The shortcomings of Ground Penetrating Radars
Yes, we all agree Ground Penetrating Radars kind of work like magic. However, there are some limiting factors that can seriously reduce the effectiveness of those devices. Here’s a few.
The first of GPRs arch-enemies. Most often this is a limitation with high frequency antennas that are normally used for concrete scanning. It’s a well known fact that water is at the top of the leaderboard when it comes to reflective materials.
Considering that moisture is water, it’s logical that a moist piece of concrete would pose a serious difficulty for radar surveying. It reflects the radar pulses thereby limiting the travel of the EM waves and subsequent data quality.
The range of the depth a GPR can achieve is limited by the electrical conductivity properties of the ground, the central frequency of the device and the radiated power. As conductivity of the material increases, the penetration distance decreases. This is because the electromagnetic energy is dissipates faster in heat, causing a loss in signal strength.
Higher frequencies do not penetrate as far as lower frequencies, although they do give a better resolution. The best penetration is achieved in dry, even sandy soils or homogenous, dry materials such as granite, limestone, and concrete.
The composition of the concrete or soil, depending on where you’re doing the radar survey is of utmost importance to achieving maximum signal penetration with high frequency antennas. As previously mentioned, the presence of moisture will limit the depth penetration of the ground penetrating radar.
Furthermore, sub-grade debris and other random anomalies can reflect the radar signal before it is able to achieve maximum penetration.
In some cases there can be reinforced steel within the concrete of a building, which can also reflect the signal and limit the ability of the GPR to penetrate the concrete.
Size of Target
There are two main ways in which a ground penetrating radar is limited when considering the size of the target.
The target diameter — a ground penetrating radar is unable to determine the diameter of the target being located. The GPR retrieves a 2D slice of the scanned material, e.g. concrete, and therefore does not detect the entire circumference of the located anomaly. This issue can be avoided with a 3D scan, which requires more traces at different angles to be made on the section of the ground we want to survey.
Level of detail — while it is possible to locate many objects with GPR there can be objects that are just too small for the radar to detect. This limitation is more widely noticeable when using low frequency antennas (providing lower resolution data), but it can also be a limiting factor when scanning concrete with a lot of reinforced steel or smaller objects buried at depths more than 25 cm.
Composition of Target
It is possible for ground penetrating radars to locate a target by detecting the difference in its electromagnetic conductivity. That said, there are objects which are found easier than others.
GPRs can locate empty plastic conduits and pipes, but it’s way easier to locate a highly reflective piece of metal. Therefore, composition of the target can be a limiting factor when locating. It’ll likely be very difficult to locate a plastic conduit running below a tightly spaced grid of steel as the rebar causes a positive reaction, seen more clearly in the data, than the negative reaction of the plastic conduit.
Of course, it’s possible to locate an object with a differing electromagnetic conductivity, but there are objects which are easier to locate than others as their reflectivity is much higher, creating a contrast with the surrounding material.