Civil EngineeringConcrete ScanningDamage PreventionData Interpretation

How Does Ground Penetrating Radar (GPR) Distinguish Between Rebar and Voids In Concrete?

By September 8, 2016 5 Comments

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  • Davide says:

    Dear Daniel

    my name is Davide, I watched one of your video on youtube and from there I discovered your website. I am not a beginner as I have been using GPR for work for few years and I have a little academic knowledge about it from uni. I am always looking for a better understanding and improvement of my knowledge and skills and I thank you for your initiative and the tips you provide.

    Just a quick note about your video on the difference between reinforcement and void GPR signature.

    I believe that talking simply about positive and negative signals could be a little bit confusing.

    The polarity of the reflected signal depends of course on the material dielectric contrast, but also on the polarity of the original GPR signal that is not mention in the video

    Not all the GPR antennas from different manufactures (probably even from the same manufacture) generate the same signal (in terms of polarity, first positive peak followed by a negative one for example), therefore I think that saying that reinforcement will show always a first positive whilst void a first negative is not necessarily correct.

    Also in my experience, using antennas that generate a pulse made of a first positive peak followed by a negative one, reinforcement shows a first negative reflection (polarity reversal).

    It is true that everything is possible, above all when many factors, that theory cannot take in account, are involved, and I am keen on expanding my knowledge about that so could you please show a radargram where reinforcement and void appear with the polarity you mention along with the direct wave for a comparison?

    thanks in advance

    Davide

    • dpbigman says:

      Hi Davide,

      Thanks for the comment. I hear everything that you are saying.

      I will say that the video is geared towards beginners and from my experience, the signatures indicated in the video are what has been typical in my career.

      There is no doubt that each instrument produces unique pulses, but even when this is the case, manufacturers often refer to reversed polarity going from low K to higher K as artifacts of the instrument (sometimes even referring to those as halo effects). A student of mine even pointed out to me recently that one manufacturer even refers to this as an “imperfection” in the data in the manual.

      Even in those cases however, I have seen the signature mentioned emphasized, which I did state in the video.

      I’ll go ahead and put a data profile in future webinars or masterclasses that we produce so stay tuned! Thanks again for the comment, it is much appreciated!

      Dan

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  • Melhem says:

    hi Dan
    when reflection coefficient it is positive , the Upper surface of the target well be negative (black line), and the lower surface positive white line ,
    and when reflection coefficient it is negative , the Upper surface of the target well be positive (white line ) and the lower negative black line.
    Dear Sir is this what you mean’t in your video ?????
    If yes you are absolutely right , because we need the true amplitude from lower and upper surface of the target , and as you know for geometrical spreading and attenuation etc etc …realy we need to know exactly where upper surface because sum time its difficult to find it.
    but my question : are you sure the positive RC give negative (black line) and the negative RC give positive (white line) .????
    thank you and best regards
    geophysicist
    Dr N.K.Melhem