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Ground Penetrating Radar vs. Electromagnetic Location for Utility Detection

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    Ground Penetrating Radar (GPR) and Electromagnetic Location (EML) are totally different technologies:

    • GPR works by sending a signal into the ground and listening for reflections from that signal, and contains its own transmitter and receiver within the unit.
    • EML works by detecting the electromagnetic signal that may be emitted by power cables, or the energy from low frequency communications signals which has ‘coupled’ onto metal services. Alternatively, in the case of a metal service which is not a power cable, it can be induced to emit a signal by accessing the service and clamping a separate transmitter onto it.

    Both types of survey need to be approached in a logical way and require training and experience to produce the best results.

    Based on our experience and controlled experiments, Ground Penetrating Radar will recover the locations of more underground features compared to Electromagnetic Location methods (within the penetration and resolution limitations of a GPR) in a like for like test with no manhole access.

    The missing information from a GPR survey is the information which is recovered by the site reconnaissance phase of a survey (by lifting manholes and identifying services) which is usually performed alongside an EML survey as part of the process of clamping onto services to induce a signal. The other difference is that EML works by inducing a known signal onto a specific utility which is then traced, therefor EML enables services to be positively identified and tracked compared to just located by GPR.

    A summary of the advantages and limitations of each method:

    • GPR provides its own transmitter and is able to detect underground features without a power source
    • GPR is able to detect most types of utilities including plastic and other non-metallic services, as well as metal
    • GPR will also detect other below surface features (rebar, voids, structural information)
    • GPR does not require manhole access
    • GPR cannot identify services
    • EML can detect smaller diameter cables
    • EML can detect metallic services which have had a signal induced onto them
    • EML can track a specific, induced signal and therefor identify individual services
    • EML requires manhole access
    • EML cannot recover any other below surface information.

    These differences mean that the final drawing from a GPR only survey will differ from the final drawing of an EML (and site reconnaissance) only survey with the GPR detecting underground features  and recovering more below surface information but unable to identify the different utilities. Whilst in the EM survey results (combined with the crucial information from site reconnaissance) services will be identified, but the drawing will lack the additional details recovered by the GPR and would not be able to detect any non-metallic services or services for which there is no manhole access.

    The best utility survey is one which combine the strengths of both methods as detailed both in PAS128 and the TSA utility survey guidelines.

    To go back to Utility Surveys click here.

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    What can a Ground Penetrating Radar survey detect?

    With GPR, you can detect a wide range of objects below ground level, including both metallic and non-metallic objects such as plastic pipework. GPR will also identify and map any voids below the surface, such as air pockets or mine shafts, as well as any other irregularities including concrete and previously excavated or back-filled areas.

    Will GPR compromise safety on my site?

    GPR equipment emits an electromagnetic pulse into the ground and records the reflected signals from subsurface structures and voids. It is entirely non-destructive and will not break the ground’s surface or affect any objects below. What’s more, it doesn’t emit any harmful levels of radiation, nor are there any other by-products created throughout the process. This means it’s entirely safe to use by its operators, and on sites of any type, including those open to the public.

    Is a GPR survey 100% accurate?

    While GPR is one of the most effective methods of non-destructive testing available, it can never be 100% accurate. One factor that can adversely affect the accuracy levels include the type of soil being surveyed. Clay soils and soils that contain high levels of salt or minerals can obstruct the GPR reading. Another factor is the experience of the equipment’s operator: interpreting the data collected can be complex, which is why it’s beneficial to commission surveys from an expert firm.

    Is GPR equipment difficult to use?

    The equipment itself is not difficult to use, but the interpretation of the data recorded tends to be complicated. The results of a GPR survey aren’t automatically translated into an easy-to-understand picture of what lies below the surface; instead, it’s a series of lines and waves and it can take both training and years of practice to master the art of correctly reading the output. Often, it is the experience of the equipment’s operator that plays the most significant role in the accuracy of the results GPR can achieve.

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