Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Hole: 909-C
Leg: 151
Location: Yermac Plateau (Arctic Ocean)
Latitude: 80° 15.894' N
Longitude: 6° 35.430' W
Logging date: September, 1993
Bottom felt: 2529 mbrf
Total penetration: 1061.8 mbsf
Total core recovered: 604.77 m (61%)
Logging
Runs
Logging String 1: DIT/SDT/HLDT/NGT
(upper and lower sections; HLDT failed to operate in upper section)
Logging String 2: FMS/GPIT/NGT
(lower section, main and repeat). No FMS/GPIT/NGT run in the upper section due
to time limitations.
Wireline heave
compensator was used to counter ship heave.
Bottom-hole
Assembly
The
following bottom-hole assembly depths are as they appear on the logs after
differential depth shift (see "Depth shift" section) and depth shift to the sea
floor. As such, there might be a discrepancy with the original depths given by
the drillers onboard. Possible reasons for depth discrepancies are ship heave,
use of wireline heave compensator, and drill string and/or wireline stretch.
DIT/HLDT/SDT/NGT
(lower section): Bottom-hole assembly at ~589 mbsf.
FMS/GPIT/NGT
(lower section): Bottom-hole assembly at ~590 mbsf.
DIT/HLDT/SDT/NGT
(upper section): Bottom-hole assembly at ~89 mbsf.
Processing
Depth
shift: The lower DIT/HLDT/SDT/NGT run has
been depth shifted with reference to the lower FMS/GPIT/NGT run and then both
lower DIT/HLDT/SDT/NGT and FMS/GPIT/NGT runs have been depth shifted with
reference to upper DIT/SDT/HLDT/NGT. Lower FMS/GPIT/NGT has been applied a
constant depth shift of - .53 m. All logs have been shifted to the sea floor (-
2526 m). This amount differs from the bottom-felt depth because the mud line on
the logs is 3 m higher than the drillers' depth.
The program used is an interactive,
graphical depth-match program which allows to visually correlate logs and to
define appropriate shifts. The reference and match channels are displayed on
the screen, with vectors connecting old (reference curve) and new (match curve)
shift depths. The total gamma ray curve (SGR) from the NGT tool run on each logging
string is used to correlate the logging runs most often. In general, the
reference curve is chosen on the basis of constant, low cable tension and high
cable speed (tools run at faster speeds are less likely to stick and are less
susceptible to data degradation caused by ship heave). Other factors, however,
such as the length of the logged interval, the presence of drill pipe, and the
statistical quality of the collected data (better statistics is obtained at
lower logging speeds) are also considered in the selection. A list of the
amount of differential depth shifts applied at this hole is available upon request.
Gamma-ray
processing: Data have been processed to
correct for borehole size and type of drilling fluid.
Acoustic
data processing: The array sonic tool was
operated in standard depth-derived borehole compensated mode, including
long-spacing (8-10-10-12') logs. The sonic logs have been processed to
eliminate some of the noise and cycle skipping experienced during the
recording. Using two sets of the four transit time measurements and proper
depth justification, four independent measurements over a -2ft interval
centered on the depth of interest are determined, each based on the difference
between a pair of transmitters and receivers. The program discards any transit
time that is negative or falls outside a range of meaningful values selected by
the processor.
Quality
Control
null
value=-999.25. This value generally appears in discrete core measurement files
and also it may replace recorded log values or results which are considered
invalid (ex. processed sonic data).
During
the processing, quality control of the data is mainly performed by
cross-correlation of all logging data. Large (>12") and/or irregular
borehole affects most recordings, particularly those that require
eccentralization (CNTG, HLDT) and a good contact with the borehole wall.
Invalid density readings are observed at 11.5, 31, and 77.5 mbsf.
Hole
deviation can also affect the data negatively; the FMS, for example, is not designed
to be run in holes deviated more than 10 degrees, as the tool weight might
cause the caliper to close. Hole 909C has a quite severe deviation from the
vertical: ~14 degrees at 600 mbsf up to a maximum of 25.6 degrees at 1010 mbsf.
The effect of the deviation is to strongly eccentralize the tool string in the
borehole; while this might not be a problem for tool strings such as the
DIT/HLDT/SDT/NGT which are usually run eccentralized, it is a serious potential
problem for the FMS string. The deviation, combined with the tool weight, might
cause the caliper arms to close. Therefore, caution is suggested when
interpreting the data recorded by the FMS string.
Hole
diameter was recorded by the hydraulic caliper on the HLDT tool (CALI)and on
the FMS string (C1 and C2).
Details
of standard shore-based processing procedures are found in the "Explanatory
Notes" chapter, ODP IR Volume 151. For further information about the logs,
please contact:
Cristina
Broglia
Phone:
845-365-8343
Fax:
845-365-3182
E-mail: Cristina Broglia