For several reasons, including signal strength and shape, the electronic depth estimate provided by locate equipment is not guaranteed. A depth estimate provided by the locate technician may give an excavator a false sense of security when crossing over or under a buried line.
The electronic depth reading is not a reading that you would want to bet anyone’s life on. The depth readings provided by handheld electromagnetic line locating equipment is an estimation of distance from the bottom of the signal receiver to the center of a locatable signal broadcasting from an underground line. The only absolutely accurate 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 – NOT a measurement of the depth of cover over a buried pipe. Since the receiver is estimating distance to the center of a perfectly round signal, if you are locating a large diameter pipe the depth calculation is to the center of the pipe. Many equipment manuals claim that in suitable conditions the accuracy of the depth reading provided by the signal receiver should be +/- 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 lines are weaker and less reliable when detected at surface level.
THE LIKELIHOOD OF ERROR INCREASES WITH THE DEPTH OF THE LINE BECAUSE SIGNALS CREATED ON DEEPER LINES ARE WEAKER AND LESS RELIABLE WHEN DETECTED AT SURFACE LEVEL.
Suitable conditions for depth measurement are when the signal transmitter is directly connected to a facility that is buried in a straight line without any adjacent facilities in the ground. The optimum locatable signal would be a strong signal with a perfectly round shape as it radiates or broadcasts out from the underground line, similar to a water ripple created by a golf ball in the middle of a pond of calm water.
When signal wave distortion exists, that signal is not perfectly round and results in the mathematical calculation of distance to line to be incorrect. In the event 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 ranging from 5 to 15 feet.
Tips for Electronic Depth Estimating
For best results, choose a point along the target line where it runs in a straight line for at least 10 feet in both directions from that point. Avoid taking a depth measurement within 15 feet of the transmitter due to interfering fields being broadcast for the temporary ground stake and wire connection leads.
OTHER AREAS TO AVOID TAKING A DEPTH READING WOULD BE
NEAR A TEE IN A PIPE OR SPLICE IN A CABLE OR TRACER WIRE.
The most accurate depth estimates are normally obtained from a buried line when the measurement is taken from a signal 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 the 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. Then establish the exact 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 12-18 inches up from the ground. 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 raised the receiver.
There are many areas along a buried line where depth measurements are not favorable, including any point that is within 8-10 feet of an abrupt turn or change of direction in the target line or areas 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. Finally, 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, including 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 the countless variables which can 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!
Bob Nighswonger is President of Utility Training Academy (UTA). Visit damagepreventiontraining.com to learn more about damage prevention training for excavators and locators.