Subsurface utility mapping and concrete scanning both utilize ground penetrating radar (GPR) to locate and map subsurface utility lines and underground features, and to find post-tension cables, rebar, and conduit in concrete. Because properly reading the variations in the signals, identifying hyperbolas, and defining features is as much an art as a science, requiring an immense amount of training and a commitment to accuracy, the industry is always looking for new ways to visualize subsurface and underground elements that “dummy-proof” the results.
That’s where the quest for “true 3D” GPR comes from – the desire to create a technology that anyone can use with little to no training – yet provides accurate, real-time as-built information in an immediately useable format.
However, if you do a quick Google search for 3D GPR, it is apparent that there is a good deal of confusion surrounding the term within the construction and related industries. The goal of this article is to clearly define 3D GPR and its uses, and to do a fair, side-by-side comparison of 3D GPR and properly conducted GPR imaging translated into a 3D data set to accurately visualize the interior features of a concrete slab for planning, design, and engineering purposes.
What is Ground Penetrating Radar?
Ground penetrating radar (GPR) is a non-destructive detection and imaging method that identifies elements that are either buried in the subsurface (utilities) or encased in concrete (PT cables, rebar, conduit, etc.).
According to the New York State Museum, GPR was invented in the 1930s as a tool for measuring the thickness of glaciers. The technology advanced to a stage where it became more affordable in the 1980s, but did not see widespread use in construction and other industries until the early 2000s.
Here’s how it works: The GPR unit sends a radio signal into the ground or the concrete slab. The radio wave will bounce off any material it encounters and create an image on a viewer. When GPR bounces off a subsurface utility or a concrete feature, it creates a hyperbola. By assessing the polarity (color), and depth of those hyperbolas, an experienced GPR technician can determine the type of material located.
GPR can detect both metallic and non-metallic objects, which gives it a wide range of applications. It reveals all types of utilities, including electrical conduit, steam pipes, telecommunication lines, gas & oil lines, water lines, and sewer & storm pipes. GPR can also locate the presence of rebar, conduit, post-tension cables, potential voids, and other structural elements hidden within concrete.
GPR is extremely accurate. However, external factors like soil conditions, the proper use of equipment, and the correct interpretation of the GPR readings can affect accuracy. In cases where GPR may not be the appropriate tool for the job, GPRS Project Managers will utilize complementary underground imaging technology such as electromagnetic (EM) locating to identify subsurface features and utilities.
What’s the Difference Between GPR and 3D Ground Penetrating Radar?
To best discuss this, first we have to understand what 3D GPR is, and what it is not. Most sites you’ll land on when searching for 3D GPR near you may say they offer 3D GPR services, but what their service actually provides is translating a series of GPR images or passes – gathered as described above – into 3D maps by using software to transpose the series of slices (created by each GPR pass) over existing as-builts or record plan drawings.
There can be drawbacks to this method of 3D data creation if it is not undertaken with the utmost care and precision. The most important drawback is that existing as-builts or record drawings are usually outdated and inaccurate because they have not been amended since initial construction and do not contain the required revisions for subsequent renovations, maintenance, and other changes. The other notable drawback is that the GPR technician applying the technology must be an expert in interpreting the GPR images so the GPR data being overlaid is accurate.
GPRS maintains a 99.8% accuracy rate on over 500,000 GPR jobs. To learn about our Green Box Guarantee, click here.
3D concrete or subsurface imaging, sometimes referred to as 3D GPR, as it currently exists, falls into one of three categories:
- Ultrasonic 3D imaging, known as ultrasonic pulse velocity (UPV) testing or tomography, as utilized by convolutional neural networks (CNNs, a type of AI) to create 3D gird images, often referred to as C-scans
- GPR coupled with augmented reality (AR) and/or CAD software to create 3D grid images for concrete scanning (i.e., Proceq's GPR Live Flexible Area Scan w/AR & 3D Visualization
- Multi-antennae, dual polarization array GPR units that cna capture 3D images of large subsurface areas for utility locating (i.e., Impulse Radar's Raptor series)
GPR technology that can couple with AR in real time can provide a snapshot view of conditions in 3D, but it merely provides a 3D grid view of the shapes encased in the slab. It does not translate those hazy hyperbolas into useable 3D objects for design and engineering purposes.
Almost all of the technology being labeled as 3D GPR requires extensive post-scan computer and CAD design of its data to create useable 3D objects.
UPV does not utilize GPR at all. It utilizes transmitters and receivers to listen for voids in the ultrasonic pulses (taps) being sent through concrete to create a kind of “heat map” of the interior of the slab. This technology is most often used in structural analysis or concrete testing, rather than to detect post-tension cables, conduit, and other reinforcements. When UPV data is fed through a CNN (which is a form of AI for imaging), it can be used to create a 3D image of the concrete, called a C-scan.
A recent journal article from Automation in Construction cites that “the process of generating C-scans is still immature, and yet to be standardized.” Further, their research shows that C-scans can “only show an approximation and not a true, full-resolution representation of subsurface objects.” Plus, it notes that the 3D imaging is cumbersome at best, allowing only “slices” of the slab to be viewed with no way to inspect the 3D data from various perspectives. While there is ongoing work to allow CNNs to utilize the GPR devices like the examples mentioned above, those involved have not yet been able to merge the two technologies.
Does 3D Ground Penetrating Radar Work?
The short answer is, yes, with a few caveats.
GPR is inherently “3D,” but what you can see via GPR is quite different from what you can use. And, while dual polarization array technology sounds like a way to capture swaths of GPR passes more quickly, it has only been perfected for large area scanning at present, and even those images require extensive post-scan processing to turn their data into useable objects.
Dual polarization means positioning GPR antennae on both the horizontal and vertical axes, to capture data. While there are some time-saving benefits to deploying a multi-arrayed GPR unit to cover acres of underground utility locates, you still need a highly skilled GPR operator to ensure accurate data capture. Even that accurately captured data requires analysis and interpretation to provide a comprehensive 3D model of what lies beneath the surface of your project so that you can utilize it.
In short, all GPR technologies require additional software, analysis, and a team of very talented CAD designers and 3D modelers to translate GPR data into useable objects that can be applied for the construction, engineering, and related industries’ use.
The difference between an accurate, usable 3D GPR image and a bad one is still just one thing: The experience and training of the person wielding the technology. Be it the Project Manager in the field applying the device to interpret the data or the CAD technician translating it all into a drawing, map, or model that can benefit you on site, the quality of the training and hands-on expertise can be the difference between a clean scan and a dangerous post-tension cable or conduit strike.
3D Technology with Millimeter Accuracy: Intelligent Visualization Via GPR + 3D Laser Scanning
There is another kind of 3D concrete imaging available – through the use of 3D laser scanning, which has proven effective in providing post-pour floor leveling and floor flatness data, as well as pre-pour PT cable mapping – to create highly accurate subsurface utility maps and 3D drawings & models of concrete slabs and their infrastructure.
GPRS has innovated a new application for this technology. By 3D laser scanning an area where a thorough, SIM-certified GPR concrete scan and mark-out has been conducted, our in-house Mapping & Modeling Team can merge the data from the concrete scan markings with millimeter-accurate 3D point cloud data to create comprehensive 3D model containing everything GPR has detected within the slab (or underground, when doing a GPR utility locate) as objects in three dimensions.
The major advantage of this variety of 3D concrete slab visualization is that it is backed with GPRS’ exclusive Green Box Guarantee and 99.8% accuracy rating. So, you know that when GPRS provides you with a 3D image of your slab, you can cut, core, or drill with confidence.
Is 3D GPR Better than Standard GPR?
At present, all “3D GPR” is limited to either hundreds of small passes of an area, or dual polarization, multi-array antennae technology that can cover large areas. While it can save time to conduct GPR grid scanning in this manner, the technology is not yet on par with the accuracy and viability of GPR for concrete imaging, especially when paired with accurate as-built data that can be supplied by 3D laser scanning technology and a highly trained, SIM-certified Project Manager.
To learn more about how GPRS can apply millimeter-accuracy with GPR to create accurate 3D concrete slab images, click here to speak to a Project Coordinator or to request a quote.
Other Frequently Asked Questions About Concrete Scanning
Does GPR work on concrete?
Yes, ground penetrating radar (GPR) uses radio waves to “bounce” off embedded items in a concrete slab without causing any damage to the structure. It can be used to find post tension cables, rebar, conduit, and other features within the slab so that you can cut, core, or drill with safety.
What machine can see through concrete?
Ground penetrating radar (GPR) and x-ray technologies are the most used to see what’s inside a cured concrete slab. Both can locate rebar, conduit, post tension cables, and other features. X-ray, however, uses radioactive isotopes to provide its images, which can create safety and health concerns on a jobsite. Ultrasonic pulse velocity and tomography technologies can also “see” inside concrete to assess the condition of a slab for cold joints, honeycombing, and deterioration.
How do you scan concrete structures or through concrete?
Generally, on a construction or renovation site, a handheld ground penetrating radar unit (GPR) is applied directly to the surface of floors, walls, ceilings and/or concrete supports to locate embedments like rebar, conduit, and post tension cables. GPRS utilizes GPR technology along with EM locating technology as needed to map the interior of a concrete slab so that customers can safely cut, core, or drill. Thanks to the application of Subsurface Investigation Technology (SIM), GPRS Project Managers maintain a 99.8% accuracy rate in concrete scanning and utility locating on more than 500,000 jobs.