Ground Penetrating Radar (GPR) is a revolutionary NDT survey technique for investigating the subsurface. In this article, we look at how GPR works, what it can detect, and how deep it can penetrate.

How does GPR work?

GPR is a geophysical survey method that uses pulses of electromagnetic waves to image the subsurface. It provides a nonintrusive and nondestructive way of surveying the subsurface. Consequently, it is a valuable survey technique for investigating many materials. Examples include ground, concrete, masonry, and asphalt.

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. A receiving antenna detects this, and variations in the return signal are recorded. The information is displayed on a radargram.

GPR 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 and reverse polarity, while reflections from a void have a high amplitude with no change in 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 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 in concrete). Ground Penetrating Radar will detect most materials, providing there is a sufficient difference in the electromagnetic properties 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 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 can also not penetrate through metallic objects, including very dense reinforcement.

A GPR surveyor scanning a masonry wall with a compact hand-held GPR antenna.
Scanning a masonry wall to detect embedded metal

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. This results in reduced 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 antennae frequency choice depends 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. For best results, scanning with more than one frequency is often necessary.

Antenna FrequencyTypical max. Penetration DepthExamples of potential use
2.6 GHz0.3 mRebar mapping, structural concrete investigation.
1.5 GHz0.45 mReinforced concrete slabs, conduit detection.
900 MHz0.9 mPipe, detection, shallow soils, construction thickness.
400 MHz2 mUtility surveys, shallow geology, buried object detection.

Typical penetration depths achieved by different frequency antennas and potential applications

The GSSI 2.6 GHz antenna
GSSI 2.6 GHz antenna

Investigations can also be undertaken using lower frequency antennae (200 – 16 MHz). Maximum penetration depths vary according to the subsurface media but can reach as deep as 30 or 40 m. Usage examples include the 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. It offers a rapid means of obtaining subsurface information. Its many advantages include the following:

  • 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.
  • Estimation of depth, dimensions of larger objects and layer thickness.
  • Site data collection is relatively quick, making it suitable for scanning large areas.
  • Only single-sided access is needed making it ideal for surveying floors, walls, decks, slabs, tunnels and balconies.
  • Different frequencies provide different resolutions and penetration depths.
  • High-resolution continuous survey data can be interpreted qualitatively, in real-time, or processed off-site.
  • Faster, safer and lower cost than radiography (X-ray).

Find out more about Ground Penetrating Radar.

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Information sheet

Download our information sheet about The uses of Ground Penetrating Radar.