What is GPR?
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  • 1. What is GPR (Ground Penetrating Radar)?

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    GPR (Ground Penetrating Radar) 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). Ground Penetrating Radar works by emitting a pulse into the ground and recording the echoes that result from subsurface objects.

     

  • 2. Why use GPR?
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    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) or external noise makes detection impossible, GPR provides an alternative. GPR provides its own source of energy, locates 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.

    GPR can also be used to Locate Underground Utilities. Traditional electromagnetic induction utility locating equipment require utilities to be conductive. They are ineffective for locating plastic conduits or concrete storm and sanitary sewers. Because GPR can detect variations in dielectric properties in the subsurface, it is highly effective for locating non-conductive utilities.

    The common sense approach for locating 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
    • Find utilities with damaged or missing tracer wire
    • Unscramble densely packed installations
    • Discover undocumented utilities
    GPR data
    Figure 1

     

  • 3. How do I use GPR for Utility Locating?
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    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). 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.

     

  • 4. What are the applications of GPR?
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    Clearing

    GPR site clearance

    Excavation best practice requires clearing the excavation area. Clearing demands that unknown underground obstructions are identified and avoided. Conflicts may involve publicly regulated utilities, privately owned buried assets or natural obstructions such as rocks and boulders. The advantage of GPR lies in its ability to detect any structure with differing properties. By scanning the area, much like mowing the grass with a lawn mower, the LMX® can identify unknown hazards or obstacles and thereby minimize excavation surprises!

     

    Verifying

    LMX GPR paint marks

    Utility locating focuses on the delivery of verifying mark-outs confirming as-built records and historical plans. Locating teams, who mark-out public and private buried structures, face many challenges and now use GPR to augment traditional locate methods.

    GPR actively illuminates the ground detecting both metallic and non-metallic targets. GPR proves most effective in the challenging cases of non-metallic or complex, closely spaced installations. GPR not only responds to the buried pipe or cable, it can also sense soil disturbances that may be associated with prior installation excavations. All these advantages enable the LMX® GPR system to deliver enhanced locating capability.

     

    Mapping

    mark out for GPR survey

    Detailed maps of underground infrastructure are regularly needed for infrastructure design. Subsurface utility mapping and engineering (SUM/SUE) service providers now turn to GPR for subsurface mapping to augment the traditional locating methods.

    By scanning an area with GPR, unknown or unrecorded buried structures can be identified and marked. Correlating responses with as-built records enables verification and identification of buried utilities. Vacuum excavation can then be targeted to expose GPR responses from undocumented infrastructure or natural obstructions. Total station or GPS surveying of mark-outs delivers updated georeferenced drawings for design needs.

     

  • 5. What is GPR effectiveness?
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    GPR effectiveness depends on local soil conditions. Some soils are transparent to GPR waves and others are not. As a result, GPR exploration depth varies from area to area. To reduce GPR user uncertainty, there is a growing need to develop what are called “GPR suitability” maps which indicate the performance expected in a given area.

    The US Department of Agriculture has pioneered this effort by transforming soil maps developed for agricultural purposes into GPR suitability maps and have made them freely available on the web. Other jurisdictions are following suit but progress is slow. CAUTION! GPR suitability maps are never a substitute for local experience. Soil conditions can vary on a much smaller scale than most maps available; GPR suitability maps should be used as a guide and never as an absolute indicator of GPR performance.

    U.S. GPR Soil Suitability Map

    Click here to see the U.S. GPR Soil Suitability Map

  • 6. How to read GPR Data?
  • The Basics of Interpreting GPR Data – Part 1

    The Basics of Interpreting GPR Data – Part 2

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