Ground Penetrating Radar

Applications
  • Image the internal structure of various materials
  • Timber condition assessment: rot, termite damage
  • Slab, beams and column details
  • Bridges, buildings, concrete and masonry structures, approach slabs, void formers, tension ducts
  • Airport runways / taxiways / railways / hardstands / footpaths / road pavement, asphalt, gravel, concrete
  • Walls and tunnels construction details, crossbreaks, delamination, voiding, deconsolidation, settlement
  • Penetrations to avoid cutting reinforcement and tendon ducts
  • Reinforcement mapping, tendon duct location
  • Tree root mapping
  • Archaeology, locating graves / burials
  • Foundation and footings
  • Services / Utility location
  • Water / sewer infrastructure, pipes condition, leakage, voiding, UST (underground storage tank) locating
  • Water retention structures
Method

Ground Penetrating Radar (GPR), also known as Surface Penetrating Radar, sub-surface radar or ground probing radar, uses reflections of transmitted radio wave pulses to create an image of the subsurface. A transmitting antenna is used to produce a continuous series of electromagnetic pulses that are reflected from boundaries and discontinuities within the material. These pulses propagate through the subsurface materials at a characteristic velocity and are reflected by any variations in material such as air gaps, moisture changes, embedded metal or different material layers. Any reflected energy is detected by a receiving antenna at the surface and recorded as individual traces or scans. This process is repeated continuously as the antenna is moved along the surface to build up an entire image profile or radar-gram along the survey line. The recorded reflections are analysed in terms of their signal shape, two-way travel time, amplitude and phase to determine depth, location, size, orientation and relative material properties of subsurface objects.

Radar Principle. Typical radar-gram profile.

The velocity with which a pulse travels through a material varies depends on the electrical properties of the material, which in turn are a function of the materials’ chemical and physical properties. The amplitude of the reflected pulses that occur at these material transitions is related to the difference in characteristic velocity between the two different mediums and the sharpness of the transition relative to the wavelength of the transmitted wave. The greater the difference in material properties then the greater will be the amount of energy reflected back.

The reflected signal strength diminishes with depth due to wave dispersion and electrical attenuation within the material. This limits the effective depth of penetration. The amount of signal loss increases at higher frequencies and varies with different materials. The size of the feature that can be resolved depends on the wavelength of the transmitted wave. Higher frequency antennas have smaller wavelengths and therefore have higher resolution but also have shallower penetration depths. Therefore most surveys are a compromise based on the target resolution and the required depth of penetration. Radar antennas transmit a radio pulse which produces a broad band signal at a particular centre frequency. In other words, they produce a wide range of frequencies. This means that the resolution of the antenna diminishes with depth as the higher frequencies are absorbed nearer the surface. To maximise the energy transmitted most radar antenna are surface coupled. For good coupling to exist the antenna must be at less than a quarter of a wavelength from the surface.

Surveys are generally carried out by profiling the structure over a grid of survey lines. This allows a 3-dimensional reconstruction of the arrangement, form and condition of the materials being investigated. Data can often be collected through a structural element (within limits) by profiling from one side. Generally more than one layer of reinforcing is visible within such a profile. Information on the element thickness, reinforcement location, concrete cover, void location and internal condition are often available from the same data profile. Access from both sides can provide clearer information, especially if the reinforcement is closely spaced. The advantage to this technique over a standard cover meter is the speed in which a large body of information, in the form of sections through the slab that can be collected, the larger depths of penetration and higher resolution.

Data analysis and presentation

Example radar profile through a suspended floor slab showing the variation in reinforcing and the total slab thickness.

Example GPR profiles through suspended floor slab

Limitations inherent with GPR include:

  • The ability to resolve subsurface detail is limited by penetration depth. Data may not be complete when collected over thick slabs or beams. This can be minimised by collected data from both sides of a structural member. Penetration depth in concrete is approximately 200 mm for a 2.6 GHz antenna. Note that both resolution and data quality diminishes with depth.
  • Radar cannot penetrate metal and as such, a dense reinforcement layer will shadow the material directly behind the bars and reduce resolution and penetration. The density of the near-surface steel layer may limit the ability to image the far side of a slab. Very dense reinforcement (typically <100 mm centres) will effectively block penetration.
  • Most transducers require direct coupling with the surface. An undulating or rough surface can reduce data quality.
  • The physical size of the transducer prevents information being collected close to the intersecting faces, or obstruction (typically within 100 mm).
  • The methods used can penetrate through most materials, except for metals. However certain materials or conditions can adversely affect penetration depth or data quality: for example, air gaps behind plasterboard, wet saturated materials and new concrete (less than 28 days) and steel fibre reinforced concrete. Certain surface finishing can prevent or adversely affect penetration. Some examples are thick carpet underlay, tiling, magnesite, terrazzo, metallic waterproof membranes and wet surfaces.
  • Highly conductive soils, (containing saturated or reactive clays) will limit penetration depth and target resolution.
  • The minimum void size that can be detected is limited by the antenna resolution (typically 30 mm for a 2.6 GHz antenna, close to the surface). The transducer also needs to pass directly over the top of the void. The minimum size of the void that can be detected will increase with depth. Smaller voids can often be detected if they are clustered close together such as occurs with extensive honeycombing. Also, honeycombing can sometimes be detected by an apparent reduction in cover depth which occurs because of the reduced density of the concrete between the surface and the steel.
  • GPR cannot resolve closely spaced bars as individual bars. There needs to be some separation between the bars. It is sometimes possible to locate lapped bars by a change in amplitude associated with the greater cross-sectional area.
  • GPR does not determine bar size. Where there is a large difference in bar size it is sometimes possible to differentiate a size difference based on the amplitude of the reflected signal, provided the bars are at similar depths.
  • The accuracy of depth measurements depends on the velocity of signal through the material. As velocity can vary significantly within a material due to variations in moisture and density some form of calibration is required for improved accuracy. Typically accuracy is better than ±15%. Alternatively, cover meter for reinforcement depth or Impulse Echo for slab thickness can be used to get more accurate results.

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