Leaders in Pipelin
e Survey Systems
C
C
C
-
-
-
S
S
S
C
C
C
A
A
A
N
N
N
2
2
2
0
0
0
1
1
1
0
0
0
SYSTEM MANUAL Version 6.03i
53
Fig 7.4 – Attenuation v Conductance for various ground resistivities
By studying the graph it can be seen that when soil resistivities are above 100k ohm.cm,
the change in attenuation between fair and bad (500µS/m
2
and 2000µS/m
2
) is very small.
There is very little change in attenuation on the 1M
Ω
.cm & 10M
Ω
.cm lines between the
two conductance values. For further information about earthing (grounding), see section
16.2 below.
7.12. Changes in Soil Resistivity
Resistivity changes during the course of a survey should not materially affect the results,
as the current stabilising circuitry in the Signal Generator will ensure that the applied signal
current remains constant despite such changes; until deliberately altered by the Surveyor.
Seasonal changes in soil resistivity (from a wet spring to a hot dry summer) will affect the
results, but experience to date suggests that changes in attenuation arising from this
cause are unlikely to exceed 5%. This variation can be allowed for and is unlikely to affect
the validity of the results. For regular ‘check’ surveys, it is suggested that any given
pipeline is always surveyed at the same time of year to minimise possible interpretation
errors from this source. If preferred, the resistivity of the soil can be measured with the
appropriate equipment and the reading can be entered into DCAPP when the data is
downloaded.
7.13. New Pipelines
It is strongly urged that new pipelines should be checked, section by section, after laying
but before backfill is complete. This will enable the surveyor to locate many of the wrap
faults and damage caused during installation so that repairs can be effected with minimum
disturbance and delay. Current values and attenuation rate (but not accurate depth
readings) can be obtained from the side of the trench by tilting the Detector so that the
antenna tube ‘points’ at the pipeline and the OVERHEAD signal is obtained on the display.