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.
An example of 2D GPR data collected over a road culvert
Why 3D?
Most GPR’s collect 2D data. That is, they use an antenna to gather a vertical slice into the ground along a travelled path.
3D GPR systems, on the other hand, gather multiple adjacent 2D scans per pass using an antenna array. The 3D data they collect are particularly useful as the dataset can be cut vertically along different orientations, or horizontally to reveal a plan view of features at different depths.
Data from multiple 3D GPR scans can also be combined to enable identification of subsurface features over large areas.
What is Noise-Modulated GPR?
Noise-Modulated GPR is a form of GPR technology that uses noise-like coded signals to perform subsurface imaging. Like more common pulsed and stepped-frequency technologies, Noise-Modulated GPR systems produce measurements of signal reflection strength versus travel time. This is achieved by emitting a special coded signal, measuring the reflected response and cross-correlating it with the emitted code to produce the familiar GPR A-scan (‘trace’) profile.
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. Importantly, however, it implements this approach using multiple simultaneous receivers that employ a unique ultra-rapid low-fidelity sampling technique.
Low-fidelity sampling?
Sampling GPR return signals in low fidelity may at first seem counter-intuitive. This approach, however, enables the use of simple hardware that can measure much more quickly and efficiently than conventional methods. The measurement capacity benefits achieved are significant and they vastly outweigh any drawbacks, considering that high quality GPR data can be produced from low-fidelity measurements through the use of dithering and stacking techniques.
The use of simplified hardware is also highly advantageous, as it enables multiple dedicated transmitter and receiver circuits to be integrated into the same GPR digitiser, without it becoming overly complex and unreliable.
An analogy to this are the cores within a modern computer’s Central Processing Unit (CPU). A CPU with more dedicated cores has greater capacity to perform calculations as it distribute the task across more resources. Likewise, having multiple dedicated radar circuits within the NM-GPR digitiser means it can transmit and receive on multiple antennas at the same time, significantly increasing the overall system collection capacity.
Why do we need a different approach?
As 3D GPRs must gather data at regular intervals on so many simultaneous channels, this places significant demand on the radar system’s collection capacity. At times conventional GPR technologies struggle to keep up with this demand, requiring lower collection speeds or compromises in penetration depth, measurement spacing or data quality to keep within operational limits.
The NM-GPR signal and sampling approach is unique within the field of GPR. NM-GPR receivers measure the return signal fast. Really fast. And there are many receivers (4 or 8) operating simultaneously within each NM-GPR digitiser unit. This superior sampling speed, use of multiple receivers and the large improvement in collection capacity that results are needed to enable much faster 3D GPR data collection that does not compromise data spacing, depth or quality.
The NM-GPR approach enables more of the reflected signal to be measured after each transmission. While conventional technologies can only sample one point or a handful of points per return signal, NM-GPR receivers sample each return signal sequentially at 0.1 nanosecond intervals.
As it is the only GPR technology capable of sampling GPR returns at anywhere near this rate, thus NM-GPR is arguably the only true real-time sampling GPR technology available today.
Its lightning-fast sampling capabilities, superior receiver efficiency, simplified radar architecture, and use of multiple simultaneous radars makes NM-GPR-equipped 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.
