Ground Penetrating Radar (GPR)

The Technology

A GPR should comprise one or more antennas that themselves comprise a transmitter and one or more receivers. The transmitter emits a pulse of radio waves the frequency of which is depends on the antenna being used.

The receiver is set to record at a specific rate. This rate is either time based or (if an odometer wheel is fitted) distance based, i.e. to sample after the antenna has moved a set distance. The receiver records the signal strength (amplitude) of radio waves reflected from within the materials and will plot this on a graphical display with axis of distance travelled against the time taken for it to be recorded (known as two way travel time).

In order to calculate depth information from GPR data, it is necessary to know or calculate the velocity of the radio waves in the material under investigation. This is described as the speed of light in the material. The most common way of achieving this is through direct calibration with real measurements of depth such as cores; however, it can be done to a degree without intrusive measurements.

The contrast in various properties, between the target material and the surrounding material (between two layers for example), governs the ability to resolve a target. The greater the difference in material the stronger the reflection will be and therefore the easier it will be to differentiate from the background signal level. Higher frequency antennas will be more sensitive to changes in material properties. The problem is that frequency is also inversely proportional to depth penetration.

Applications

GPR systems can either be operator by hand such as on a trolley or pulled on a sled across the ground or wall surface (slow speed) or can be attached to a survey vehicle so that large distances and areas can be covered easily such as a traffic speed survey of a pavement.

GPR and Wall Penetrating Radar have many applications for engineering projects, the following is not an exhaustive list but shows the common uses for the technology:

• Traffic Speed Surveys

•  Location of major construction changes
•  Bound and unbound material layer thicknesses and profiles

• Slow Speed GPR and WPR Surveys
As well as the two applications above, it can be used for:

• Voiding within unbound materials, particularly below concrete slabs
• Location and depth of buried obstructions such as sub-surface utilities, rebars in concrete or misalignment of dowel bars in concrete joints
• Location and depth of buried wall foundations
• Variation in subbase moisture content
• Delamination between normally bonded layers
• Condition of steel in concrete
• Crack depth measurement

Compliance

TRL is licensed by Ofcom to carry out GPR surveys in the UK .  A copy of our licence can be provided upon request.
TRL is an active member of the European Ground Penetrating Radar Association and as such complies with the association's code of ethics and code of practice for the use of GPR systems in potentially sensitive radio environments. A copy of our membership certificate can be provided upon request.

TRL performs all of their traffic speed surveys of pavements in accordance with The Highway Agency Design Manual for Roads and Bridges Volume 7, HD29/08.

Further Information

For more information regarding the GPR services offered by TRL please download the following information sheet or contact:

Adam Cook
Telephone: 01344 770573
Email: acook@trl.co.uk