Disappearing Pipes
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Disappearing Pipes

GPR operators quickly learn that GPR penetration depth depends on the local ground conditions. If you have attended a Sensors & Software GPR training course, you will have heard us say “If the answer is not obvious, you may be far better off to move over a few inches and collect another line of data to see if the interpretation is easier”. In this article, we show a dramatic example.

Data were collected running parallel to a building where at least on utility was expected to exit. To provide complete coverage, data were collected on a grid. A 16 x 30-foot grid was set up along the side of the building to investigate the area. GPR survey lines were positioned every 1 foot in both the X and Y directions.

A NOGGIN® 250 SmartCart™ system was used to conduct the survey. The 250 MHz Noggin system is optimal for investigating utilities. It has maximum depth penetration with sufficient resolution to delineate objects smaller than 2 inches in diameter. In Figure 1 (see figure 1), Utility 1 can be seen on all the cross-sectional images, while Utility 2 is visible on survey lines A, B, F, and G but weakly or not at all on C, D, E. The area was covered with an asphalt layer with no visible reason for the difference.

The data were processed using EKKO_Project™ software to create Figure 1 (left). Utility 1 can be seen on all cross-sectional images A to G, while utility 2 is clearly visible only on A, B, F, and G survey lines. A plan map in Figure 1 (right). Utility 1 is visible as a continuous linear response across the map but Utility 2 exhibits a gap.

mapping and marking underground utility pipe
Figure 1
(Left) Utility 1 can be seen on all cross-sectional images A to G, while Utility 2 is clearly visible only on A, B, F and G survey lines.
(Right) Utility 1 is visible as a continuous linear response across the map but Utility 2 has a gap in the response.

Why does the pipe disappear? There are several explanations, some more plausible than others:

  1. The soil conditions changed
  2. The utility changes direction
  3. The utility goes deeper, out of the range of the GPR signal
  4. The material composition of the utility changes and is not detectable with GPR.

Considering most construction practices, explanations 2-4 are not likely. The data itself suggests that asphalt/soil conditions have changed because signals from the soil stratigraphy and even the direct ground wave that travels at the surface directly from the GPR transmitter to receiver show reduced in signal strength. Most likely soil conditions above the pipe have changed resulting in the GPR signal absorption preventing detection of the utility pipe.

The penetration of the GPR signal is dependent on the electrical conductivity of the soil being scanned. The natural soil composition, water content, contaminants, and overlying construction materials all can affect the conductivity. In this case, without testing by digging or drilling, it is difficult to determine what factor has reduced the penetration in this area. The key points from this case study are:

  • Proper survey technique is extremely important.
  • Without the data on a grid, we may not have collected a portion of the survey area and missed important utilities. Also, if we had only collected one or two data lines we may not have recognized the anomalous area of increased absorption.

  • Use all the knowledge available to you when making an interpretation of the data.
  • Familiarity with local construction practices makes interpreting data easier because you will understand what objects are likely to be in the ground and how they were buried.

  • Soil conditions can change in a matter of inches.
  • Don’t give up too quickly. An adjacent survey line may be the key to understanding what is going on in the subsurface.