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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 objects. Find out how we can help you.

What is Ground Penetrating Radar?

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

Due to continued development and advances in technology, GPR is used for an ever-increasing range of applications. It has even been used on missions to the Moon and Mars. During the 1970's Apollo missions to the Moon, it was used to explore the near-surface geological structure, 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.

GPR is now widely recognised as a highly cost-effective non-intrusive testing (NDT) technique, offering a rapid means of obtaining subsurface information from a variety of materials including concrete, brick, masonry, wood, asphalt and soil.

The advantages of Ground Penetrating Radar

GPR detects changes in the sub-surface. It is not limited to the detection of metallic objects. It will also detect most other 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 surrounding material.

GPR will also provide estimates of depth and layer thickness. It can, therefore, be used to determine the depth and extent of objects, the thickness of walls and slabs, and the thickness of asphalt and screed layers.

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?

Ground Penetrating Radar uses electromagnetic pulses usually in the range 10 MHz to 4 GHz, to detect changes in the sub-surface. The equipment comprises a control unit, power supply (often incorporated within the control unit) and an antenna. The antenna (usually a combined transmitter and receiver) is used to transmit the electromagnetic energy into the sub-surface. When this energy encounters changes, notably different permittivities, some of the power is reflected back to the antenna. By recording the time taken and variations in the return signal, the information can be interpreted to provide information about the sub-surface.

Several factors affect Ground Penetrating Radar, and the quality of the data received:

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

In GPR surveys, there is always a trade-off between resolution and 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.

Ground Penetrating Radar in the Construction Industry

Due to recent progress in the development of GPR equipment, particularly at the high-frequency end of the spectrum, the use of GPR is increasingly common within the Civil Engineering and Construction industries.

Sandberg were amongst the first to recognise it as an investigative survey technique in these industries. It was the perfect fit for the testing and inspection services of construction materials in which Sandberg specialise.

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.

Uses

Sandberg have developed expertise in the use of Ground Penetrating Radar for many different applications including:

The application of GPR is limited only by the imagination and skill of the GPR operative conducting the survey and interpreting the data. Availability and use of the most suitable Ground Penetrating Radar equipment is key to overcoming many of the problems and challenges we encounter on site.

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