretation of Ground Penetrating Radar.

Ground Penetrating Radar being used to detect embedded metal on a church bell tower

Ground Penetrating Radar being used to detect embedded metal.

Sandberg now use it extensively for many different applications including:

  • Concrete imaging and rebar detection
  • Concrete floor slab surveys
  • Chimney flue location
  • Historical building characterisation (incl. embedded steel location)
  • Post-tensioned tendon location
  • Locating underfloor heating pipes
  • Mapping of services and conduits in concrete floor slabs
  • Retaining wall surveys
  • Structural investigation
  • Site mark-up surveys
  • Wall and slab thickness measurement
  • Shallow foundation and pile-cap location
  • Railway ballast thickness and evaluation
  • Grave and crypt location
  • Voids in construction and below slabs
  • Verification of construction detail

We believe that the application of GPR is limited only by the imagination and skill of the surveyor. Their knowledge and experience in conducting the survey and interpreting the data are crucial. We also believe that by using the best and most suitable Ground Penetrating Radar equipment, we can overcome many of the problems and challenges we encounter on site.

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