What is GPR?
GPR stand for Ground-penetrating radar which is a geophysical method that uses an electromagnetic energy signal to image the subsurface. A GPR usually comprises of a transmitter, a receiver and a signal encoder. When the emitted energy encounters a buried object or a boundary between the materials having different permittivities, the signals are returned to the receiver and it records a two-way travel time and the strength of the signal. For best results a contrast in electrical properties is needed between the target and host material.
Radar travels at different speeds through different mediums and the more conductive or saturated the medium, the more the signal will attenuate and may limit the effective depth range of GPR investigation.
High and Low Frequency
GPR comes in many different sizes and shapes depending upon the application. High frequency GPRs are for shallow depths and have an improved resolution which is ideal for concrete scanning, whilst lower frequencies are ideal for deeper depths and larger targets such as utility mapping applications.
A few things worth noting with GPR surveys:
- The depth of ‘standard’ GPR for utility mapping is ‘approximately’ 2-3 metres. So it is enough to detect most services underground, however it does have range limitations depending upon the conductivity of the ground.
- The device can’t tell you what the items are – The results are interpreted by the surveyor, not the GPR
- The GPR can’t see through items, so if a large pipe is set directly above two smaller pipes, it is unlikely that the equipment will be able to see them underneath the large pipe
- GPR surveys usually go hand in hand with standard topographic surveys. This gives the utilities survey context and essentially gives spatial meaning to the Utilities Survey. Without the topo aspect of the survey there is nothing to relate the utilities spatial data back to.
Prevent Asset Strikes
One of the main aims of our utilities surveys is to prevent asset strikes when digging on site. Obviously striking assets can be disastrous and extremely dangerous, GPR therefore is used to detect assets, map them out relative to a topographical survey and identify where assets are so they are not accidentally struck during the project, making them safer and cost effective.
Features that can be found using GPR are
- Buried metallic and non-metallic utilities
- Underground voids or air pockets, including basements and culverts
- Buried objects, such as underground storage tanks, foundations or unmarked burials
- Concrete reinforcement bars
What is Post-Processing?
When we undertake a GPR survey that requires post-processing, radar data is collected in a tightly spaced orthogonal grid, stored, and taken away for analysis away from site. Post processing is a desktop exercise by which an analysist can take considerably more time to analyse and assess the obtained data as well as building up a bigger picture of the area by making comparisons, and joining together the hyperbolas generated by the radar data to create strings in CAD that represent a likely utility service or underground feature.
Please get in touch with Senior Survey Manager, Michael Wood, to find out more