User's Manual

Locating
DigiTrak
®
F5
®
Operator’s Manual 63
Interference: What It Is and How to Check for It
Before drilling (preferably before bidding on a project) the interference potential at your site should be
evaluated. Interference can reduce the transmitter’s range or cause variable readings and possibly result
in job slowdowns. Interference comes from two different types of sources: active and passive.
Active interference is also known as electrical interference or background noise and can have varying
effects upon the F5 locating equipment. Most electrical devices emit signals that can affect your ability to
locate the tool accurately or to get good pitch/roll readings. Some examples of active interference are
traffic signal loops, buried dog fences, cathodic protection, radio communications, microwave towers,
cable TV, fiber-trace lines, utility data transmissions, security systems, power lines, and phone lines, to
name a few. You can conduct a test for the presence of active interference with your F5 system; see
“Conducting a Background Noise Check” below.
Passive interference can reduce the amount of signal received from the transmitter, which results in
deeper than expected depth readings or a completely blocked signal. Some examples of passive
interference are metal objects (such as pipes, rebar, trench plate, chain-link fence, or vehicles). Two other
examples of passive interference are saltwater/salt domes and conductive earth, such as iron ore. You
cannot conduct a test for the presence of passive interference with your F5 system. Conducting a
thorough site investigation prior to drilling is the best method of identifying passive interference sources.
The first step in familiarizing yourself with the interference potential along your intended bore path. The
next step is to conduct an electrical interference/background noise check.
Conducting a Background Noise Check
With the transmitter off, power up the receiver and walk the bore path while monitoring the signal strength
on the receiver’s screen, noting any locations where it changes. A general rule is that the background
noise should be at least 150 points less than the transmitter’s signal strength when measured at the
maximum depth for that bore. In the figure below, the red flag area denotes an increase in background
noise.
One-Person Background Signal Strength Check (No Transmitter)
Intended Bore Path
Background Noise