24
78-8130-6151-8 Rev G
accuracy along with an effective identification. There are two main types of “no access”
or “blind” Induction Sweeps;
Passive Sweeps:
These require only the use of the Receiver with passive frequency
detection capability. This method uses existing external signal sources, such as 60 Hz/50
Hz for electric power, 15 kHz to approximately 30 kHz for low frequency radio signals,
120 Hz / 100 Hz CPS impressed current Cathodic Protection signals and the less
commonly occurring CATV signal of 31.25 kHz NTSC with CRT TV turned on.
Active Sweeps:
These require both the Receiver and Transmitter with Induction
capability. This method refers to using a 3M™ Dynatel™ Advanced Locator Transmitter
as the signal source through induction, instead of relying on passive frequencies that
may exist on the buried conductor or metal pipe. Active Induction allows for detecting
buried cables and metal pipes in the absence of passive signals and at deep depths
and short sections. Dynatel transmitters provide a choice of induction frequencies and
output levels. Higher induction outputs are needed for detecting deeper depths and
longer ranges. The Dynatel 2200M Series 3 Watt transmitters provides two induction
frequencies: 33 kHz and 133 kHz. The 12 Watt transmitter provides three induction
frequencies: 8 kHz, 33 kHz and 133 kHz. The 133 kHz frequency is commonly used for
deeper cables/pipes and the lower frequencies are used to give longer locate distances.
The 8 kHz (low frequency) induction helps in locating shallow facilities, such as risers.
C. Passive Sweeps
Select Peak mode on the receiver and then select the frequency (source; for example,
power, LF, etc.) to be traced. Passive sweeps at power frequency will detect buried
power cables and conductors carrying 60 Hz/50 Hz power signal in the ground. Other
passive sources exist, such as Cathodic Protection, LF and CATV cable (with NTSC TV
CRT turned on).
Walk in a grid pattern over the sweep area holding the receiver as shown in the
following illustration. Stop when there is a response increase, locate the position of the
maximum signal, follow the conductor path all the way out of the sweep area while
putting location marks on the ground. Resume the sweep until another cable or pipe is
detected, or the whole area is completely swept. Switch to another available passive
frequency in the receiver and sweep again.
D. One-person Active Induction Mode Sweeps
One-person active induction mode sweeps require that the transmitter be set on the
ground over the suspected path of the buried infrastructure. The receiver is used to
detect signals induced by the transmitter on buried long conductors. Proper placement
