Wireline Standard Data Processing
ODP logging contractor: LDEO-BRG
Hole: 820B
Leg: 133
Location: Central Great Barrier Reef (tropical SW Pacific)
Latitude: 16° 38.219' S
Longitude: 146° 18.218' E
Logging date: September, 1990
Water depth: 290.3 mbrf
Total penetration: 400 mbsf
Total core recovered: 324.9 m (81.2 %)
Logging
Runs
Logging string 1: DIT/SDT/HLDT/NGT
Logging string 2: ACT/GST/NGT
Logging string 3: FMS/GPIT/NGT (2
passes)
Logging string 4: WST
The
logging report did not provide any information about use of wireline heave
compensator.
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:
Bottom-hole assembly at ~74 mbsf
ACT/GST/NGT:
Bottom-hole assembly at ~74 mbsf
FMS/GPIT/NGT:
Recorded open-hole.
Processing
Depth
shift: Original logs have been
interactively depth shifted with reference to NGT from ACT/GST/NGT run and to
the sea floor (- 290.3 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 pass 2 (longer pass) have been
processed to correct for borehole size and type of drilling fluid. During
geochemical processing, NGT from ACT/GST/NGT has been corrected for effect of
bottom-hole assembly.
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 (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.
Hole
diameter was recorded by the 3-arm mechanical caliper device (MCD) and by the
caliper on the FMS string (C1 and C2). The lithodensity tool caliper was not
operational.
Additional
information about the logs can be found in the "Explanatory Notes" and Site
Chapter, ODP IR volume 133 and in the Data Report in ODP SR volume 133. For
further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: chris@ldeo.columbia.edu