HOW DOES GPR WORK?
Every GPR system contains one or multiple antennas. These antennas transmit and receive radio frequency waves. Upon transmission, these radio waves penetrate the surface that the user intends to investigate if and when the antenna is touching or in close proximity to the surface. While much of the signal dissipates, a percentage of the signal comes back to be received by the antenna. This return signal brings data that can be recorded, interpreted and manipulated. The data shows up on a control unit as images that give the operator the ability to see the size of objects and the depths at which they are located.
GPR has the same basic principles as a metal detector. A metal detector sends energy into the earth in up to 17 frequencies. When that energy meets a metallic object, it is translated into a recognizable tone. The GPR sends out millions of frequencies that return to the antenna and translate material composition definition in the subsurface.
Radar is sensitive to changes in material composition. Detecting these changes requires movement. In the case of air traffic control radar, the targets are moving, so a stationary transmitter works. In the case of GPR, we are looking for stationary targets, so it is necessary to move the radar to detect the target.
WHAT CAN GPR FIND?
US Radar GPR Systems are designed to display differences in material composition. They can be used to locate any object that has a different composition than it's surrounding materials. For example, a PVC pipe will have a different composition than the surrounding soil. Voids and excavations that have been filled in will also have different compositions than the surrounding soil. However, GPR does not know what the actual materials are that it is imaging. For this reason, it is not suited to locating gold, precious gems, and treasure.
Here is a list of things GPR can find:
HOW DEEP DOES GPR GO?
The depth of your findings will be determined by three factors:
- Soil type
- Antenna frequency
- Size of Target
The radar signal is attenuated or absorbed differently in various soil conditions. Dense wet clays are the most difficult material to penetrate whereas clean dry sand is the easiest. Lower frequency antennas will yield greater depth penetration, however, the minimum size of an object which is visible to the radar increases as the antenna frequency decreases.
|Antenna||Approx. Penetration in Dense Wet Clay||Approx. Penetration in Clean Dry Sand||Example of smallest visible object|
|100 MHz||20ft (6m)||60ft+ (18m+)||Tunnel @ 60ft (18m) depth
2ft (60cm) Pipe @ 20ft (6m) depth
|250 MHz||13ft (4m)||40ft (12m)||3ft. (90cm) Pipe @ 12m
6in. (15cm) Pipe @ 13ft (4m)
|500 MHz||6ft. (1.8m)||14.5ft. (4.4m)||4in. (10cm) pipe @ 4m
3/16 in. (0.5 cm) hose 1.8m and less
|1000 MHz||3ft (90cm)||6ft (1.8m)||3/16 in. (0.5 cm) hose @ 3ft. (90cm)
Wire mesh, shallow
|.5 ft. (15cm)||2ft. (60cm)||Monofilament fishing line|
The 500 MHz antenna is the antenna which is most widely used for locating utilities.
The 1000 MHz antenna is the most widely used for locating rebar and utilities in walls and floors.
Note that in many cases if it is not possible to penetrate to the depth of a buried utility due to soil conditions, it is still often possible to detect the disturbed soil from the original excavation.
A common misconception is that the size of the antenna affects the amount of area covered. This is not the case. The size of the antenna relates to the frequency of the antenna and subsequently, the depth that it can penetrate (for more information see: How deep does it go?). While the signal from a GPR antenna does spread in the direction of travel, the lateral width which it scans per pass is razor thin regardless of the antenna used. Furthermore, targets are most easily identified with GPR when the survey path is perpendicular to the orientation of the target. For this reason, surveys are usually conducted on a grid in two perpendicular directions:
|A typical GPR survey pattern for ground surveys||A typical GPR survey pattern along a proposed trenchline|
The spacing of the grid is determined based on the size of the targets that need to be identified and what sort of results are going to be produced from the survey. Typical grid spacings can be 1m, 3ft, 5ft, 10ft, 20ft for ground surveys and 1in-1ft. for walls and floors.
The speed at which data can be collected along a survey line is limited by two factors: 1) any time spend interpreting real-time data and/or spent doing on the spot markout 2) The Seeker SPR is capable of capturing data at highway speed, so the main practical limitation is keeping the antenna in smooth contact with the ground.
CAN IT SEE THROUGH EVERYTHING?
Almost. Radar is the only remote sensing technology that can detect both conductive and non-conductive materials. Although radar can easily see conductive materials such as metal and salt water, it cannot see through them. Also, concrete is conductive when it is fresh, but becomes non- conductive as it cures.
Generally, GPR will reveal the horizontal positioning of targets in their exact locations, however, there are a number of factors which can affect the accuracy of the depth measurements. The speed of the radar signal is dependent upon the composition of the material being penetrated. The depth to a target is calculated based on the amount of time it takes for the radar signal to be reflected back to the antenna. Radar signals travel at different velocities through different types of materials. The moisture content of the material also affects the velocity of the signal. It is usually not possible to know the exact velocity that the radar signal travels through a material, however it is usually possible to estimate this to within +/- 10%. It is possible to use a depth to a known object to determine a precise velocity and thus calibrate the depth calculations. This technique only works well however, when the material being investigated has a consistent composition such as concrete. When investigating underground, the inescapable limitation is that due to natural differences in the composition of the geological layers, the exact velocity will vary from one point to the next. There are some techniques for modeling the variations in velocity along the path of a survey, however, ultimately these are all estimations and none are completely precise.
WHAT SYSTEMS DO YOU RECOMMEND?
The system that a user will use depends largely on what application they are using it for. The application often determines the depth of penetration the user will need, as well as the resolution, which represents the overall detail of the data. Users who need data from different depth ranges, who want more comprehensive data, and who have broader needs require multi-frequency systems. Users who are locating specific objects within one depth range will only require single-frequency systems.
Here is a chart of where our products fit within particular depth ranges, applications, and other factors one may consider in purchasing ground penetrating radar:
If you want to learn more about our products, please visit our products page.
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