Standard Wireline Data Processing
DSDP operator
and logging contractor:
Scripps Institution of Oceanography
Hole: 440B
Leg: 57
Location: Japan Trench (NW Pacific)
Latitude: 25° 13.3' N
Longitude: 85° 59.5' W
Logging date: November 1977
Sea floor
depth ("bottom felt"):
4517 mbrf
Sea floor depth (step in GR log): 4518 mbrf
Total
penetration: 814 mbsf
Total core
recovered: 401.9 m (49 %
of cored section)
Oldest
sediment cored: Diatomaceous claystone (Late Miocene)
Lithologies: claystone, sandstone, siltstone,
conglomerate, and breccia ; dacitic and ryolitic conglomerates
The logging data
was recorded by Schluberger. The original tape contained only partial data; the
analog data was digitized by Centerline Data in 2004 and processed at the
Borehole Research Group of the Lamont-Doherty Earth Observatory in March 2005.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. MCD/GR |
Pass 1 |
0 |
749.8 |
105.5 |
BHC failed |
| BHC/MCD/GR |
Repeat 1 |
Bad quality BHC |
|||
| 2. FDC/CNL/GR |
Main |
0 |
766 |
105.5 |
No caliper |
| Repeat |
666.7 |
772.4 |
No caliper |
||
| 3. BHC |
Pass 2 |
No MCD and GR. Bad quality BHC |
|||
| BHC |
Repeat 2 |
No MCD and GR. Bad quality BHC |
|||
| 4. HRT |
0 |
497 |
Logging
conditions at Hole 440B were far from ideal: due to a gale-force sea the ship
heave was about 2 m every 9 seconds during the first three runs and 1 meters
every 10 seconds during the last run. The BHC failed during the first pass and
was run again after the FDC/CNL/GR string; it is not clear if caliper and gamma
ray were recorded during the second pass, as they are not on the blueprints.
The High Resolution Temperature (HRT) tool was run last, almost 25 hours after
circulation stopped; only the uppermost 500 m were logged, due to the presence
of a bridge.
The depths in
the table are for the processed logs (after depth matching between passes and
depth shift to the sea floor). Generally, discrepancies may exist between the
sea floor depths determined from the downhole logs and those determined by the
drillers from the pipe length. Typical reasons for depth discrepancies are ship
heave, wireline and pipe stretch, tides, and the difficulty of getting an accurate
sea floor from the "bottom felt" depth in soft sediment.
Depth match
and depth shift to sea floor:
The original logs were depth-matched to the GR log from the main pass of the
FDC/CNL/GR tool string, and were then shifted to the sea floor (- 4518 m). The
first repeat pass of the BHC/MCD/GR could not be depth matched or shifted
because of a recurring technical problem that would cause the process to abort.
It is not included in the outputs, as the data of Pass 1 are of better quality.
Depth matching
is typically done in the following way. One log is chosen as reference (base)
log (usually the total gamma ray log from the run with the greatest vertical
extent and no sudden changes in cable speed), and then the features in the
equivalent logs from the other runs are matched to it in turn. This matching is
performed manually. The depth adjustments that were required to bring the match
log in line with the base log are then applied to all the other logs from the
same tool string.
The sea floor depth
was determined by the step in gamma ray values at 4518 mbrf. This differs by
only 1 m from the sea floor depth given by the drillers (see above).
Sonic data. As shown in the above summary table,
three acoustic logs were acquired at Hole 440B; the logs show poor
repeatability and no correlation with the density data. For this reason they
have not been processed.
The quality of
the data is assessed by checking against reasonable values for the logged
lithologies, by repeatability between different passes of the same tool, and by
correspondence between logs affected by the same formation property (e.g. the
resistivity log should show similar features to the density or sonic velocity
log). Some of the log data acquired at Hole 440B are of acceptable quality. The
gamma ray and density logs show some repeatability between passes, while the
acoustic log is of very poor quality and does not correlate with any of the
other logs.
Gamma ray logs
recorded through bottom hole assembly (BHA) and drill pipe should be used only
qualitatively, because of the attenuation on the incoming signal. The
thick-walled BHA attenuates the signal more than the thinner-walled drill pipe.
(The CNL porosity can sometimes be used qualitatively through the BHA and pipe,
but most of the other logs will not give usable data. The FDC/CNL/GR and MCD/GR
strings entered the pipe at 105.5 mbsf.
A wide
(>12") and/or irregular borehole affects most recordings, particularly
those that require eccentralization and a good contact with the borehole wall
(FDC, CNL). Hole diameter was recorded by the 3-arm MCD on the sonic tool
string. No caliper was recorded by the FDC tool because the maximum opening was
only 8 inches. The MCD showed that hole conditions were generally good and washouts
correlate well with lithologic unit boundaries.
A null value of
-999.25 may replace invalid log values.
Additional
information about the drilling and logging operation can be found in the
Operations section of the Site Chapter in DSDP Initial Reports volume 57. For
further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia