TIPS: Beware of the (GPR) shadow!
O
ne of the first lessons GPR practitioners learn about GPR is that radio signals cannot penetrate through metal objects. In this TIPS article, we show a few examples of this phenomenon and how it affects GPR data interpretation.
Metal objects provide a reflectivity of 100% (Figure 1). The equation for calculating reflectivity is shown alongside, using a dielectric permittivity value of K1=5 for dry soil.
Figure 1
Using the equation for reflectivity (R) and dielectric permittivity values for metal mean that K2 = ∞ and non-metal (K1 = 5), the reflectivity of metal is 100%. In fact, it is always 100% for a buried metal object, no matter the what the host material is.
This means that all the GPR signal that hits a metal object reflects from it, and no signal transmits through it and continues deeper. This results in a “shadow zone” of no signals under metal objects (Figure 2). For small targets energy can be diffracted and scattered so can appear under the metal target.
Figure 2
When a GPR system travels over a metal object, all the signals reflect from the metal object, resulting in a shadow zone, free from GPR reflections, underneath.
Example 1: One of the most common places to see the shadow zone under a metal object is under rebar in concrete. In this suspended slab example, the strong, horizontal reflector from the bottom of concrete (at ~0.2 m depth) is interrupted with shadow zones directly under each rebar. Since these are small interruptions, we can still easily see the strong flat-lying reflection from the bottom of concrete.
Example 2: When a deeper, large diameter, metal utility crosses or runs parallel to a metal shallower utility, it is sometimes possible to see the shadow of the shallower utility at the top of the hyperbolic response from the deeper utility. This provides confirmation that the two hyperbolic responses are from two separate objects and not the top and bottom reflections from a non-metallic utility (see TIPS: Determining pipe diameter from GPR data).
Example 3: This concrete image is like Example 1 except the spacing between the rebars is significantly less. This results in a series of very short, flat reflections from the bottom of concrete interface that is only visible between each pair of rebars. Since the bottom of concrete reflector is so discontinuous with large gaps in it, it is easy to miss it and not interpret it; meaning that estimating the slab thickness, which could be important information for your client, is not included in your project report.
Example 4: This example is like Example 3 but on a much larger scale. The responses between the metal tanks shows a series of short, flat reflectors that could be interpreted as the bed of material that the tanks were placed on. If this reflector is recognized, and the interpretation that it is, in fact, the tanks bed, this additional information provides a way to estimate the diameter of the tanks.
