September 2022 Vol. 77 No. 9

Directional Drilling

Three Tips to Help HDD Professionals Select the Correct Frequency to Avoid Interference

Rodolpho Cabello  

With nearly 2 million miles of underground utilities in the United States, according to leading industry association NASSCO, and even more around the world, there is no questioning the growth of the underground construction industry. And as our world continues to modernize, it is crucial that underground construction crews know how to navigate the complicated network of buried utilities. 

To accomplish this, crews need to understand how to correctly identify interference on the horizontal directional drilling (HDD) job site. This is a crucial step to ensuring operators use the proper frequency when mapping an HDD project. Equally important for job-site success is having the appropriate equipment and knowing how to use HDD guidance tools to identify interference. 

Following are three key tips to help HDD professionals identify and select the best frequency to avoid interference that can cause costly damage and downtime. 

Understand the types of interference 

There are two types of interference that impact operating guidance equipment. 

Active interference includes any type of infrastructure with a live current running through it. Some of the most common examples include powerlines, fiber optic lines and cellular lines. While underground construction professionals are mainly focusing on what is beneath the surface, it is also important to identify if there is any overhead active interference, like powerlines, that could impact frequency. 

On the other hand, passive interference often is a result of current or previous structural developments, such as metal, guard rails, chain link fencing, underground steel drains, reinforced concrete or rebar. Like active interference, sources can also be above ground, causing disruptions to your guidance equipment. 

Correctly identifying interference 

As underground construction environments continue to become increasingly congested and complex, it is important to first identify what types of interference are on the job site and how to properly track each type. While this may initially seem easy, each job site is unique and could have an abundance of different interferences, both underground and overhead, that could affect the correct frequency selection. 

Here are a few best practices. 

First, visually identify and note any interference sources. Today’s tracking equipment has the ability to detect active interference, but passive interference requires visual inspection of the job site. 

Start with identifying any possible passive inference, such as guard rails, chain link fences or reinforced concrete. Then, check for active interference sources by using the guidance equipment to walk the planned bore path, noting spikes in interference and employing the analyzer feature to help select recommended frequencies. 

Characteristics to keep in mind are that lower frequencies do not couple to adjacent utilities or other metallic objects as easily as higher frequencies, making them a good choice for mitigating passive interference issues. Mid-range frequencies are the most used, when not dealing with interference from passive situations or powerline interference. Higher frequencies can help mitigate interference from high-power transmission lines – both overhead and buried. 

Choosing the correct equipment 

With a wide range of unique variables on any job site, HDD guidance equipment should have two key features. The first is to ensure it has a wide range of frequencies and the ability to change frequencies in the middle of a job. 

Second, equipment needs to have multiple, simplified interfaces. This will enable operators to select the correct frequency, quickly and easily, thereby increasing productivity. 

Having the right tools on hand will help ensure that HDD professionals are able to select the right frequency or frequencies needed for the jobsite, resulting in mitigating damage and increasing efficiency. UC 

ABOUT THE AUTHOR: Rodolpho Cabello is product manager for Subsite, Perry, Okla. 


Subsite Electronics, (800) 846-2713 

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