What is Ground Penetrating Radar and how does it work?

Ground Penetrating Radar is a revolutionary NDT survey technique to investigate the sub-surface. Find out how it works, what it can detect, how deep it can penetrate and the advantages of GPR.

Ground Penetrating Radar (GPR) is a geophysical survey method that uses pulses of electromagnetic radiation to image the subsurface. It provides a non-intrusive and non-destructive method of surveying the sub-surface. It is a useful survey technique to investigate many materials including the ground, concrete, masonry, and asphalt.

How does it work?

A GPR transmitter emits pulses of electromagnetic energy into the subsurface. Changes in the sub-surface are detected based on differences in permittivities. When a change in the sub-surface is encountered, some of the electromagnetic energy is reflected back to the surface. This is detected by a receiving antenna and variations in the return signal are recorded. The information is displayed on a radargram.

Although Ground Penetrating Radar can detect changes in the sub-surface, it can’t determine their exact nature. Some features exhibit specific characteristics in the reflected wave pattern; for example, reflections from metallic surfaces have a high amplitude, while reflections from a void are reverse polarity. These help with the identification of the detected features. However, in some cases, it may be necessary to supplement a Ground Penetrating Radar survey with absolute data from boreholes, sample cores, trial pits, etc.

What can Ground Penetrating Radar detect?

Ground Penetrating Radar (GPR) can effectively be used to locate and distinguish a wide variety of metallic and non-metallic materials. GPR works best when there is a big difference in the electromagnetic properties of the materials being surveyed. For this reason, metallic objects make ideal targets (e.g. reinforcement within concrete). Ground Penetrating Radar will, however, detect most materials providing there is a sufficient difference in the electromagnetic property between the target and surrounding material. Some of the more common target materials include:

• Metal
• Plastics
• Changes in ground strata and geological features
• Concrete
• Air pockets or voids

Excavated areas, back-filled areas and any other ground disturbances can also be identified and mapped.

Ground Penetrating Radar will not work in certain ground conditions such as heavy clay soils, particularly if they are waterlogged. De-ionised water does not pose a problem to GPR, however, water with a high mineral content (e.g. seawater) attenuates the signal making it an unsuitable medium. Ground Penetrating Radar is also unable to penetrate through metallic objects, including dense reinforcement.

Ground Penetrating Radar Penetration Depth

The electrical conductivity of the scanned medium, the transmitted centre frequency, and the radiated power all influence the penetration depth.

An increase in electrical conductivity attenuates the GPR electromagnetic wave, resulting in a decrease in the penetration depth.

Higher frequencies provide a higher resolution; however, the penetration depth is limited. Conversely, a lower frequency provides greater penetration depth, albeit at a lower resolution. The choice of antennae frequency is dependent on the investigation objectives, including the width of the survey path; for example, an antenna frequency of 400 MHz has a 0.3 m survey path width. Generally, it is advisable to use the highest frequency possible, although often it is necessary to scan with more than one frequency to provide the best results.

The following table shows examples of typical penetration depths achieved by different frequencies and potential application:

Investigations can also be undertaken using lower frequency antennae (200 – 16 MHz). Maximum penetrations depths vary according to the subsurface media but can reach as deep as 30 or 40 m. Examples of potential use include detection of groundwater tables, depth to bedrock, mapping of landfills and commercial assessment of sands and gravels in glacial landforms.

Advantages of Ground Penetrating Radar

Ground Penetrating Radar is a highly cost-effective non-disruptive survey technique, offering a rapid means of obtaining subsurface information. Its many advantages include:

• GPR is non-intrusive, non-destructive and benign, making it safe for use in public spaces.
• It detects metallic and non-metallic objects and voids.
• It can resolve construction layer interfaces.
• Depths, dimensions of larger objects and layer thicknesses can be estimated.
• Site data collection is relatively quick making it suitable for scanning of large areas.
• Only single-sided access is needed making it ideal for surveying floors, walls, decks, slabs, tunnels and balconies.
• Different frequencies can be used to provide different resolutions and penetration depths.
• GPR provides high-resolution continuous survey data that can be interpreted qualitatively, in real-time, or processed off-site.
• Faster, safer and lower cost than radiography (X-ray).