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TM2-T2-02

Avoiding Interference - a Unique Approach to Dealing with a Common Problem in HDD Installations

Siggi Finnsson, Digital Control Incorporated, Kent, Washington

Craig Caswell, Digital Control Incorporated, Kent, Washington

1. ABSTRACT

Interference, in particular active interference, is a common problem with walkover locating systems used with

Horizontal Directional Drilling (HDD) installations. Current methods of dealing with interference include the use of

more powerful transmitters or manually selecting between a few discrete frequencies if the locating system supports

more than one frequency.

This paper briefly describes current methods of identifying and contending with interference during HDD locating.

It then discusses a unique new approach to dealing with interference which has proven to be significantly more

effective than what is used today. The paper describes the underlying technology and how it is deployed in the field.

It then discusses how this new approach can be used to improve the performance of the locating system in the

presence of active interference.

A few recent HDD projects where the new technology has been used are described, detailing results from the field

together with a qualitative view of the benefits of this new technology.

2. INTRODUCTION

HDD locating or tracking systems generally consist of three components, a transmitter (also known as beacon or

sonde), a receiver (also referred to as locator or tracker) and a remote display. The transmitter resides inside a drill

head at the front of the drill string, and the receiver receives the signal emitted by the transmitter and in turn sends

data back to the remote display situated at the drill rig. All of these data and signal transmissions are wireless and

therefore subject to outside interference. This paper will discuss the data transmission between the transmitter in the

ground and the handheld receiver. We will discuss the inherent issues associated with interference, how interference

is dealt with currently and a novel new approach to dealing with this obstacle.

3. FREQUENCY CAPABILITES OF LOCATING SYSTEMS

Current walkover locating or tracking systems typically operate on a single or small number of distinct frequencies

that are preselected by the manufacturer. The HDD transmitter generates a magnetic field at the preselected

frequency which is used for locating the transmitter (e.g. depth). The transmitter also transmits a data signal to

communicate information such as transmitter roll position, inclination or pitch, temperature, battery life and in some

instances fluid pressure. The reliability and accuracy of these transmissions is critical for accurate location of the

transmitter inside the transmitter housing in the ground.

North American Society for Trenchless Technology (NASTT)

NASTT’s 2016 No-Dig Show

Dallas, Texas

March 20-24, 2016

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Current HDD locating systems offer up to five preselected frequencies ranging from 1.5 to 38 kHz. For example,

Digital Control Incorporated (DCI) has historically offered single or dual frequency transmitters with preselected

frequencies ranging from 1.5 to 33 kHz. Dual frequency transmitters typically allow the operator to switch from one

frequency to another while the transmitter is underground. If interference results in loss of data at the first

frequency, the crew can switch to the second frequency without the expense and lost time associated with “tripping

out”.

4. INTERFERENCE

When discussing interference in the context of HDD locating, it is common to refer to active and passive interference.

Active interference is often defined as “anything that emits a signal or generates its own magnetic field” while passive

interference can be described as “anything that blocks, absorbs or distorts a magnetic field”. In this paper, we will

primarily deal with active interference.

Active interference emits a signal which competes with the transmitter signal. Some of the effects of active

interference include erratic signal strength and depth readings, impaired depth readings (e.g. depths may appear less

than they actually are), loss of pitch and roll data and inaccurate receiver calibration (which may lead to depth errors).

Some examples of active interference include power lines, traffic signal loops, cathodic protection, fiber trace lines,

security systems and invisible dog fences. With many of these (and other) sources of active interference being

commonly found on or around job sites, active interference is one of the most prevalent issues faced by HDD

contractors. When working in an area where the locating signal is being significantly interfered with, accuracy of the

locating information and therefore accuracy of the installation can be affected. Inaccurate installations lead to

increased risk of breaching other underground utilities, posing safety risks to the crew and to the public, as well as

risk of significant property damage.

5. IDENTIFYING ACTIVE INTERFERENCE USING CURRENT LOCATING SYSTEMS

According to HDD industry best practices, a necessary step prior to commencing drilling operations is to walk the

planned bore path. This serves two purposes. First, this allows the crew to verify that all utilities to be crossed have

been appropriately located and potholed and to inspect the bore path for any other potential obstructions or issues.

The second purpose is to scan for interference. This scan is performed by turning the receiver on and keeping the

transmitter powered off. As the crew walks the bore path from entry to exit, signal strength readings on the receiver

are monitored. Since the transmitter is absent, any signal picked up is active interference. The higher the signal

strength readings (displayed on a numerical scale), the greater the interference and likelihood of interfering with the

transmitter signal. The effects of interference will vary depending on the site in question and in some cases can vary

along the intended bore path.

Let’s assume that after walking the bore path, an area is identified where signal strength readings are high. It should

be noted that “high” is a relative term since on deep bores, while the level of active interference may be low it may

still significantly impact the readings, while on a shallow bore in the same area the effects of that interference might

not be felt at all. As a rule of thumb, the signal strength from the transmitter (which is displayed on an arbitrary

signal strength scale) at the desired depth should be at least 150 counts greater than the interfering signal.

In order to gauge the effect of interference on the locating system, the following above ground test can be

performed. (It should be pointed out that this test is merely an approximation of what happens with the drill head

underground but does serve as a good indicator.)

a. One crew member holds the receiver at the end of the bore path, with the receiver facing the launch end. b. A second crew member installs batteries in the transmitter to power it on, and holds the transmitter at a

distance away from the receiver approximately equal to the maximum depth of the intended bore (back

along the bore path).

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c. The crew then walks together in parallel back along the bore path toward the launch end, maintaining the

separation distance constant. They periodically stop and change the transmitter’s pitch and roll orientation

so that the speed and accuracy of these readings can be verified on the receiver. This is particularly

important in the area where the highest inference is identified. The crew also notes any locations where the

display information becomes erratic or disappears.

6. CURRENT WAYS OF DEALIING WITH ACTIVE INTERFERENCE

Assume the crew encounters an area along the bore path where the effects of interference are such that the depth and

data readings are marginal or possibly unusable. There are several ways to address this situation. The first is to try

to achieve separation between the receiver and the interfering source. For example, move to the other side of the bore

path where roll and pitch signal reception might be better. There are more advanced locating methods such as target

steering and off track guidance, both of which can achieve separation from the interfering sources.

A second method is to try using a different (preselected) frequency (assuming the locating system supports multiple

frequencies). Whether an alternative, preselected frequency will work better depends primarily on how broad-based

the interference is and how close the available, preselected transmitter frequencies are in relation to the interference.

Preselected frequencies may work well at one location and/or point in time, but may not work as well at a different

location and/or point in time due to varying interference.

A third option is to resort to a more powerful (stronger signal) transmitter. Here the assumption is that by transmitting

at stronger signal, the receiver may be more successful in picking up the transmitter signal over the interfering sources.

The ability to boost transmitter power is inherently limited by the size and design of the transmitter and the power

source (battery). Boosting power drains the battery quicker, which may not leave enough battery life to last through

the entire bore path. Also, interference is often so strong that no amount of power boost will overcome it. Accordingly,

using a more powerful transmitter, by itself, may not be enough to overcome active interference in many

circumstances.

7. A DIFFERENT APPROACH, CHARACTERIZING ACTIVE INTERFERENCE

Over the years, manufacturers have searched for one or a few “best” frequencies. The rationale underlying these

selections have varied by manufacturer. Throughout DCI’s history these decisions have been both design and field

performance driven. For example, which frequencies seemed to be least affected by traffic signals, which gave the

best depth range, which worked best around power lines. Ultimately, however, when using distinct frequencies, a

crew is invariably faced with situations where the transmitters they have at their disposal will simply not perform

optimally or in some cases not at all. It was clear that a better understanding of the characteristics of active

interference crews face on a daily basis was required. The first step was to modify an existing receiver so that it

could receive data over a wide range of frequencies. A new software-based system was created to log the frequency

data (interference) received.

Figures 1 and 2 show two frequency plots gathered in this manner after the data was exported to a laptop.

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Figure 1. Data taken close to a 750 kVA transformer

Figure 2. Data taken by light rail line

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The spectrum covers the frequency range from 0 to 50 kHz. It is clear that the spectrum can vary widely given the

surrounding environment. In the first example there are distinct signal peaks at generally regular intervals while in

the second plot the interference is the highest at the lower frequencies but much less towards the upper end of the

frequency range.

Over a period of several months, a large number of these tests were performed in multiple locations. The initial

assumption was that interference would be relatively predictable, concentrated around power line harmonics.

However, as data was being gathered, what stood out was that each test yielded a unique interference profile. An

analysis to try to identify optimum operating frequencies revealed that a given frequency which would have been

appropriate for a given test site, would not have worked nearly as well at other sites. In other words, there is no

“best” frequency or set of frequencies that will work for every drilling environment.

DCI also performed comparative testing with more powerful transmitters, in an effort to gauge which approach –

changing frequency or increasing power – yielded better results over a range of varying drilling environments.

While increasing power yielded benefits in performance in some cases, the test data indicated that in a majority of

cases, changing frequency provided a significant performance advantage relative to increasing power.

It became clear that the best way to deal with interference is to identify site specific optimum frequencies that can

efficiently carry the signal from the underground transmitter given the interference profile at a particular place and

time. This became the focus of the DCI engineering team. Although the frequency plots were a vital part of the

testing and research, it was clear that this type of information was not going to be useful for the average user.

Figure 3 depicts a close up of the data from the transformer test site (Figure 1) and can be used to illustrate the

nature of the interference identified. In this case the spectrum between 24 and 27 kHz is being highlighted. This

allows for a closer view and also illustrates the receiver’s capability to measure very discrete frequencies.

Figure 3. Narrowing in on the frequency band between 24 and 27 kHz from transformer test

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As can be seen in Figure 3, a relatively small change in frequency can significantly impact the measured signal.

Around 25.4 kHz there is a peak and about 100 hertz higher (25.55 kHz) the interference is significantly lower. This

clearly demonstrates that picking fixed frequencies, as has been the practice until now, is not the best approach since

it is impossible to choose one that is consistently free from interference.

The interference signal is measured in dB. For every 18dB of transmitter signal above the interference, the locating

range is effectively doubled. It can therefore be seen that at this particular site, a locating system operating at 25.4

kHz would have fared significantly worse than one operating on 25.55 kHz but better than one operating at 24.6

kHz. DCI has concluded that the operating frequency is the paramount factor in overcoming interference for an

HDD locating system. An optimum system would ideally be able to adapt to varying conditions and therefore be

capable of operating at a large number of different frequencies.

8. DCI’s NEW SYSTEM: “FALCON” TECHNOLOGY

DCI has developed a new technology to augment its existing DigiTrak F2 and F5 locating systems, branded

“Falcon.” Falcon is a technology that allows a user to measure interference at a job site, identify one or more

optimum frequencies or bands of frequencies, and pair these frequencies or bands with a transmitter. It was

important to minimize the amount of new learning required for operators who have invested in learning existing

locating systems. Falcon technology is intended to be a (significant) feature upgrade representing continued support

of existing lines of locating systems, as opposed to a new model that requires the user to start over.

The initial implementation of Falcon technology involves splitting the roughly 40kHz wide operating range (4.5 to

45 kHz) into 9 bands, each band spanning 4.5 kHz. The bands are identified by their approximate center frequency

and are 7, 11, 16, 20, 25, 29, 34, 38 and 43. This simplifies the user interaction as well as communications related to

frequencies. The user selects which band to use based on readings from the spectrum analyzer (in general, the band

with the lowest amount of active interference, with exceptions for the visible presence of passive interference such

as rebar). Within the selected band, the system identifies several specific frequencies customized for that site. The

system also allows the operator to select a second band to use as a backup. An operator might use the primary band

for the first half of the bore but then encounter interference (because, as indicated previously, interference shifts over

distance and time). This feature allows the operator to shift to the second selected band while the transmitter is

downhole, to continue the bore without having to trip out.

9. HOW DOES IT WORK IN THE FIELD?

The first step in using a locating system with Falcon technology involves a visual inspection of the job site in order

to identify the portion of the bore that might entail the greatest locating challenge. This could be the deepest part of

the bore, or the area with the most obvious active interference such as power lines, railway crossing, traffic lights or

transformers (to name a few examples). Once this point on the bore has been located, the receiver is powered up

parallel to the running line. It is important that the

transmitter be powered off or be at least 100 feet (30.5 m)

away. The user selects the menu item “Frequency

Optimizer” and the system will start scanning frequencies.

After about 15 seconds, the user is presented with a screen

display similar to Figure 4. This screen represents a “live”

indication of the average interference levels within each

frequency band and the red band at the top of each of the

interference bars represents the high point measured. Based

on this initial scan, bands 11, 20 and 43 have the lowest

measured interference. The next step in the process is to

walk the entire length of the bore with the receiver.

As the operator walks the bore path, the operator should note

areas where interference peaks. The taller the bar is, the

greater the active interference. As a rule of thumb, Figure 4. Frequency Optimization Scan display

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interference levels between -90/-72 dB would be considered low, -72/-54 dB represent moderate levels and -54/-18

dB represent interference that will become an issue as depth increases.

As the interference levels will often vary along the bore path, the display will show the bars increasing and

decreasing in height. Figure 5 illustrates what the display screen will look like in this case. After walking the entire

bore path, the scan (Figure 6) has revealed that bands 11 and 20 appear optimum and have been selected for use on

this particular bore. Although the area for the original frequency scan was based on a visual inspection, other areas

might emerge as having greater interference based on scanning the bore path. In that case, the user could simply

choose to rerun the Frequency Optimizer in that area.

Figure 6. Selecting optimum frequency bands

When selecting frequency bands, there are other considerations besides active interference. When sources of

passive interference are present, lower frequencies will often work better. Since Falcon technology does not

measure passive interference, user judgment is required to override the recommend band indicated by the screen in

Figure 5. The choice of band is always left to the user.

Once the user has selected the frequency

band or bands to use, the receiver and

transmitter need to be paired. Both the

receiver and the transmitter have Infrared

(IR) ports. By holding the two close

together, the two can now be paired (Figure

7). Once pairing has taken place, the

transmitter can be calibrated and the locating

system is ready for use. At this point, the

system operates the same as the previous F2

and F5 systems, with an objective of limiting

the amount of new training to use the system

with Falcon technology and making adoption

of this new technology as seamless and

convenient for the operator as possible.

10. SHORT PROJECT STORY – FIBER INSTALLATION IN OLYMPIA, WASHINGTON STATE

In October of 2015 HDD contractor Trenchless Technology was working on a fiber installation project in Olympia,

the capitol of Washington State. One bore in particular looked like it was going to be a challenge due to the

Figure 5. Interference high points

Figure 7. Pairing the receiver and transmitter

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multitude of existing utilities and the target depth of about 20 ft. (6.1m). The bore involved installing 4” HDPE

conduit over a distance of 740 ft. (225 m). In walking the bore path scanning for interference, one area in particular

appeared to be problematic. This area included a steam pipe feeding the capitol building. Once this area had been

identified as potentially problematic, above ground range tests using an F5 and Falcon F5 were performed. The

results of the tests are found in Table 1 below. In summary, the Falcon F5 receiver had about 40% greater depth and

data range than the F5 receiver.

Table 1. Range tests comparing F5 to Falcon F5 at Olympia job site

Locating System/Frequency Depth Range Data Range

F5/12 kHz 35 ft. (10.7 m) 45 ft. (13.7 m)

F5/19 kHz 45 ft. (13.7 m) 50 ft. (15.2 m)

Falcon F5/Band 20 65 ft. (19.8 m) 70 ft. (21.3 m)

Falcon F5/Band 38 55ft. (16.8 m) 60 ft. (18.3 m)

As a note on range testing, depth range is defined as the range where the system reads depth within the specified

accuracy, in this case plus or minus 5%. Data range is defined as the range where the data signal is lost.

After performing the range tests, the decision was made to select bands 20

and 7. Band 7 was selected since it offers the lowest frequencies, it was the

best option to deal with potential passive interference from the metal steam

pipe. Additionally, potential interference from reinforced concrete in the

street supported that choice. This bore required drilling in the curb lane of a

four-lane road and across a major intersection at the entrance of the Capital

building. The crew decided to start the bore in band 20 with the hopes that

the lower band 7 would not be required for the rebar since it was clear the

passive interference from the steam pipe would be the bigger challenge in

the deeper parts of the bore. Signal was solid during the entire bore at 20 ft.

(6.1 m) of depth, even in the intersection directly on the traffic light loops

the signal barely wavered which is a true testament to the ability to optimize

and avoid the active interference. The last 200 ft. (61 m) of the bore allowed

for a shallower depth and the crew performed a below-ground frequency

change to band 7 and verified the ability to locate around the passive

interference of storm drains and rebar in the sidewalk area near the exit pit.

All together the crew completed 6 total bores to complete this leg of the

fiber project and there were no issues with locating in a very busy section of

Olympia Washington.

Figure 9. Bore profile as monitored at the drill rig

Figure 8. Olympia job site

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11. SHORT PROJECT STORY – RAILROAD CROSSING IN FREUDENSTADT, GERMANY

In December of 2015, Leonhard Weiss, a large German contractor, was faced with a fairly typical railroad crossing

in the town of Freudenstadt, Germany. Railroad crossings tend to be problematic due to interference typically

caused by signal transmissions along the tracks. The project was to install 160 mm (6.3”) PE protective pipe for a

110 mm (4.3”) HDPE water line at a maximum depth of 9 m (29.5 ft.) over a distance of a 100 m (328 ft.)

underneath two sets of railroad tracks. There is a minimum depth requirement of 5 m (16.4 ft.) below the rails for

such crossings.

Weiss had previously attempted the crossing using a

DigiTrak F5 receiver and three different transmitters. The

first attempt involved the standard F5 transmitter, which

has a range of 19.8 m (65 ft.) and as a dual frequency

transmitter operates on 12 and 19 kHz. Due to the high

interference at the tracks, signal from the transmitter was

lost at depth of about 6 m (19.7 ft.). This represents the

difficulties that interference presents in that the usable

range of the transmitter on that site was less than a third of

its rating which is based on industry standard tests in an

interference-free environment.

Two later attempts, using more powerful transmitters, rated

at 25.9 m (85 ft.) operating at 12 and 19 kHz were similarly

unsuccessful indicating that those two frequencies were

being particularly hard-hit.

The final attempt involved using a Falcon F2 receiver. By

using the frequency optimizer the lowest interference was found on bands 25 and 43 so those were selected for the

crossing. Weiss elected to follow one of the bore holes

from the previous attempts but now that the receiver

readings were more accurate than before, it became clear

that this bore path would end up being much deeper than

planned. Therefore the bore was relaunched but as they got

close to the first set of rails at a depth of 9 m (29.5 ft.)

signal became intermittent. The foreman on site then made

the decision to change the bore plan and cross the rails at

the shallower depth of 6 m 19.7 ft.). At this depth with the

optimized frequencies of the Falcon F2 system, both the

locating signal and roll pitch data were very stable and no

issues were experienced in tracking the head and

controlling the bore.

The main take away for the contractor was that by being

able to select appropriate frequencies and modifying the

bore plan to suit the site conditions, a bore that had been

very troublesome was finished successfully in relatively

short amount of time.

Figure 11. Falcon F2 frequency optimization

Figure 10. Freudenstadt railroad crossing

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12. SUMMARY

Advancements in locating the drill head underground have focused primarily on enhancing the usability of complex

electronics packages that allow the drill rig to be used in a productive manner. Locating systems have become easier

to use and train on, while at the same time adding more sophisticated features.

Recently, attention has turned away from user interaction and features to the challenge of fighting active

interference. As underground (and above ground) become more crowded, interference has become one of the

primary obstacles to completing HDD projects around the world. Active interference affects a locating system´s

ability to receive data necessary for a crew to navigate the drill head underground in an efficient and productive

manner. A number of approaches have been investigated including the impractical solution of enlarging the

transmitter diameter and lengthening the encasement for the electronics. Obviously, this type of approach would

require great expense to the industry in the form of new tooling standards and the forced investment associated with

new equipment and tooling purchases.

A novel approach to addressing active interference has been developed that doesn’t require a change in industry

standards or investment in new underground tooling. Rather than using a single-frequency transmitter, the new

approach allows the user to select bands containing site-specific frequencies from within a wide range of frequencies

between 4.5 kHz and 45 kHz making the system effective across a variety of jobsites. Because interference has no

common or fixed signature, the use of a single frequency to carry a signal becomes problematic unless there are

many single frequencies that have been preselected for optimum performance under ever changing interference.

Until now, an underlying technical design to solve this problem has been elusive.

For the first time in many years, the underlying approach to addressing active interference at the jobsite has

fundamentally changed.