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5.2 Verifying depth measurements
Check a suspect or critical depth reading by lifting the
locator 50mm (2 inches) above the ground and repeating
the measurement. If the measured depth increases by
the same amount it is a good indication that the depth
reading is correct.
Depth measurements should be accurate to ±5% if
conditions are suitable. However, you may not always
know if the conditions are suitable so the following
techniques should be used to check critical readings:
•
Check that the route of the line is straight for at least 2
meters (6'6") to either side of the measurement point.
•
Check that the signal response is reasonably constant
over the 15 meter distance to the transmitter and make
depth measurements to either side of the original point.
•
Check that there are no adjacent lines carrying a
significant signal within 1 to 2 meters of the target
line. This is the most common source of error of depth
measurements as a strong signal coupled to
an adjacent line can often introduce ± 50% error.
•
Make several depth measurements at points slightly
displaced from the line's apparent position. The
shallowest indication will be the most accurate and
will also indicate the line's position most accurately.
5.2.1 Rough depth calibration check
This check is a quick and easy way of verifying that the
depth reading on the locator is within acceptable limits.
It can be used if you are getting inaccurate depth
readings from a cable or pipe for which you know the
approximate depth. Inaccurate depth readings could be
the result of the locator picking up a stronger signal such
as another cable or pipe running in close parallel to the
target pipe or cable.
There are two ways of checking the calibration of the
locator in the field. Both methods require the use of a
transmitter:
Method 1
Place the transmitter on top of a non-metallic object, such
as a cardboard box, on the ground and away from any
buried lines. Switch the transmitter on, ensuring that no
accessories are connected and that the transmitter is in
induction mode. To reduce the effects of ground coupling
the transmitter, when it is placed on the box, must be
more than 500mm above the ground
Hold the locator with the blade horizontal and pointing
towards the front of the transmitter and approximately five
meters from the front of the transmitter.
1.
Switch the locator on.
2.
Select the same induction frequency as is selected on
the transmitter.
3.
Select sonde mode on the locator.
4.
Move the locator from left to right and when the
maximum signal is obtained place the locator on top
of a non-metallic object, such as a cardboard box,
on the ground. Ensure that the blade is horizontal
and pointing towards the transmitter. To reduce the
effects of ground coupling the locator must be more
than 500mm (18 inches) above the ground when it is
placed on the box.
5.
Note the depth as indicated on the locator.
6.
Measure the distance from the base of the locator to
the center of the transmitter using a tape measure.
7.
Compare this reading with the depth reading on the
locator.
The locator can be considered accurate if the difference
between the depth reading on the locator and the
distance measured with the tape is less than 10%.
Method 2
1.
Apply a signal to a cable or pipe of known depth.
2.
Locate the cable or pipe; the locator will show depth
automatically.
3.
Compare the depth reading on the locator with the
actual depth.
RD7000+ Operation Manual 17
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