Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 553A
Leg: 81
Location: Rockall Plateau (central N Atlantic)
Latitude: 56° 5.32' N
Longitude: 23° 20.61' W
Logging date: August 1981
Sea floor depth (drillers' mudline): 2339 mbrf
Sea floor depth (loggers': the downhole logs did not cross the mudline)
Total penetration: 682.5 mbsf
Total core recovered: 288.97 m (54.4 % of cored section)
Oldest sediment cored: late Paleocene/early Eocene
Lithologies: ooze, chalk. tuffaceous mudstone (sediments), basalt (basement).
The logging data was recorded by Schlumberger in LIS format. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in March 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. LSS/GR/MCD |
1
main
|
104
|
670
|
122
|
|
|
1
repeat 1
|
487
|
671
|
|
|
|
|
1
repeat 2
|
632.5
|
671
|
|
|
|
|
1
repeat 3
|
649
|
670
|
|
|
|
| 2. DIT/LSS/GR |
2
main
|
93
|
651
|
121
|
|
|
2
repeat
|
118
|
187
|
121
|
|
|
| 3. LSS/GR/MCD |
3
|
119
|
650
|
|
|
|
4
|
120
|
520
|
SWF recorded.
|
|
|
| 4. HRT |
main,
rep
|
0
|
669
|
|
|
| 5. CNL/GR |
main
|
116.5
|
655
|
|
|
|
repeat
|
593
|
654
|
|
|
|
| 6. FDC/CNL/GR |
main
|
107
|
654
|
117
|
reference
|
|
repeat
|
591
|
653
|
|
|
Logging at Hole 533A started after flushing the hole with 50 barrels of gel and 40 barrels of GUAR; then the hole was displaced by 75 barrels of gel. The shipboard scientists were concerned about noise in the LSS logs, and ran the DIT/LSS/GR tool string without centralizers, but with a noticeable degradation in the sonic logs. The third tool string was supposed to be the FDC/CNL/GR, but FDC tool failure meant that the tool string was not run until the end of logging operations. Meanwhile, new pads were mounted on the LSS centralizers, and the LSS/GR/MCD was re-run in the hole, collecting good sonic logs. The CNL/GR tool string was run while the FDC was being repaired. Ship heave affects the logs in this hole, as can be seen when comparing the gamma ray log from different runs.
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 then shifted to the sea floor (- 2339 m). The FDC/CNL/GR main pass was chosen as the reference run because it covered nearly all of the logged interval, and the cable speed remained near-constant over the whole run. The GR logs from the other passes were matched to the GR log from the reference run, and the matches were cross-checked with the velocity and porosity logs.
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 is usually determined by the step in gamma ray values at the sediment-water interface, but in Hole 553A, the logs did not cross this interface. Therefore we used the drillers' water depth of 2339 mbrf.
Sonic data: The transit time data were processed using an in-house program that compares the slowness derived from the 8 different transmitter-receiver combinations at each depth, and discards those times that are significantly different from the majority as bad data. The "points" column in the LSS data files is a measure of confidence: it records the number of transmitter-receiver pairs retained - a value of 8 means that no data was discarded. This processing leads to improved compressional wave velocity logs that are mostly free of the artifacts present in the velocities derived directly from DT and DTL.
Sonic waveforms were acquired in 5 sections between 120-520 mbsf - these sections were spliced together and are identified here as LSS pass 4, although it is unclear from the DSDP volume at what stage in the operation they were acquired.
Velocity logs from the lower part of the second LSS pass (DIT/LSS/GR tool string) were of poor quality Ð velocity data from passes 3 and 4 should be used instead.
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 sonic velocity log).
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.)
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 hydraulic caliper on the FDC tool (CALI) and by the 3-arm MCD tool (CALI). The hole is 9-11 inches wide in the basement (below 498 mbsf), 11-14 inches wide from 204-498 mbsf, and wider than 14 inches shallower than 204 mbsf.
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 81. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia