GPR (Ground Penetrating Radar) is still the most non-destructive accurate testing method used for utility marking and locating. To make sure you are getting the best results from GPR surveys you should be adhering to the best practices concerning target characteristics, system setup, and the right antenna selection.
Knowing Your Target Characteristics
Before you start a GPR survey, assess both the target along with any surrounding materials very carefully.
GPR equipment is designed to look for any differences in the materials, which can make surveying difficult if the materials have similar properties or the way they are reacting to the GPR signal.
The electrical conductivity of surrounding materials will have an effect on how deep a surveyor can see.
Precision Utility Mapping GPR Surveys is best suited to inspecting materials that have a lower electrical conductivity, such as sand, concrete, asphalt, and wood. If the materials are conductive (wet concrete or clay soil) GPR energy won’t be able to penetrate that far into these materials.
Over and above the electrical characteristics of the target, there are also other types of target characteristics that need to be considered. This includes the utility line orientation, type and size, the post-tension cables, and if any other objects are present such as graves or tanks.
When it comes to size, the rule of thumb for GPR is that it can image targets 1-inch-in-size per a foot-of-depth. So at 2-feet, ideally the target needs to be 2-inches in size or even greater, or at 5-feet, the target should ideally be 5-inches or greater.
Survey Design Considerations
In the situations where absolute confidence becomes necessary, the design of the survey must include something known as a “bi-directional grid”, and the spacing between each line must be equal to “the smallest dimensions” of each target.
If this is not possible, due to either budget or time constraints, you can also use a larger overview grid, along with one or two smaller-focused grids. The bi-directional grid is the best option to investigate complicated utility areas or cylindrical objects such as old buried drums or oil tanks.
The survey should use either a 2D or 3D scanning method. With a 2D scan, a surveyor will be collecting basic individual profiles where the data is interpreted on a live screen. 2D surveys are a lot easier and faster than 3D surveys.
With a 3D scan, the operator will use a grid placed over the ground. The scanners then collect information along perpendicular and parallel lines. Then, software (either onboard the system or in-office) will take each individual profile and then model these together which provides a birds-eye top-down view similar to an MRI of that piece of ground.
This bi-directional 3D survey is the most effective for when targets are moving in an unknown direction, when high confidence is required in the target location, when better visual imagery is required for the report, or when there might be a potential danger on site.
The Correct Antenna Will Make A Difference
Choosing the correct antenna frequency will be vital to obtaining the most precise information for a GPR target location. The choice of an antenna will depend on the depth and size of the target, and the material-imaging through. A low-frequency antenna such as 200 MHz or lower allows a surveyor to view a bit deeper, but the resolution will be lower. Targets have to be larger in order to be detected and seen.
For the purpose of scanning utilities, a mid-range antenna is in most cases the best choice. For example, GSSI recently developed a technology known as HyperStacking to view deeper targets along with operating in conditions regarded as too “noisy” for a conventional system.
The design of the antenna comes with a newly-developed accessory that assists with locating exact power sources. This latest technology has added the capability to pick up power that is radiating from and induced radio frequency or electrical cables and fuses these together with GPR data on a single display.
On the radar data, a user can also produce overlays that will show if specific pipes are present. Many of the GPR systems also come equipped with GPS integration, which is often used to provide location information within a radar-data file.
System Setup Is Vital
Operators must ensure the system is set up correctly by choosing the right depth level. They should also collect the correct scan numbers to obtain the data density that is needed to find exactly what the surveyor is searching for. You can start off with as-built plans where you can look for the site features which will provide an idea of the pipe orientation or location, such as catch basins or manholes.
Set the boundary for the site-survey correctly to capture everything that is required. The correct scan numbers will vary according to the target size, the soil, along with the materials one has to work with. Smaller scan spacing (more than one scan per unit) will slow the survey down, so it is best to collect the highest number of scans while maintaining acceptable survey speeds.
Smaller targets will require more scans. For example, when a target is just 1-inch, take at least 2 to 3 scans per inch which will increase the likelihood of hitting the target.
