A success story of technology being used in the contracting world
With innovating technology, it is now possible to identify a break in a water main. Contractors in the industry have utilized many different tools to try and locate utilities underground. Ground Penetrating RADAR (GPR) is one of the best geophysical applications used to identify subsurface features. GPR uses an antenna of a certain frequency to transmit electromagnetic pulses into the ground, and based on the changes in physical properties of the subsurface materials, the waveform reflects back to the receiving end of the GPR to be analyzed and interpreted by a geophysicist or a trained GPR technician.
By surveying in a grid pattern of multiple transects, the GPR technician was able to accurately locate the water main, as well as the electrical lines and gas lines in the area of the suspected break. Once all the utilities in the area were identified, the GPR technician was able to take a closer look at the water main and the amount of saturation in the subsurface.”
Materials within the subsurface have distinct physical properties. The change in physical properties is what allows the waveform to “bounce” back to the GPR unit to be analyzed and interpreted. In this case, a cast iron water line within surrounding sand. Once a survey area on the surface is identified, the GPR is deployed in a grid pattern of transects. A transect is the path the GPR is moving towards. Multiple transects are conducted in order to confirm the presence of any obstructions that may be in the underlying surface. This approach allows for objects consistent with utilities, underground storage tanks, void spaces, and the identification of buried objects to be located (Figure 1).
REAL WORLD EXAMPLE
In March of 2019, in Oakland County, Michigan, GPR was used on an emergency call from a contractor in need of locating a water main with a suspected break. The contractor was tasked with locating the water line, locating all utilities in the surrounding area prior to excavation, identifying the location of the water main break (if possible), and fixing the water line. The problem was exacerbated by the amount of utilities that surrounded the area of the suspected water main rupture (Figure 2).
The surface conditions of the survey area consisted of an accumulation of water over asphalt and grass. By surveying in a grid pattern of multiple transects, the GPR technician was able to accurately locate the water main, as well as the electrical lines and gas lines in the area of the suspected break. Once all the utilities in the area were identified, the GPR technician was able to take a closer look at the water main and the amount of saturation in the subsurface.
This allows the GPR and the operator to zone in on a more localized survey area in order to more accurately analyze subsurface conditions. After analyzing the degree of saturation within the sands and determining the depth and trajectory of the water main, the GPR technician was able to use this information to identify the pipe itself. A transect taken directly above the water line indicated an interface change between pipe, water, and soil which suggested a break in the water line. By utilizing all this information, the GPR technician was successful in determining the exact location and depth of the ruptured pipe. This allowed the contractors to address the issue, providing a great example of how using GPR can be an effective and efficient tool in the construction world.
Stephen Tatum is a geophysicist with Great Lakes GPR. He can be reached at firstname.lastname@example.org.