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Frequently Asked Questions
  
  1. What is GPR?
  2. Why use GPR?
  3. How do I use GPR for Utility Locating?
  4. What are the pitfalls of GPR?
  1. What is GPR?
    Ground Penetrating Radar (GPR) is the general term applied to techniques which employ radio waves, typically in the 1 to 1000 MHz frequency range,  to map structures and features buried in the ground (or in man-made structures).
    • acronym for Ground Penetrating Radar
    • ground can be soil, rock, concrete, wood - anything non-metallic
    • emits a pulse into the ground
    • records echoes
    • builds an image from the echoes


    GPR Image Creation


     
    As the above animation illustrates, point targets shows as inverted Vs in the GPR data. The apex of the hyperbolas indicate the location and depth of the utility.Back to top
  2. Why use GPR?

    The tools and methods of locating buried utilities are quite diverse. The most common approach is energizing metal pipes and cables with electric currents and using a magnetic field sensor to detect the current. Provided the target object can be exposed for connection or current can be induced, sufficient current remains on the object and the magnetic field at the detector is strong enough, then this technique works well and is very cost effective.
     
    When access is difficult, the electrical current does not flow (i.e. non-metallic element or broken connection), external noise makes detection impossible, GPR provides an alternative. GPR provides its own source of energy, detects both metallic and non-metallic objects, detects disturbed soil conditions and other buried structures.

    Other direct approaches are to trench, hand dig, or vacuum excavate to expose features. A priori knowledge and accurate as-built drawings are needed to be effective with these techniques.  Generally, these are not available or sufficiently accurate. 

    The common sense approach for a locating group is to use all of the tools available.  Understanding where and when a particular approach is most cost effective comes from experience, business practice and local construction.


    Expand your locate capabilities

    • Locate plastic and concrete
    • Resolve damaged or missing tracer wires
    • Unscramble densely packed installations
    • Discover undocumented utilities
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  3. How do I use GPR for Utility Locating?

    Locate & Mark

    The locate and mark is most common and is very similar to the use of traditional current tracking utility detectors.  The GPR sensor is moved along sweeps perpendicular to the anticipated utility axis. When the GPR unit crosses the utility, the image shows a hyperbolic shape (inverted V) such as shown in the below animation. The location of the top of the "V" is the position of the utility. The depth to the top of the "V" is an estimate of depth.



     
    By moving the GPR back and forth and marking the ground where the top of the V is observed, the alignment of the subsurface utility can be traced out as the X's in Figure 1 indicate.  


     

    Figure 1: GPR locates utility line by crossing its alignment.
     

    Example: Tracking a Storm Sewer case study.
     

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  4. What are the pitfalls of GPR?
    GPR is not without its limitations. GPR radiowave signals are absorbed by the ground with some soils (clays, saline) greatly limiting exploration depth. GPR effectiveness is thus site specific and varies greatly from place-to-place. GPR can also see many soil and buried structures which can make identification of the desired target difficult.  (i.e. can't see the individual tree in the middle of the forest!!).Back to top
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