Wireline Standard Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 566C
Leg: 84
Location: Mid America Trench Slope (tropical NE Pacific)
Latitude: 12° 48.84' N
Longitude: 90° 41.53' W
Logging date: January 1982
Sea floor depth (drillers' mudline): 3673 mbrf
Total penetration: 136.6 mbsf
Total core recovered: 5.82 m (8.8 % of cored section)
Oldest sediment cored: late Miocene
Lithologies: Siliceous mud (sediments), breccia, and serpentinite (basement at 109 mbsf).
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 January 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. LSS/GR/MCD |
pass
1
|
16
|
71
|
20
|
|
|
pass
2
|
0
|
70
|
19.5
|
reference
|
|
|
pass
3
|
19
|
68
|
SWF recorded
|
|
Although the hole was 136.6 m deep, the tool string could not pass below 71 mbsf because of an impenetrable bridge. After the LSS/GR/MCD run, the pipe was lowered through the bridge so that the lower part of the hole could be logged, but the tool string could not get out of the pipe.
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 Pass 2 of the LSS/GR/MCD tool string, and were then shifted to the sea floor (-3673 m). Pass 2 was chosen as the reference run because it was the only run to cross the sea floor. The GR logs from the other passes were matched to the GR log from the reference run.
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 could not be determined from the gamma ray log because of the lack of a clear step in the log, therefore the driller's sea floor depth of 3673 mbrf was used. LSS/GR/MCD Pass 3 had to be shifted downwards by 11m to bring it in line with the other two passes.
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. The repeatability between the velocity logs from the three passes is only moderate, and it appears that the quality of the data from Passes 1 and 2 is superior to that from Pass 3.
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.
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 3-arm MCD tool (CALI). The hole was mostly wider than 14 inches, with some thin narrow intervals. No caliper was recorded during Pass 3.
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 84. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia