Ground penetrating radar (GPR) is one of the most accurate non-destructive testing methods available for utility locating and marking. You can get the most from GPR surveys by following best practices regarding target characteristics, proper survey design, correct antenna selection, and system setup.
Know Your Target Characteristics
Before beginning a GPR survey, carefully assess the target and the surrounding material.
GPR equipment is looking for differences in materials, so surveying may be more difficult if the two materials are similar in properties and the way they react to the GPR signal.
Electrical conductivity of the surrounding material affects how deep surveyors can see.
GPR is best for inspecting materials with low electrical conductivity, like concrete, sand,
wood, and asphalt. If a material is conductive (clay soil, wet concrete), GPR energy cannot
penetrate very far into the material.
In addition to the target’s electrical characteristics, other target characteristics to consider
include size, type and orientation of utility lines, post tension cables, and presence of objects like tanks and graves. With regard to size, the general industry rule of thumb is GPR can image targets one inch in size per foot of depth. So at two feet, the target should ideally be two inches in size or greater. At five feet, it should be five inches or greater.
Survey Design Considerations
In situations where utmost confidence is necessary, the survey design should include a bi-directional grid, with spacing between the lines equal to the smallest dimension of the targets. If this is impossible, due to time or budget constraints, consider use of a large overview grid, followed by one or more smaller focused grids. Bi-directional grids are best for investigations of complex utility areas and cylindrical objects like old oil tanks/buried drums.
The survey can use either 2D or 3D scanning methods. With 2D scanning, the surveyor collects simple individual profiles and interprets data live on the screen. 2D surveys are much quicker and easier than 3D surveys. With 3D scans, operators place a grid on the ground and scanners collect information on parallel and perpendicular lines. Then, software (either onboard the system or in office) takes the individual profiles and models them together to get a top-down birds-eye view, almost like an MRI of the ground. This type of bi-directional 3D survey is effective where the targets move in unknown directions,
if one requires high confidence in a target’s location, where a good visual image is needed for a report, or if there are any potential site dangers.
The Right Antenna will Make a Difference
Selecting the right antenna frequency is critical to getting the most accurate GPR target location information. The choice depends on the target size and depth, as well as the material imaging through. Lower frequency antennas (200 MHz or lower), will allow surveyors to see deeper, but at lower resolution. Targets must be larger to be seen.
For scanning utilities, mid-range antennas are often a good choice. For example, GSSI
recently developed technology, called HyperStacking, to see deeper targets and operate in conditions considered too “noisy” for conventional systems. The antenna design also includes a new accessory which helps locate specific power sources. The new technology adds the ability to detect power radiated from electrical cables, or induced radio frequencies, and fuses it together with the GPR data onto a single display. Users can produce an overlay on the radar data that shows if a specific pipe is present. Many GPR systems are also equipped with GPS integration, which can be used for providing location information in the radar data file.
System Setup is Critical
Operators must set up the system to ensure they are selecting the correct depth level and
collecting the proper number of scans to obtain the data density necessary to find what they are looking for. Start by consulting as-built plans and look for site features that provide clues to pipe location or orientation, like manholes or catch basins. Set the site survey boundary properly to capture what is required. The proper number of scans varies depending upon the size of the target, the soil and the material one is working in. A smaller scan spacing (more scans per unit) slows down the survey, so the rule of thumb is to collect as many scans as possible while maintaining an acceptable survey speed. More scans are required for smaller targets. For example, if the target is only one inch, take two or three scans per inch to guarantee getting a hit on the target.
Gather All the Site Information You Can Find
Gather all available site information, including site plans and as-built drawings, as well as
library and town hall records. Talk to site maintenance people and those familiar with local site conditions. Useful resources include the U.S. Department of Agriculture’s Ground- Penetrating Radar Soil Suitability Maps, and the USD A Natural Resources Conservation Service Web Soil Survey, which provides online soil maps and data.