20
Besides overlapping each sweep, it is important to keep the
coil close and parallel to the ground. Pendulum-swinging the coil
results in loss of depth, and if the ground balance isn’t perfectly
set the variation in loop height will create a ground response.
The following illustration shows improper and proper coil sweep
motion.
Wrong Right
Dealing with Noise
Because PI detectors have wideband receivers they are
more prone to noise and interference than narrowband VLF
designs. There are a few strategies for dealing with noise. First,
TDI has a
Frequency Offset
control that slightly adjusts the
overall pulse rate. With the
Threshold
and
Gain
turned up to the
point of mild chatter, make very small incremental adjustments
to the
Frequency
control to minimize the chatter.
Some sources of noise cannot be dealt with electronically,
such as electric fences. Try to identify the noise source and dis-
able it. Indoor testing is notoriously difficult to do with PI
