What are the common underground utility location methods?
What are the common underground utility location methods? Since April is Safe Digging Month, we have been reminding everyone to call 811 to have your utilities located before you dig. Routinely we are asked about the different methods available to locate utilities. Within this article, we will focus on electromagnetic utility locating, ground-penetrating radar, acoustic utility locator, hydro excavation, and even dowsing.
Electromagnetic Utility Locating
Gas lines are generally constructed of medium or high-density polyethylene pipe, or plastic, which is why tracer wire is commonly run alongside or on top of the pipeline to make the line “locatable.” In these situations, by having access to the system (e.g., a tracer wire box), a current can be induced on the tracer wire, and the tracer wire can be located.
This technology also doesn’t perform well beyond the depth of 10-15 feet. Natural gas distribution lines are usually installed 3-4 feet deep while transmission lines run deeper, though it would be unusual to be buried more than 10 feet.
Ground Penetrating Radar
Ground-penetrating radar or GPR is a technology used for subsurface utility locating, which also uses high-frequency pulses. The radio waves are emitted into the ground and facilities deflect the radio wave back up to the operator where the objects will be displayed on the equipment. This type of locating equipment is generally used for buried pipes, tanks, manholes, cables and, other buried objects that cannot be located with other more common methods, such as electromagnetic locating. Operating this type of equipment and understanding the results require experience and extensive training.
There are some limitations of GPR:
- Some soils like saline, shale, or clay have high conductivity, which limits the GPR from penetrating the soil.
- High-frequency GPR antennas (300-1000+ MHz) can only penetrate a short distance down.
- A common rule applied when using ground-penetrating radar is that for every foot in-depth, the buried facility must be an inch in diameter. For example, a six-foot deep pipe would need to be at least 6” in diameter to locate it via GPR.
- Soil density, environment accessibility, and crowding of surrounding utilities can also influence the GPR’s effectiveness.
Hydro or Vacuum Excavation (potholing)
Vacuum excavation is a fast and non-destructive way to safely locate and expose underground utilities. Here, high-pressure air is used to break up the soil, which is then vacuumed into a tank. Hydro excavation is used in difficult soil conditions where pressurized water breaks up the soil, which is then vacuumed into a tank. This is more commonly known as potholing, hydro-digging, hydro-trenching, or soft digging.
While dowsing may not be recognized by many of the larger utility location services, many experienced locators will admit that they have and still use dowsing as a reliable way to locate underground utilities and water. Here dowsing rods, which can be made from coat hangers, can indicate whether you are standing on top of an active pipeline. It will not indicate the depth of the pipeline, but according to many experienced locators, it is a dependable way to locate pipelines. Skeptics claim that there is no science behind this method, and some claim intuition is the driving force in dowsing. No matter who you believe, dowsing has been used to detect underground water since the Middle Ages.
As you can see, there is a wide range of methods used to locate utilities, from state-of-the-art equipment to coat hangers and intuition.