Ground Penetrating Radar – GPR

What is Ground Penetrating Radar?

Ground Penetrating Radar (GPR) is a non-destructive and rapid geophysical method that operates by transmitting electromagnetic waves from an antenna and reflects off layers and objects hidden in the ground. These reflections are collected as data which generates an image of the subsurface. A typical GPR system configuration consists of one or more antenna elements, a control unit, and a monitor or external Tablet/PC, for storage and display of data.

 

Why do I need a GPR?

Our MALÅ range of geophysical solutions can be used for multiple geological applications. The GPR method is commonly used when digging, coring, or drilling is not allowed, or needs to be reduced to minimize cost. GPR can quickly add useful information when other sparsely sampled information is not detailed enough.

Some of the application areas where our MALÅ solutions and GPR are being used includes:

 

  • Utilities: Location and depth of pipes, cables, barrels etc. Pre-studies for horizontal directional drilling etc.
  • Concrete investigation: Location and depth of rebars and other installations in concrete, thickness of concrete slabs/elements, void detection, concrete fracture mapping etc.
  • Roads and Railroads: Road/asphalt layering, railroad ballast surveys, internal structure analysis, geological investigations prior to construction, bridge decks surveys, runway investigations at airports etc.
  • Archeology: Mapping of archaeological landscapes, sites, features and objects.
  • Geology: Stratigraphic mapping, structure analysis, cavity and sinkhole detection, depth to bedrock investigations, mapping of bedrock fractures, mining hazards etc.
  • Water: Mapping of lakes and rivers, mapping bathymetry and river / lake floor structure Geological investigations for the detection of groundwater etc.

 

  • Earth and concrete dams: Internal structure and layer investigations, void detection, concrete and rebar mapping etc.
  • Tunnels and Mining: Fracture detection, structural investigations, concrete thickness evaluation etc.
  • Environmental: Hazardous waste mapping, underground storage tank and other utilities detection, bedrock surface mapping for estimating flow directions, lake sediment surveys etc.
  • Military: UXO (unexploded ordnance) detection, runway integrity analysis, clearing of trenching routes etc.
  • Ice and Snow: Structure analysis, ice and snow thickness estimations, crevasse detection etc.

How does a ground penetrating radar work?

In principle, GPR is similar to a fish-finder (or echolocation device) in a boat. The transmitter antenna radiates repetitive short-duration electromagnetic signals into the subsurface as the antenna moves across the surface. Electromagnetic waves are reflected back to the receiver by interfaces between materials with differing electrical properties, e.g. sand and bedrock, or rebar and concrete. The collected traces (also called scans) of the reflections form a ‘radargram’, the result of the GPR investigations, and is presented to the user, in real-time, as data is being collected.

The depth and resolution that can be achieved with GPR depends on the frequency of the transmitted electromagnetic pulse. A higher frequency results in higher data resolution, whilst a lower frequency results in better depth penetration.

Primarily the depth/resolution requirements and the soil conditions at the site determine the choice of antenna frequency. The table below is a guide when selecting antenna frequency based on the depth interval of interest. The table is calculated using recommendations from D.J. Daniels (Ground Penetrating Radar, 2004) where the depth of penetration before the full energy of the transmission is lost, is defined as 20 wavelengths or less.

Note! It is important to remember that this is only a guide and the results at a certain site will very much depend on local conditions, i.e. the specific electrical properties of the media investigated. For instance, in ice and snow the depth penetrations can be considerably higher.

Antenna centre frequency (MHz) Suitable target size (m) Approx. max. penetration​ depth (m) at 100µs/m​
25 1 80
50 0.5 40
100 0.1 20
250 0.05 8
500 0.04 4
750 0.03 2.7
1000 0.025

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To improve the performance of GPR antennas Guideline Geo / MALÅ has developed and patented the HDR (High Dynamic Range) technology. The HDR technique will increase the bandwidth of an antenna to roughly 120% compared to a non-HDR capable antenna. As a result, the systems will have better depth penetration and resolution than a comparable non-HDR antenna.

As a rule of thumb the resolution decreases with depth. The objects to be mapped need to be at least 10% of the investigation depth. This means that small objects cannot be detected at larger depths with GPR.

On ground or in ground?

GPR can be applied both on the ground surface or in boreholes. For the two different types of investigations there are several different antenna frequencies available. 

 

 

 

 

Field examples

Depending on the application and field environment, the GPR antennas can be pulled or pushed, by hand or vehicle. Choose the sled for a pull-optimized system, or select a rough terrain cart for a push-optimized system. The GPR system can most often be adjusted to fit your purpose, for easy data collection in any environment.

Considerations

Ground Penetrating Radar is a very versatile geophysical method, reaching from rebars to deep ice investigations. However, like any geophysical method, it is useful to understand the limitations. For GPR the conductivity of the ground/construction is critical. If the conductivity is too high, the electromagnetic waves are efficiently subdued, energy gets lost and no information is gained from depth.  

The conductivity can be a problem in areas with clayey, and silty soils, salt or brackish water (both ground water and lake/sea/river), in areas with dissolved ions (different types of ground contamination), in newly lain concrete (due to dissolved ions), newly salt sprinkled roads 

Most often a short test measurement or knowledge of the electrical conductivity can tell if GPR is a way forward or if another investigations technique should be considered.  

As a rule of thumb, GPR is good for any geological medium when the resistivity is above 100 Ohm-m (conductivity below 10 mS/m). GPR will, in most cases, be ineffective in a geological medium with a resistivity lower than 50 Ohm-m (above 20 mS/m).    

Note! GPR waves do not penetrate metallic objects but sometimes can give a typical signature, so called “ringing”, which can be used to separate metallic objects from non-metallic objects.

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