Damage PreventionData CollectionData InterpretationUtility Locating

The #1 Reason for False Positives in GPR Data (Ground Penetrating Radar)

By June 6, 2017 4 Comments

There are several problems that GPR practitions can encounter while conducting ground penetrating radar projects. These can include gradual transitions between materials, high attenuation rates, and the inability to estimate depth. One of the most dangerous is dealing with air waves. Why? Because air waves can produce false positives in your GPR data set!

Air waves are can produce responses in your GPR data that appear to be buried objects, but in reality they are signals from your gpr that reflected off of an object that is on the surface. For example, while collecting a line of data past a tree, the gpr wave can move across the ground surface, reflect off the tree, and be recorded by your receiving gpr antenna. The problem is that this reflection event appears in your gpr profile as if it occurred below the ground surface.

The first way to handle this issue is be aware of your surroundings. Recognize that the GPR antenna produces a pulse of energy that not only goes into the ground, but along the ground surface and even up in the air. Second, match the hyperbola to get a dielectric value or a velocity. A dielectric of 1 will indicate that the hyperbola was produced by an air wave.

Be alert, be cautious, and survey on!

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Join the discussion 4 Comments

  • Bob perry says:

    Airwaves…..Great subject! Thanks for adding the part at the end about graves; it touches on a topic that I was not aware of. Interesting enough, I did note during my field work that sometimes happens when scanning next to headstones looking to confirm a burial and position – the anomaly seems to be to shallow for the period.

    I’d like to send you one of my “Bone Finder – Tracking the Dead” ball caps if interested. Here is what my logo looks like:

  • Hello, Dan.
    Very interesting for people who don’t know much about GPR. I think about this matter of air waves, that there is a very simple way to identify them: air waver are wider than subsoil reflexions. This is produced as air waves have a velocity of 0,3 m/ns, the air velocity (so the reflection/refractions arrive before than ground ones), meanwhile subsurface materials has a velocity not bigger than 0,16 m/ns (ice).
    Second point that I have seen (better heard) about reflecions in building. I think you hava had explained that above this air reflecion from corners, can be found reflections of interest: that produced by foundations, such us shoes. This is very required by engineers when hasen’t data of foundations of old buildings, to calculate if them can support the charge of another floor.

    • dpbigman says:

      Great point. Knowing the speed is critical. I did a live periscope one time showing the speeds. Some units don’t give velocities and only give dielectric values. So we should add to your great comment that the dielectric of air is 1. This way folks can get this either way. Thanks again Ricardo. I always appreciate the commentary!

  • A mistake in my last post: in air, we can’t talk about ‘refractions’, only of ‘reflections’ (trees, etc…)