3D NM-GPR
What is GPR?
Ground Penetrating Radar (GPR) is a well-established technique that uses ultrawideband electromagnetic (EM) signals to image subsurface features within soils and other non-conductive materials, including asphalt, road-base materials, timber and snow / ice.
Common civil engineering uses include locating subsurface utilities, mapping the pattern and depth of reinforcing steel within concrete, detecting subsurface voids or other defects within or below roads, detecting pits and other buried assets, and determining ‘as-constructed’ details of roads, bridges and airport pavements.
2D GPR data collected over a road culvert
Why 3D?
Most GPR’s collect 2D data. That is, they use an antenna to gather one vertical slice into the ground along a travelled path. A 3D GPR dataset, on the other hand, combines multiple 2D scans into a 3D data volume. That data volume can then be sliced vertically or horizontally to reveal subsurface features at different depths and spatial locations.
Gathering 3D data can be slow and tedious if you’re only gathering one scan per pass. Dedicated 3D GPR hardware makes collection of these data much faster and easier by using an antenna array to gather multiple scans at the same time. Data from multiple 3D passes can then be combined to cover large & complex sites. For instance sites incorporating multiple roads, airport runways, parks, etc.
3D GPR datasets are particularly useful because horizontal slices (i.e. plan views) cut through the 3D data are often often more intuitively understood by clients unfamiliar with GPR, while retaining the ability to cut the data vertically. Nonetheless, GPR interpretation can be a tricky affair, and expert guidance is recommended to ensure observed features are correctly interpreted.
What is Noise-Modulated GPR?
Noise-Modulated GPR is a form of GPR technology that uses coded signals. Like impulse and stepped-frequency technologies, Noise-Modulated GPR systems produce measurements of signal reflection strength versus two-way travel time. Unlike those other methods, however, it does so by emitting a special coded signal, measuring the reflected response and cross-correlating the emitted and measured signals to produce A-scan (‘trace’) profiles.
Is NM-GPR the same thing?
Partially. ‘NM-GPR’ refers to a particular type of 3D GPR technology developed by CodedRADAR. As the name implies, it uses a form of noise-modulation (i.e. coded signals). Importantly, however, it implements this approach using novel techniques and a radically different approach to GPR hardware.
Why use a different approach?
Fast 3D data collection places significant demand on GPR hardware. At highway speeds, conventional GPR technologies struggle to keep up with this demand. As a result, GPR operators often need to reduce collection speed or compromise the penetration depth, measurement spacing in the direction of travel, number of GPR channels, frequency range, or data quality, to keep the equipment within its operational limits.
NM-GPR’s unique approach enables it to measure significantly faster than all other 3D GPR technologies, avoiding the need to compromise. It achieves this through techniques and hardware that enable much faster signal transmissions, more rapid return signal measurements & the ability to listen on multiple antennas at the same time.
Signals emitted by NM-GPR systems are also much weaker than those from conventional GPR systems. As a result, they can be transmitted much more quickly while remaining within regulatory limits. Return reflections are then sampled at a blisteringly-quick rate of 10 Gigasamples/second (0.1ns intervals) using low-fidelity hardware. Sampling in low-fidelity appears counterintuitive, however it is so fast that high-quality data can still be produced (via appropriate techniques) while massively boosting the system 3D data collection capacity.
As it is currently the only approach capable of measuring each GPR return signal sequentially at 0.1ns intervals, NM-GPR is arguably the only true high-frequency real-time sampling GPR technology available today.
Furthermore, each NM-GPR system operates multiple dedicated hardware receivers (4 or 8), enabling it to listen to many receive antennas at the same time.
This combination of much faster signal transmissions, lightning-fast sampling, multiple receivers, and use of comparatively simple hardware makes these systems blisteringly quick, reliable and capacious. As a result, NM-GPR systems are unmatched for rapid 3D subsurface imaging applications.
Surface + subsurface
In addition to gathering 3D NM-GPR data, the Kerberos™ and Lynx systems can also capture geolocated surface imagery in the same pass. This provides a wealth of benefits for site investigations, including:
- Detailed snapshot: Provides a detailed record of current surface conditions throughout a site, to document or monitor changes over time.
- Relate cause and effect: Compare the location of subsurface defects and subsurface features or changes that may be causing them.
- Better site context: Use positions of manholes, pits, drainage inlets, bridge joints and other surface features to better inform & guide interpretation of 3D GPR data.
- Reliable positioning: As the surface and subsurface images are synchronized, it is possible to locate subsurface features by measuring their location relative to surface features. This feature is particularly useful for locating voids or defects within ‘urban canyons’ where GPS reception is often unreliable.
TerraVerse™
CodedRADAR’s TerraVerse™ software enables 3D NM-GPR and surface imagery data from hundreds of scans to be combined into large geospatially-corrected mosaics. This enables large assets such as roads, bridges, airports, ports, parks, gas stations, carparks to be covered in detail and later viewed and analysed as one model, for a holistic understanding of surface and subsurface features.
Auxiliary sensors
For more than 15 years we have created systems that integrate 3D NM-GPR with cameras and a variety of off-the-shelf and custom sensors. This includes sensors used for transverse laser profiling (e.g. measuring ruts), inertial measurement of vehicle motion (IMU), illuminated road surface imaging (i.e. for site context; mapping surface defects), roughness measurements (IRI), LiDAR, and a patented photometric stereo technique.
When you need advanced sensors that complement and make better use of 3D GPR data, CodedRADAR is your one-stop-shop.
