Vivax Metrotech vLocDM, Руководство пользователя

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iii) Cross Bonding
Pipes can often be cross bonded. This is a deliberate action designed to reduce the number of CP stations
and to limit the effects of stray currents.
If the pipes are laid in the same path and are in close proximity the currents flowing on the two pipes will
interfere with each other and cause interference. For best results the cross bonding should be disconnected
for duration of the survey.
iv) Passing Vehicles
The sensing devices used to detect the 3Hz vLocDM profiling signal are very sensitive to low frequencies.
Vehicles passing very close to the receiver will disturb the earths magnetic field and cause distortion of the
received signal. Try to take measurements when there is a gap in passing vehicles.
In areas where traffic is passing, wait until three constant consecutive readings have been shown before
saving result.
v) Pipe Sleeving
It is common practice to insert a pipe in a protective steel sleeve. This may distort the signal and cause errors.
Take readings before and after the sleeve. If they are the same, the pipeline is well insulated through the
sleeve. Sleeves are very often used at road crossings.
ii) Adjacent Pipes or Cables
Adjacent pipelines or cables very often have a signal induced onto them by capacitance or inductive effects.
These signals will interfere with the signal on the target pipe and will cause distortion. See diagram below.
8 Interpreting Results
8.2.2.2 Checking for Distorted Fields
Check to see if the signal is being distorted by other radiated fields. Locate the pipe, first in the “Peak” mode,
and then in the “Null” mode. The two locates should indicate that the pipe is in the same place. If they do not,
the signal field is distorted.
If the antenna mode, “Peak with arrows”, is selected the same can be achieved without having to switch
between modes. To do this, first find the position of the pipe using the largest bar graph reading as above. Then,
find the position as indicated by the arrows. The arrows will indicate the same position as the null response of
the bar graph. Hence, if the arrows and bar graph indicate the same position there is unlikely to be significant
field distortion.