Ground Penetrating Radar is often misunderstood as an NDT survey and investigation technique in the Construction Industry. In this Ground Penetrating Radar FAQ we have listed the most frequently asked questions below; we hope you find the answers and links to further information useful.
Table of Contents
What is Ground Penetrating Radar?
Ground-penetrating radar (GPR) is a geophysical non-destructive survey technique which can be used to characterise the sub-surface. Sandberg specialises in the use of GPR as an investigative survey technique within the Civil Engineering and Construction industries.
To find out more about Ground Penetrating Radar, our homepage is a good place to start.
How does Ground Penetrating Radar work?
GPR is a geophysical survey method that uses pulses of electromagnetic radiation to image the subsurface.
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.
For more detailed information, see What is Ground Penetrating Radar and how does it Work?
On what materials can Ground Penetrating Radar be used?
GPR works on most materials, most commonly soil, concrete, masonry and asphalt. It detects changes in the sub-surface; therefore, it is not limited to the detection of metallic objects but will also detect most other materials such as plastic, clay pipes, voids, wood, disturbed ground, layer interfaces, etc. In order for the target material to be detected, it must have sufficiently different electrical properties from the surrounding material.
The most significant performance limitation of GPR is not being able to penetrate metallic objects or, to be more precise, highly conductivity materials. This includes materials such as clay soils, which have high dissolved mineral content, making them conductive.
Is Ground Penetrating Radar data easy to interpret?
The latest all-in-one systems are mostly pre-configured and hence simple to use. Data is often processed automatically. They are, however, also very limiting. They lack versatility and can’t be modified to suit the varying site and survey conditions. For this reason, they tend to be one-trick ponies and are designed for a specific survey type.
At Sandberg, we favour a modular system approach comprising a control unit, usually including a rechargeable battery to power the system and antenna with an appropriate central frequency. GPR data is processed off-site using specialist software. This approach is very versatile and is suitable for the majority of GPR surveys.
Depending on the objectives of the survey and the features being investigated, GPR radargrams can be easy or extremely difficult to interpret. Interpretation of radargrams is generally regarded as non-intuitive and a mystery to inexperienced users.
For a successful GPR site survey, we strongly believe that it is crucial the survey is conducted by a suitably trained and experienced surveyor using the most appropriate GPR equipment available. Considerable expertise is required to plan, undertake and interpret GPR surveys.
How deep can Ground Penetrating Radar penetrate?
The following factors determine GPR penetration depths:
1) The electrical conductivity of the material being scanned.
2) The central frequency of the antenna used; and the radiated power – see Typical penetration depths of different antenna frequencies.
In the UK, GPR penetration depth into the ground can vary from less than one metre (in clay) to several metres (in chalk or dry sandy soils).
Heterogeneous materials, such as rocky soils or reinforcement in concrete, also limit GPR penetration depth due to signal scatter.
Trial scans may need to be conducted to assess the penetration depth of the GPR signal under specific site conditions.
Can Ground Penetrating Radar measure depth or thickness?
Yes, GPR can measure depth and thickness.
The speed at which the GPR signal travels depends on the permittivity of the material being scanned. The depth to a target can be calculated based on the length of time it takes for the GPR signal to travel from the transmitter to the target and back to the receiver.
If the depth to a specific object is known, a calibration can be conducted in situ to determine the coefficient of permittivity of the material being scanned. This information can then be used to calculate the depths of other objects at unknown depths under similar site conditions.
Is Ground Penetrating Radar equipment portable?
GPR equipment is very portable. Several different systems are available, ranging from all-in-one units (the size of a brick) to cart-mounted systems (small lawnmower size). All the systems we use are battery-powered and can be easily moved around the site.
For jobs further afield and abroad, the equipment can be packed in cases and shipped as airline baggage.
What Ground Penetrating Radar equipment does Sandberg use?
Sandberg uses a variety of GPR systems according to a project’s needs. Our most common set-up is a GSSI SIR-4000 control unit with an antenna of appropriate frequency. This is the most versatile set-up, and using an antenna with the appropriate central frequency will provide the best results for most jobs. If scanning at a wide range of depths is required, we will use several different antennas or a dual-frequency (DF) antenna.
For more information about the equipment we use, see our GPR Equipment page.
What GPR surveys does Sandberg undertake?
Our most common surveys include the following:
Concrete imaging and rebar detection
Concrete floor slab surveys
Chimney flue location
Historical building characterization (incl. embedded steel location)
Locating tendon ducts
Locating underfloor heating pipes
Locating services and conduits in concrete floor slabs
Retaining wall surveys
Site mark-up surveys
Wall and slab thickness measurement
Shallow foundation and pile-cap location
Railway ballast thickness and evaluation
Voids in construction and below slabs
Verification of construction detail
Find out more about the Ground Penetrating Radar surveys undertaken by Sandberg.
Can GPR differentiate between rebar and service conduits?
GPR does not identify the exact nature of the detected feature, however, in most cases, it is fairly straightforward to differentiate between rebar and service conduits.
Rebar generally follows a regular pattern, both in spacing and depth. Anything which clearly breaks the pattern is likely to be a service or conduit. In floor slabs, services are nearly always located within the screed layer, not within the structural slab.
Sandberg has an extremely high success rate in detecting and mapping services in concrete floor slabs. It is essential that all services are accurately located prior to any cutting, drilling or other intrusive works being undertaken.
Can GPR differentiate between rebar and tendons in a post-tensioned slab?
GPR does not identify the exact nature of the detected feature, however, in most cases, it is fairly straightforward to differentiate between rebar and tendons.
In post-tensioned structures, the rebar reinforcement layers are nearer the surface with the tendons behind. Rebar tends to follow a uniform pattern and depth, whereas the depth of tendons follows a hyperbolic path, being near the top of the member at supports and near the bottom at mid-span.
Sandberg GPR surveyors are fully trained and understand these key structural differences. You can be confident that any detected tendons will be correctly identified.
What can Ground Penetrating Radar detect?
GPR provides a non-intrusive and non-destructive method of surveying the sub-surface and can be used to locate a wide variety of metallic and non-metallic materials effectively. It will 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, reinforced concrete and voids.
Can GPR determine the diameter of reinforcing bars?
No. There are several research papers, which suggest possible methods, but these are not accurate or reliable.
Alternatively, there are cover meters which use pulse induction, which can provide rebar diameter estimates. In our experience, these are not particularly reliable and are prone to be affected by other adjacent reinforcement.
Sandberg strongly recommends a small local break out to expose the reinforcement at an intersection. This is reliable and determines the reinforcement detail of both the longitudinal and transverse bars.
Can Ground Penetrating Radar detect voids below concrete slabs?
Yes, GPR can detect voids below concrete slabs. Voids produce a strong characteristic reflection. The planar extent of the void can be determined; however, the actual depth of the void itself (from the top of the void to the bottom of the void) cannot be accurately measured.
Sandberg successfully carries out many such surveys, warehouse slabs being the most common. It is normal to combine the void survey with a reinforcement survey to identify different construction detail and missing or poorly placed reinforcement.