Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Hole: 850B
Leg:
138
Location:
East Pacific Rise (tropical NE Pacific)
Latitude:
1° 17.827' N
Longitude: 110° 31.286' W
Logging date: June, 1991
Bottom felt: 3797.8 mbrf (used for depth shift to sea floor)
Total penetration: 399.8 mbsf
Total core recovered: 393.6 m (99.2 %)
Logging Runs
Logging string 1: DIT/SDT/HLDT/NGT
Logging string 2: ACT/GST/NGT
Logging string 3: FMS/GPIT/NGT (2 passes)
Wireline
heave compensator was not 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
and drill string and/or wireline stretch.
DIT/SDT/HLDT/NGT:
Recorded open-hole
ACT/GST/NGT:
Bottom-hole assembly at ~46.5 mbsf
FMS/GPIT/NGT:
Recorded open-hole.
Processing
Depth
shift: Original logs have been
interactively depth shifted with reference to NGT from FMS/GPIT/NGT main pass
and then 0.9 m upward to match the GRAPE values. Finally, they have been depth
shifted and the sea floor (- 3797.8 m). 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: NGT data from
DIT/SDT/HLDT/NGT, ACT/GST/NGT, and FMS/GPIT/NGT main pass 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 are of excellent
quality and velocity has been calculated after minor editing.
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 (HLDT) and a good contact with the borehole wall. 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.
Data
recorded through bottom-hole assembly should be used qualitatively only because
of the attenuation on the incoming signal. Invalid gamma ray readings were
recorded by the ACT/GST/NGT tool string in the 23-25.5 mbsf interval.
Invalid
resistivity readings were recorded in the 360-365 mbsf interval.
Hole
diameter was recorded by the hydraulic caliper on the HLDT tool (CALI) and on
the FMS string (C1 and C2).
Additional
information about the logs can be found in the "Explanatory Notes" and Site
Chapter, ODP IR volume 138. For further questions about the logs, please
contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia