9
In older wells, the casing may be corroded or rough
, causing high attenuation and unreliable
water level readings.
In this situation, the higher sensitivity
DEEP
mode will minimize the problem. However, to
use this setting, the water level must be above 60 meters.
An error in the measurement can occur
if the well casing has discontinuities
, causing an
incorrect reflection of the sonic pulse. This situation can occur
in rock wells if there are voids
or cracks in the rock face.
It can also occur in wells with continuous casing if the casing
diameter changes abruptly somewhere in the well.
Perforations or cracks in the casing that are above the surface of the water
can reflect the
sonic pulse resulting in a shorter measurement of the actual water level.
Obstructions in the well housing
, such as torque stop devices, cable shields, or anything that
exceeds half the surface of the enclosure, can also cause erroneous level readings.
Sleeves in the shaft case can sometimes cause a problem
depending on how far it is at the
top of the sleeve because the sonic pulse can reflect from the top edge of the sleeve.
Wide
cuffs can cause an additional problem
because the sonic pulse can split between the inside
and outside of the cuff, reducing the signal level.
In some cases the signal coupled to the well casing can be poor.
To avoid this, make sure
that the measurement line passes completely through the well plug or gasket. Cover plates
should normally be used. Make sure there is no large gap between the plate and the well
casing.
Gases in the wellbox or bore other than air can cause a measurement error.
The amount of
error will depend on the air / gas ratio. Measuring the water level depends on knowing the
speed of sound in the well case. This parameter is different in different gases. The meter is
calibrated for enclosures that are rarely filled with a nominal humidity. (For a container filled
with methane the difference can be up to 30%).
