NoDig 2016 TM2-T2-02 Final

Paper TM2-T2-02 - 1
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).

Paper TM2-T2-02 - 3
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

Paper TM2-T2-02 - 7
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.

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