by: Mark Gallucci, Digital Control Inc. (DCI)
Since the inception of HDD, drillers have had to contend with signal interference that may wreak havoc with the locator. High-pressure gas lines with cathodic protection, fiber optic tracer tones lines and metal objects near the drill path mean that HD drillers are tasked with threading a high-stakes needle. Squirrely information can baffle the locator person, resulting in lost productivity or, worse yet, a bore that deviates significantly from the intended drill path. In this article, we’ll cover the types of interference you’ll encounter, how it affects your transmitter and receiver, and how new technology developments in directional drill products have helped mitigate these challenges.
Two types of interference
Before you can tackle your interference problem, you need to understand the interference itself. There are two types of interference that can play tricks on your walkover locater: active and passive. Active Interference – Active interference is anything that creates its own magnetic field, like tracer tone lines, underground or overhead electric power lines, two-way radios, and invisible dog fences. The rule of thumb is: if it’s conducting electricity, it’s emitting a radio frequency. Active interference causes problems with your locator when its frequency matches (or comes close to) the frequency at which you’re transmitting and receiving. When active interference is overwhelmingly strong (like in the case of a nearby power plant), its frequency can be off from yours on the spectrum but still interfere with your equipment. 
Passive Interference – Sources of passive interference don’t create their own magnetic signal; instead, they block, distort, absorb, or weaken the signal that you’re trying to pick up. One of the most common culprits is the rebar found in reinforced concrete.
How your locator works
This is a good time for a quick refresher on what exactly your locator is measuring when it takes a reading before transmitting information back to the drill rig operator. According to your locater, signal strength equates to depth. Locaters simply measure the amount of signal being received. Then, based on your above-ground calibration, the locater converts “x” many counts of signal to number we are familiar with, 120 inches. The bottom line: if interference affects your signal strength, you’ll notice a repercussion on your depth readings. Erratic signal strength leads to erratic depth numbers.
Big challenges lead to new advancements
In the old days of directional drill products, we would recommend dealing with the problem by getting a bigger, louder transmitter with more signal– or switching to one of the few alternate transmitter frequencies. This solved some of the interference problems, but contractors would still struggle on many drill sites. Massive amounts of active and passive interference sources overwhelmed the best equipment offered by the industry leaders. Ultimately, these challenges opened the door to extensive R&D that has now benefitted the entire HDD industry.
Active interference meets its match: The Falcon F5
To mitigate the interference problem, DCI engineers developed a single transmitter that has the ability to broadcast anywhere in the range, from <4.5 kHz up to 45 kHz. The Digitrak Falcon F5 breaks this spectrum down into 9 different bands. 
To find the least interfered with (or cleanest) frequencies, the operator puts the handheld receiver into frequency optimization mode. The receiver runs a 10-second scan in the environment where drilling is set to occur. At the end of the scan, a graphic appears on the display that shows the noise level of each frequency band. Operators easily identify the bands with the least interference. The Falcon F5 enables the operator to select two of the least noisy bands on the receiver. The primary can be locked into the “up” mode. The “down” mode can be set to the next lowest noise-free band or selected with a specific portion of the drill path in mind. When the transmitter is paired to the handheld receiver, the receiver becomes programmed to listen to those specific frequencies, not unlike pre-set function on a car radio. Once underground, the operator can switch between the “up” and “down” frequencies without having to trip out. Frequency Optimizing can be conducted on each drill site, so drill crews always use the most appropriate frequency, every shot, every day.
Passive interference is no match for Falcon Rebar
We also discovered a method of overcoming the problem of passive interference. The lower the frequency scale, the better a locator is able to overcome the negative effects of passive interference sources. Falcon Rebar transmitters can broadcast at Sub-k frequencies–less than 1kHz–to deal with interference resulting from rebar, mesh, chain link, etc. In Rebar mode, Falcon systems transmit and receive signal on 6 bands: 3 wide and 3 skinny Sub-k bands. There is a depth tradeoff when broadcasting at these ultra-low frequencies, but for most jobs, the range usually exceeds the required depth. The improved accuracy in spots with high passive interference improves productivity unlike anything ever offered by Locator or Tracker manufacturers. Identifying and dealing with sources of interference is a part of any HDD job. But with the right knowledge and superior technology, you can tackle even the toughest drilling challenge with accuracy and keep production on schedule.
Contact the directional drill product experts at Melfred Borzall to get the right tools for your next job.
Mark Gallucci is General Manager of In-House Customer Service Group/Training Manager at Digital Control Inc., manufacturers of the best-selling HDD locators on the market, including the new F5 Falcon system.
