Page 21 - West Virgina 811 Magazine 2020 Issue 3

How deep is the line buried? During my years of line locating, this is a question that I’ve probably been
asked about a thousand times. I was
not allowed by my employer to provide depth readings to excavators for liability reasons. My normal response was, “I can’t see it so I don’t know how deep
it is.” There are several reasons why
an electronic depth reading provided
by line locating equipment can be wrong. This makes it impossible for
the equipment manufacturer or the operator of the line locating equipment to guarantee an accurate depth reading is being provided by the machine.
The electronic depth reading is not
a reading that you would want to bet anyone’s life on. The depth readings provided by the hand held line locating equipment is an estimation of distance from the bottom of the signal receiver to the center of a locatable signal being broadcast from an underground line. The only sure way to know the depth of any buried line is to safely expose the line and see it with your own eyes.
Electronic Depth Estimation and Signal Fields
An electronic depth measurement is a distance calculation from the bottom
of the locator’s signal receiver to the center of the signal field being detected. This reading is not a depth of cover over a buried pipe. Since the receiver
is estimating distance to the center
of a perfectly round signal, if you’re locating a large diameter pipe the depth calculation is to the center of
the pipe. It has been claimed in many equipment manuals that in suitable conditions the accuracy of the depth reading provided by the signal receiver should +/- 5% for lines up to 10 to 15 feet in ideal conditions. The likelihood of error increases with the depth of the line because signals created on deeper
How Deep is It?
lines are weaker and less reliable when detected and calculated at surface level.
Suitable conditions for depth measurement are when the signal transmitter is directly connected to a buried line that is buried in a straight line without any adjacent lines in the ground. The locatable signal would be
a strong signal that is perfectly round in shape as it radiates or is broadcast out from the underground line. It would be similar to a water ripple created by a golf ball in the middle of a calm pond. In areas of signal wave distortion, the signal is not perfectly round which causes the mathematical calculation
of distance to line to be incorrect. In
an area of signal distortion caused by target signal coupling with a nearby line, the depth reading can be in error up to 50% off the actual depth. That means a line buried 10 feet deep can produce a depth reading of either 5 feet or 15 feet.
Tips for Electronic Depth Estimating
Choose a point along the target line where it runs in a straight line for
at least 10 feet in both directions. Avoid taking a depth measurement within 15 feet of the transmitter due to interfering fields being broadcast from the temporary ground stake and wire connection leads.
The most accurate depth estimates are normally obtained from the target line when the signal is created by a signal transmitter that is directly connected
to the targeted line. The depth assumptions are that the receiver is directly over the top of the line. Another assumption is that the receiver’s handle is aligned with the direction of the line or orientation of the signal field.
Use the guidance indicators and signal strength readout to pinpoint the exact location directly over the line. This will be the top dead center of the round
signal field. Next, establish the exact line direction of the line. Some receivers align the handle of the receiver to the direction of the buried line. Set the bottom of the receiver on the ground while maintaining alignment and
obtain an electronic depth reading by either the push of a button or full time display. Note the depth at ground level and then raise the receiver directly up about a 12 to 18 inches above surface. Check the depth reading again and do the math. The reading should equal the sum of the depth at ground level plus the distance you lifted the receiver from the surface.
There are many areas along a buried line where depth measurements are
not favorable. A few of these areas include any point along the line that is within 8 to 10 feet of an abrupt turn or change of direction in the target line. This includes the area where the line could be at a downward or upward pitch from the surface of the ground. Other areas to avoid taking a depth reading would be near a tee in a pipe
or splice in a cable or tracer wire. In these areas, the signal splits in multiple directions and will collide and distort. Any point along the path of a target line where signal may bleed over to another nearby line or metallic object, like a fence or heavy equipment, will also produce unfavorable results. This also includes joint trench scenarios where your targeted line is buried with several other lines. These areas may cause the target signal to become substantially distorted due to signal coupling.
With countless variables which cause an electronic depth reading to be in error, it will never be as reliable as safely exposing a buried line prior to digging across, above or beneath a buried line. If the depth of the line is important, the only way to guarantee it is to see it. Dig Safe!
2020, Issue 3 West Virginia 811 • 19