round penetrating radar (GPR) mapping surveys are commonly used to identify the presence of underground storage tanks (USTs). A locating service company was called in to perform a GPR survey to map the location of the underground storage tanks at a gas station. The EKKO_Project™ software was later used to view the grids as one large data set.

Challenges

Environmental monitoring and spill management is a priority at high risk sites where hazardous substances are stored. As part of an environmental risk management initiative, a local gas station was advised that they should install additional monitoring wells.

Since the gas station did not have records of the exact perimeter of the underground fuel storage tanks, it was necessary to map them and other buried infrastructure so the wells could be safely installed.

Solution

The GPR survey identified five storage tanks and the pipes connecting them. The GPR survey results have been presented in several ways,each providing different perspectives and understanding of the data:

Cross-Section: The cross-section clearly shows five USTs; four grouped together on the right and one further apart on the left. The hyperbolic response suggests that the tanks are round in shape and it is even possible to see the slight differences in burial depths. The tank on the far right is the shallowest at approximately 3 feet in depth.

Figure 1:

Noggin® 250 GPR cross-section showing underground storage tanks.

Depth Slices: The position and length of the tanks are evident in the depth slice images. Data had to be collected around a pump island resulting in a blank area within the grid data. This obstacle was easily avoided with the flexible grid collection features of the Noggin® 250 SmartCart. Product lines, leaving the island and going to another tank (not shown here), are also visible in the depth slices.

Figure 2:

Underground storage tanks and product lines visible on GPR depth slices

Google Earth™: The EKKO_Project software automatically integrates GPS data and can export depth slices as Google Earth™ compatible KMZ files. This is a very powerful tool for plotting geo- referenced depth slices of buried utilities. Users launch Google Earth™ and immediately “fly” to the surveyed location. They can then view and toggle between multiple depth slices.

Figure 3:

GPR depth slice displayed in Google Earth™

3D: GPR grid data were exported from EKKO_ Project™ in a 3D format and plotted with the Voxler 3D visualization software. All the GPR data collected in the separate grids could be displayed in a single view, making interpretations easier. Users can rotate and cut into the data volume or make weaker GPR signals translucent or invisible to highlight signals from strong GPR reflectors.

Figure 4:

3D visualization of the GPR data

Results

This GPR survey clearly located the buried storage tanks and provided the subsurface insights needed for boring to occur and the monitoring wells to be installed.

This is just one example of how GPR can locate underground structure to ensure safe construction for environmental projects. It illustrates how many professionals are deploying GPR systems to address similar problems around the world.

Key results to note:

• GPR was a practical solution when no other method could locate these USTs.
• SmartCart® configurations are optimized for rapid grid surveying.
• The field crew were easily trained in the whole methodology in less than a day.
• The whole project was completed by a two person crew in less than half a day.
• Integrated operation of EKKO_Project™ software and field data acquisition makes on-site mapping quick and easy.
• GPR image mapping results can also help locate take-offs and product lines.

Download the case study: Mapping underground fuel storage tanks at a local gas station

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