Download GTILT® PLUS - Geokon, Incorporated

Page 6-8
GTILT USER’S MANUAL Version 3
Getting the Data into GSPIRAL
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Using a serial cable connection, transfer the results of the survey to a PC using
the UNI_DOWN program which is supplied with the logger. Store the information
in a file with the extension .NUM, e.g. 5432.NUM.
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Start GSPIRAL and use File/Import NUM File to import the .NUM file. As part of
the importing process, GSPIRAL averages the readings in groups of three, so
that only a single reading appears for 99, 97.5, 96 m and so on. The reading for 0
m depth is ignored.
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Before saving the data to disk or making any plots, be sure to enter the spiral tool
offset (average of the values measured in steps 5 and 6) in the appropriate field
on the GSPIRAL SubHeader. Remember to include the sign, if negative. You are
now ready to create plots of cumulative and incremental twist versus depth, and
by using File/Save As, you can create an .SPI file for use with GTILT PLUS, as
described in section 6.2.4.
Sample File
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A sample of data in NUM format is supplied in the file 5432.NUM.
6.3
Accelerometer Rotation Error
This type of error often manifests itself as a rotation of the plot about the base of the
inclinometer, so that a significant deflection appears to have built up by the time the plot
reaches the surface, but there is no obvious shear plane. This type of error is usually
associated with casings which have been installed at two degrees or more from vertical.
According to the instrument manufacturers, it is difficult to align an accelerometer to
better than about 0.5 degrees of azimuth, equivalent to about 0.010 radian. If a casing is
installed with a significant tilt (say 3 degrees) in the B direction, the readings taken in
the A direction are likely to suffer from accelerometer rotation errors, especially if the
probe is subjected to rough handling, if the accelerometer mountings are adjusted for
any reason, or if the same probe is not used for successive sets of readings. The
problem shows up on the cumulative displacement plot in direction A as a divergence
from the expected result. This divergence takes the same shape (but not necessarily
the same sign) as the absolute position plot in the B direction.
The B direction CD plot can similarly be affected by changes in the alignment of the B
accelerometer if the casing is out of vertical in the A direction.