What are the common underground utility location methods?

Posted April 15, 2013

What are the standard underground utility location methods? This article focuses on electromagnetic utility locating, ground-penetrating radar, acoustic utility locator, hydro excavation, and even dowsing.

Electromagnetic Utility Locating

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The most common method to locate private utilities is known as electromagnetic utility locating. In this technique, the locating equipment generates an electromagnetic radio frequency, and when applied to the ground, the equipment detects the subsurface utilities containing conductive material on its receiver. This method is accurate and commonly used to detect gas, electric, telephone, cable, propane, water, sewer, storm, and irrigation lines. The most significant limitation of this technology is that it cannot locate unmarked plastic, asbestos, concrete, terra-cotta, or non-ductile pipes.

Gas lines are generally constructed of medium or high-density polyethylene pipe or plastic, so tracer wire is commonly run alongside or on top of the pipeline to make the line “locatable.” In these situations, by accessing the system (e.g., a tracer wire box), the operator can induce a current on the tracer wire and locate the tracer wire.

This technology also doesn’t perform well beyond 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 equipment displays the objects. This locating equipment is generally used for buried pipes, tanks, utility holes, cables, and other buried objects that typically cannot be located using ordinary methods, such as electromagnetic locating. Operating this 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, limiting 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 the buried facility must be an inch in diameter for every foot in-depth. For example, a six-foot deep pipe must 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 locate and expose underground utilities safely. Here, high-pressure air breaks up the soil, then vacuumed into a tank. Hydro excavation is used in difficult soil conditions where pressurized water breaks up the soil, then vacuumed into a tank. This method is known as potholing, hydro-digging, hydro-trenching, or soft digging.

Dowsing

While dowsing may not be recognized as a traditional utility location service, 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, 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.

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