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415-553-0129
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909-609-5265
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858-395-1085
North County
760-574-0436

 
 
 
 


Limitations of GPR / Definition of GPR

Definition/Description of Ground Penetrating Radar:

  • Ground-penetrating radar (GPR) is a tool that uses radar pulses to image the subsurface.
  • This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR uses high-frequency (usually polarized) radio waves and transmits into the ground. When the wave hits a buried object or a boundary with different dielectric constants, the receiving antenna records variations in the reflected return signal. (credit: Wikipedia)  

    Definition of GPR KY         
  • The spectrum of GPR reflectivity (electromagnetic conductivity) is defined where air is the least reflective and water is the most reflective to the radar pulses.

It is important to have a basic working understanding of ground penetrating radar as the limitations are in direct correlation to the technology and science.

GPR Limitations with HIGH frequency antennas (often used for concrete scanning applications):

  • Moisture – It is known that water is the most reflective material a radar pulse will encounter. Given this information it is easy to understand why moisture presence in a concrete slab would inhibit the effectiveness of GPR. Moist or ‘green’ concrete can be problematic for GPR as the presence of moisture will reflect/inhibit the passage of the radar pulse and thereby limit penetration and data quality.
    • Elevated Concrete: Scanning of elevated concrete slabs is most commonly done prior to core drilling for the purpose of mapping embedded objects and reinforcing steel. It is recommended that an elevated slab be allowed to cure up to 3-4 weeks where slab thicknesses of <12” are present. Further, 6-8 weeks of curing is recommended for slabs with a thickness of >12”.
    • Slab-on-Grade Concrete Applications: Scanning a slab-on-grade is most commonly done prior to saw cutting or demolition for the purpose of mapping shallow utilities and electrical conduits. The same recommendations are suggested with regards to moisture presence in the concrete. Further, if moisture is present in the soils/sub-grade immediately below the concrete the radar pulses can be inhibited and accuracy/penetration will be limited.
  • Depth Penetration – The depth range of GPR is limited by the electrical conductivity of the ground, the transmitted center frequency and the radiated power. As conductivity increases, the penetration depth decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth. Higher frequencies do not penetrate as far as lower frequencies, but give better resolution. Good penetration is achieved in dry sandy soils or massive dry materials such as granite, limestone, and concrete. (credit: Wikipedia)
      • Frequency – High frequency antennas (1000MHz – 2.6GHz) are typically able to achieve signal penetration ranging between 12”-30”. 
        • Rule of Thumb: The higher the frequency the less the signal is able to penetrate the medium. However, higher frequency antennas will provide a higher image resolution with more data detail for interpretation. This trade off is present across all GPR antennas.
      • Medium/Composition - The composition of both the concrete and sub-grade soils is paramount to achieving maximum signal penetration with high frequency antennas. As noted above, moisture presence will limit the depth penetration of GPR. Further, sub-grade debris and other random anomalies can reflect the radar signal before it is able to achieve maximum penetration. Lastly, the quantity of reinforcing steel present within the concrete can also reflect the signal and thereby limit its ability to penetrate the concrete.
  • Size of Target – There are two main ways in which GPR is limited when discussing the size of a target.
    • Diameter of Target: GPR technology is unable to determine the size/diameter of the target being located. Ground penetrating radar is collecting a 2-Dimensional slice through the scanned medium, such as concrete, and therefore does not detect the entire circumference of the anomaly being located.
    • Level of Detail when locating a Target: While it is possible to locate many objects with GPR there can be objects that are simply too small for the radar to find. While this limitation is more widely experienced while using low frequency antennas (which provide lower resolution data) it can be an inhibitor when scanning concrete with a high congestion of reinforcing steel or small objects buried a depths more than 10”.
  • Composition of Target – It is possible for GPR to locate any target/anomaly possessing a differing electromagnetic conductivity.  That being said, there are objects which are found more easily. It is true that GPR can locate empty plastic (PVC) conduits and pipes; however, it is easier to locate a highly reflective piece of metal reinforcing steel. Therefore, composition of the target can be a limitation with regards to locating. For example, it will likely be very difficult to locate a plastic conduit running below a tightly spaced grid of reinforcing steel as the rebar is a positive reaction, showing more clearly in the data, than the negative reaction of the plastic conduit. Again, it is possible to locate any object with a differing electromagnetic conductivity but there are objects which are easier to locate as their reflectivity is much higher or lower, by contrast, to the surrounding scanned medium or located anomalies.
  • Quantity of Anomalies – Our GPR limitations noted above have briefly mentioned this constraint. The quantity of reflective objects found in the scanned medium (concrete or other) can have a direct impact on GPR signal penetration and the radar’s ability to locate objects at greater depths. This is a direct result of the signal not being able to penetrate (as it is reflected back to the surface) beyond the initial layer of anomalies. This is often experienced, when scanning concrete, on an elevated slab near a column.
  • Concrete Conditions – The surface conditions of the concrete can also pose limiting factors to ground penetrating radar. Concrete surfaces where standing water is present are unable to be scanned as the standing water will immediately reflect the signal before it penetrates the concrete. Further, concrete surface that are rough or pitted can affect the antennas ability to directly couple with the surface and thereby limit the signals ability to penetrate the concrete. The best concrete scanning conditions are found over bare, smooth concrete where there is little to no moisture present.
  • Data Interpretation – The technician providing the GPR scan is potentially the biggest limiting factor involved with this science. Training an individual to operate ground penetrating radar technology can be done in a few hours; however, teaching an individual how to interpret the data he or she receives with the equipment can take long periods of time. The highest quality equipment operated by an inexperienced technician will offer little information to the customer as the ability to interpret the data is essential. In short, the technology/science of ground penetrating radar is only as good as the operator’s expertise in data interpretation. 

GPR Limitations with LOW frequency antennas (often used for utility locating applications):

  • Moisture – It is known that water is the most reflective material a radar pulse will encounter. Given this information it is easy to understand why moisture presence in the soils would inhibit the effectiveness of GPR.   Wet soils encountered while conducting an underground utility search are problematic and limiting. Further, the elevation of the water table present in the soils will, likely, directly correlate to the maximum depth penetration of the radar.
  • Depth Penetration – The depth range of GPR is limited by the electrical conductivity of the ground, the transmitted center frequency and the radiated power. As conductivity increases, the penetration depth decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth. Higher frequencies do not penetrate as far as lower frequencies, but give better resolution. Good penetration is achieved in dry sandy soils or massive dry materials such as granite, limestone, and concrete. In moist and/or clay-laden soils and soils with high electrical conductivity, penetration is limited. (credit: Wikipedia)
    • Frequency – Low frequency antennas (100MHz – 900Hz) are typically able to achieve signal penetration ranging between 4’-30’. 
      • Rule of Thumb: The lower the frequency of the antenna the further the signal is able to penetrate the medium. However, lower frequency antennas will provide a lower image resolution with less data detail for interpretation. This trade off is present across all GPR antennas.
    • Medium/Composition - The composition of the soil is paramount to achieving maximum signal penetration with low frequency antennas. As noted above, moisture presence will limit the depth penetration of GPR. Further, sub-grade debris can reflect the radar signal before it is able to achieve maximum penetration.
  • Size of Target – Low frequency antennas experience limitations regarding the size of target being located as there is a loss of resolution regarding the image available for interpretations. There are two main ways in which GPR is limited when discussing the size of a target.
    • Diameter of Target: GPR technology is unable to determine the size/diameter of the target being located. Ground penetrating radar is collecting a 2-Dimensional slice through the scanned medium, such as soil, and therefore does not detect the entire circumference of the anomaly being located.
    • Level of Detail when locating a Target: While it is possible to locate many objects with GPR there can be objects that are simply too small for the radar to find. This limitation is most frequently experienced with low frequency antennas and specifically in reference to utility locating projects.  Given the noted resolution loss, variance of soil compositions and other factors the following rule is a general guideline for locating small objects with ground penetrating radar.
      • Rule of Thumb: For every foot deep an object is buried it will need to be at least one (1) inch in diameter. In order for us to locate a utility at 4’ (48”) deep the pipe would need to be at least 4” in diameter.
  • Quantity of Anomalies – The quantity of reflective objects found in the scanned medium, such as soil, can have a direct impact on GPR signal penetration and the radar’s ability to locate objects at greater depths. This is direct result of the signal not being able to penetrate (as it is reflected back to the surface) beyond the initial layer of congestion.  Areas containing a high quantity of utilities may be problematic as the radar is unable to locate smaller objects or objects buried below other existing utilities. Again, if a shallow object reflects the radar pulse back to the surface it can, in essence, eclipse the utility buried just below.
  • Composition of Target – While it is possible for GPR to locate any target/anomaly possessing a differing electromagnetic conductivity there are, often, objects that are found more easily. GPR can locate empty plastic (PVC) conduits and pipes even though plastics are considered non-conductive.  It is, however, easier to locate conductive anomalies, such as metal pipes, as they posses greater reflectivity. Therefore, composition of the target can be a limitation with regards to locating. For example, it will likely be very difficult to locate a small plastic conduit running below three large metal water lines in the ground.  The positive reactions will likely make the plastic conduit invisible given the strength of their reaction and the way in which they reflect the GPR signal. Again, it is possible to locate any object with a differing electromagnetic conductivity but there are objects which are easier to locate as their reflectivity is much higher or lower, by contrast, to the surrounding scanned medium or located anomalies.
  • Soil Composition – The composition or materials which make up the soil through which the radar pulse is attempting to travel is a limiting factor. Highly conductive soils, such as clays, can limit the penetration ability of the radar pulse. By contrast, non-conductive soils, such as sandy porous soils, can greatly increase the ability of the radar pulse to penetrate the ground. The less conductive soils offer less ‘interference’ or reflection of the radar pulse and thereby allow it to penetrate to greater depths. The opposite is true of highly conductive soils. This can be problematic as soil composition can change wildly across any given inspection area. It is best to consult a Soil Suitability Map when considering GPR services.
    • Soil Suitability Map – See map included with this document for more information.
  • Data Interpretation – As noted above, the technician providing the GPR scan is potentially the biggest limiting factor involved with this science. Training an individual to operate ground penetrating radar technology can be done in a few hours; however, teaching an individual how to interpret the data he or she receives with the equipment can take long periods of time. The highest quality equipment operated by an inexperienced technician will offer little information to the customer as the ability to interpret the data is paramount. In short, the technology/science of ground penetrating radar is only as good as the operator’s expertise in data interpretation. 

  GPR Soil Suitability Map

GPR Soil Suitability Map

 

 

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