Ground Penetrating Radar
Ground Penetrating Radar (GPR) is a non-destructive means of generating a profile of subsurface structures and features. GPR produces an image of underground features which can be used to identify the location and depth of buried structures. The images produced by GPR can also aid in determining the type, structure and condition of soils.
GPR units consist of an antenna which is dragged or rolled over the soil in a cart, and a video screen/data logger to record the received signals. The GPR antenna transmits pulses of high frequency radio waves down into the soil. The transmitted radio waves are then reflected back to the antenna. Variations in soil conditions and buried objects alter the reflected radio waves, which in turn produce the digital images that are recorded by the GPR. The GPR does not actually produce an image of a buried object. This instead produces a reflection of the object. This is read and interpreted by our trained technicians to indicate the presence of a specific object or utility.
Although GPR is very useful for scanning large areas and for determining the location and depth of previously unknown objects or soil conditions, it does have limitations that must be understood.
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Soil Conductivity - This is a measure of how well an electromagnetic signal may pass through a material. The lower the soil conductivity the farther a signal may travel. In terms of GPR units, the lower the soil conductivity the deeper they can “see” into the soil. Dry, sandy or granite soils have a low conductivity. Clays, shale or soils saturated with water have a high conductivity and consequently may limit or even eliminate the ability of the GPR to penetrate the soil.
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Antenna Frequency - The frequency of GPR antennas affects how deep they can see and how much detail they can resolve. Low frequency antennas (25-200 MHz) can penetrate deep into soils but provide low image resolutions. These are typically used to identify soil geologies and structures such as soil stratification, sinkholes and fractures. High frequency antennas (300-1000+ MHz) can only penetrate a short distance into soils (from 10-15 feet down to a few inches) but can produce very detailed images of the soil and buried objects.
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Pipe / Cable Diameter - The signal that is broadcast from a GPR antenna extends downward and outward, much like the ripples in a pond. The further the pipe or cable is from the surface and the GPR antenna the bigger in diameter it must be in order to be “seen” by the GPR unit. The rule that is generally applied is for each foot in depth that an object is from the surface it must increase by an each inch diameter. For example a pipe buried at four feet deep must be at least four inches in diameter. Small diameter pipes or cables or those that are buried very deeply may not be locatable by means of a GPR unit.
