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TIPS: Determining pipe diameter from GPR data

W

hile most utility locators simply want to know where a pipe is located, there are times when more detailed information is required. One example is knowing the diameter of a buried pipe.

To estimate pipe diameter, three important conditions must exist:

1. The pipe must be non-metallic – since the GPR wave will not penetrate metal, it will never see the reflection from the bottom of the bottom
of the pipe.
 
2. You must get a reflection from the top and bottom of pipe (Figure 1) – even if a pipe is non-metallic, it doesn’t necessarily mean you will get a reflection from the bottom. Reasons for this include the pipe diameter being too small (so the travel time within the pipe is too fast) or the reflection being too weak to be detected by the GPR

    To confirm that the 2 hyperbolas are from the top and bottom of the same pipe, look for two things:

  • the two hyperbolas (from top and bottom) should be exactly located one on top of the other (Figure 2).
  • the bottom hyperbola has the opposite polarity of the top hyperbola. In Figure 2, the hyperbola reflection from the top of pipe is caused by the GPR wave travelling from soil to water, resulting in bands that are black-white-black. The reflection from the bottom of pipe is caused by the GPR wave going from water to soil, so the bands are the opposite polarity: white-black-white. This has to do with the reflection coefficient, moving from a lower to higher dielectric constant in the first case, and then, vice-versa.

3. The contents of the pipe are known – most typically, it would be empty (air or gas) or water-filled.

 
Once you satisfy all the criteria above, we can calculate the estimated diameter. Let’s start with our familiar depth-time equation:

gpr underground utility pipe diameter formula

Figure 2 shows a zoomed image of a water pipe, where the top and bottom of pipe are clearly seen.

gpr utility pipe diameter
Figure 1
Schematic representation of the change in time, ΔT, measured between the top and bottom reflections from the pipe with diameter, d.

Ground penetrating radar hyperbola signal
Figure 2
Reflection from top and bottom of a water-filled plastic pipe.

 

First, we need to determine the value for ΔT. From the image above, we can see that the time difference between the hyperbola from the top of the pipe and the hyperbola from the bottom of the pipe is 6 ns.
Next, we need to know the velocity that GPR waves travel in the material inside the pipe. We knew that this pipe was water-filled, so the GPR velocity in water is 0.033 m/ns. Plugging these values into the equation yields:

gpr underground utility pipe diameter formula

Water-filled, non-metallic pipes are easier to estimate their diameter since water slows down GPR waves, making the bottom hyperbola (originating from the bottom of the pipe) clearly distinguishable from the top hyperbola. With empty (air-filled) pipes or pipes with a very small diameter, the bottom reflection occurs too close in time to the top hyperbolic reflection, thereby making it impossible to distinguish the two.

Remember that this calculation, while handy at times, is only an estimate and requires prior knowledge of pipe contents. It also assumes that the pipe is completely full of that material, which is not always the case. Ground truthing would be the only way to get an exact diameter of the pipe and GPR should only be used to get a size estimate. If you need to be 100% certain of the pipe diameter, you will need to daylight it.

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