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9 Interpreting Results
• Cross Bonding
Pipes can often be cross bonded. This is deliberate 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 nearby 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 the duration of the survey.
• Passing Vehicles
The sensing devices used to detect the 3Hz vLoc3-DM profiling signal are very sensitive to low frequencies. Vehicles passing
very close to the receiver will disturb the earth’s magnetic field and distort 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 results.
• 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.
9.2.2.2
Checking for Distorted Fields
Check to see if other radiated fields are distorting the signal. Locate the pipe, first in the “Peak” mode and then in the “Null”
mode. The two 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 switching 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 indicates
the same position there is unlikely to be significant field distortion.
2
1
3
1
Null Position
2
True Position
3
Peak Position