When you’re starting a new project, utility locating should be at the top of your list of tasks to ensure that the project goes smoothly, without compromising the structure of your existing utility grid. But what if you’re not sure where the utility lines are on your project or 811 services just don’t provide the level of detail you need to get the job done safely? Active and passive utility locating services can help make all the difference between a successful, profitable project and one where you spend entirely too much time on guesswork. Here’s a look at the differences between active and passive utility locating.

What’s the Difference Between Passive vs Active Utility Locating Services?

Underground utilities help protect vital services from a number of potential hazards, including bad storms, flooding, car accidents and any number of other possible issues that could cut off service or cause serious harm. However, unless the utility lines were all put into place using the Subsurface Utility Engineering standards, there’s no way of telling exactly where those utilities may be located. Fortunately, there are a few different options to locate your utility lines, some of which are active and some of which are passive.

Active Utility Locating

Active utility locating works well when you need to locate a specific utility that has an unbroken connection from one end to the other and is metallic. An electromagnetic frequency is introduced to the pipe or line on one end and transmitted down its length, allowing a surface receiver to pick up on the signal and trace it across the surface of the soil or concrete. This makes it easy to mark the specific utility’s location at the surface so that you can avoid it while digging or only dig where you want to pull up that specific line.

Electromagnetic (EM) location uses a transponder to transmit a very low-voltage alternating-current frequency into a conductive probe, like a steel pipe, or by attaching directly to the utility line itself. This process creates an EM field around your property’s underground utility lines. This field is then detected by a receiver, allowing the line to be mapped for later reference. This can be accomplished in two different ways:

  • Conductive location involves attaching the transmitter to the utility line directly, creating an EM field on the outside of the line, which is then traceable.
  • Inductive locating applies the signal to the utility line by placing the transmitter into the ground directly over the utility line.

Passive Utility Locating

Though active utility locating provides the best accuracy and specific information about each of the utility lines that cross a property, there are some situations where it won’t work. If a pipe or line is broken, the frequency can’t transmit across the entire length, making it difficult if not impossible to fully trace. Plastic or concrete pipes that don’t conduct electrical frequencies are another area where active utility locating won’t work well. In these cases, using a passive location technology is a better bet.

Ground Penetrating Radar

One example of passive locating is ground penetrating radar. This allows you to get a full map of the surface under your soil, including utilities and many other features that you may not be aware of, such as voids, old wells or graves. You can then use the information you receive to develop a full map of the underground features of your property, making future planning much easier.

Ground penetrating radar works by transmitting a microwave-frequency radio wave into the ground. When signals are reflected after coming into contact with subsurface structures, including changes in soil composition. We have a range of different frequencies available, from low-frequency 250 MHz units that reach far into the ground to high-frequency 1.6 GHz units that provide much higher resolution and accuracy. Single-frequency ground penetrating radar units are well-equipped to detect utility lines up to around 10′ deep, multi-frequency 3D units work well in congested areas and high-frequency units that work well for inspecting concrete slabs up to 20″ thick.

Once the radar wave is reflected, refracted or scattered back to the receiving antenna on the surface, variations in the return signal can be recorded. The system does have some limitations, however, that can lead to false readings. Saltwater, soils that have a high salt content and tightly-packed clay such as a hardpan layer and rock soils can impact your readings.

Because it relies on radar rather than EM energy, ground penetrating radar works well for situations where you need to map non-metallic utility lines, such as plastic or concrete water and sewage lines. It also works well when you’re taking on a new property and need all of the underground structures and utilities mapped at once before starting on a new project. It can be used prior to purchasing a property to verify the thickness of concrete slabs and the reinforcing rebar that’s located within it. It makes it easy to determine whether there are issues such as bedrock, sinkholes, karst or void formations on the property, and can even be used to find the exact location of a broken pipeline.

EM Location

Another example of passive utility locating is using EM location in a passive setting. Passive location uses sensitive equipment to detect EM signals that the line you’re tracing already produces. It doesn’t need a transponder to detect the line, but to trace this type of line, it will need to be an active electrical power or communications line. If it’s not actively carrying a signal, this method of location will not work.

Utility Locating Services

If you’re having problems with accurately locating public utility lines or private utility locating of your electrical, communications, gas, water or sewage pipe utilities, we can help! At Soft Dig, we have a wide range of services available to help you with utility locating for your project, including ground penetrating radar and active utility locating technology. Please feel free to reach out today to request an estimate.

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