By Steve Benzie
When we apply a locate tone to a buried utility, we create an electromagnetic field around the utility. Its this field that we detect when we are utility locating. In an ideal world, the signal radiates out from the utility in a nice predictable way. All cable locators are adjusted and calibrated assuming this is the case. However, in the real world, the signal is very often distorted which can lead to services being located in the wrong position or depth
There are a number of techniques that can be used to identify a distorted field. I will explain a few ways. I am sure there are others, but these are the ones I use:
Most modern cable locators have horizontal antennas that give a good precise signal that maximizes the bar graph reading when it is directly over the utility. They also have a vertical antenna that creates a minimum bar graph reading when over the utility. The two are referred to as peak and null modes. The null mode tends to be affected by distortion more than the peak, so we tend to encourage the use of the peak mode for accuracy. However, we can use the null mode to help identify distorted fields as the two locate positions will coincide when there is no distortion but will be displaced in a distorted field. The more the peak and null locate positions are displaced, the more signal distortion.
This is a good technique but can be fooled by symmetrical distortion. This can occur, for instance, where there are two locate signals, one directly above or below the target line. If this is suspected, then using the “Extended depth technique” can help.
Extended depth technique
A simple but very effective method. Take a depth reading at ground level, raise the locator a known distance, say 1ft, note the new depth reading. If the indicated depth has not increased by that amount, the information should be treated with caution. This simple method will identify distorted field even in situations where the peak/null technique could struggle, for instance where the interference is coming directly from a utility buried directly above or below the target utility.
This sounds complicated, and I guess it is, as it uses an array of six antennas to analyse the field shape in three dimensions. It’s only available in the very latest cable locators, such as the vLoc3 from Vivax-Metrotech. The good news is that design engineers have done all the hard work for us and presented the information in a simple color coded display.
Green bar graph indicating no/minimal distortion
Blue bar graph indicating some distortion so treat the information with caution.
Red bar graph indicating excessive distortion so do not trust the information.
Distorted signals are a major reason for mis-locates, but if we understand what causes them, we can mitigate their effects.
I believe distortion is caused in three main ways:
- The utility changing direction or depth
- Electrical cross bonding of cabling or pipes
- Inductive and capacitive bleed over of locate tone
Changing direction is the easiest to deal with. The answer is to take readings where the utility is not deviating. So, stay away from bends “T’s” or sudden changes in depth. The deeper the utility, the further away from the deviation you need to be.
Electrical cross bonding. The best way to deal with this is to remove the cross bonding so that the utility you want to locate can be isolated. Removing cross bonding is usually possible on pipelines and telephone cabling but is not usually possible with power lines. Never tamper with cabling unless authorised to do so.
Bleed over. This usually occurs where signal bleeds across from the target utility onto an adjacent line. This causes a secondary current to flow which causes distortion. The best way to deal with this is by reducing the frequency of the locate tone as the effects of capacitance and inductive coupling reduces with lower frequencies. Secondly make sure the position of the ground is well away from utilities in the same vicinity as the signal will choose to hitch a ride on the one nearest the earth stake.
Lastly, remember that it is very rare to have a perfect, distortion free signal. So always check for distortion and dig with care.