This will serve as an initial reference level. The operator then
scans the earth with the instrument, moving it in a direction
which will cross over the expected run of the cable. The full
scale reference level signal will remain until the instrument
crosses over the energized cable. As it crosses to a minimal
level, the cable lies directly under the point where the signal
response “nulls out”
(Fig. 7 and 8)
. (Note -- if the nulling of
the signal occurs over an area which is too broad for deter-
mining accurately where the cable lies, the amplifier’s gain
should be increased; increasing the gain will “sharpen” the
response and make the null-area narrow enough to allow
precise pinpointing of where the cable lies. Conversely,
if the null-area is so “sharp” that it is difficult to detect it at
all. The gain of the amplifier must be reduced.) Finding a
sharp null-point thus “locates” where the cable lies.
The “located” cable may now be “traced” along its entire run
by the same minimizing technique. By finding two null-
points, the operator can determine the direction in which the
cable runs. He then walks with the “Kab-L-Tec” in the direc-
tion of the cable’s run and moves the “Kab-L-Tec” to his left
and to his right as he walks. Each time that the detection rod
crosses over the energized cable, the signal response will
“null out” to a minimum, and each time that the detection
rod moves either to the right or to the left of the energized
cable, the signal response will rise sharply to a high level.
By following the sequence of these null points, the operator
traces out the line under which the cable lies.
12
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