Wireline Standard Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 621
Leg: 96
Location: Middle Mississippi Fan (Gulf of Mexico)
Latitude: 26° 43.86' N
Longitude: 88° 29.76' W
Logging date: October 1983
Sea floor depth (drillers' mudline): 2485 mbrf
Sea floor depth (step in GR log): 2481 mbrf
Total penetration: 214.8 mbsf
Total core recovered: 175.29 m (42% of cored section)
Oldest sediment cored: Pleistocene
Lithologies: Clay, silt, sand, gravel
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 October 2003.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. LSS/GR/MCD |
Pass
1
|
55
|
216
|
62
|
|
|
Pass
2
|
0
|
220
|
62
|
Reference run
|
A second tool string (FDC/CNL/GR) was run, but did not record any useful data because the cable was damaged at the cable head.
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 shifted to the sea floor (-2481 m). The DIT/LSS/GR main pass was chosen as the reference run because it was the longest with this tool string. The GR log from the repeat pass was already well matched in depth to the reference run, so no differential depth matching was required.
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 2481 mbrf. This differs by 4 m from the sea-floor depth given by the drillers (see above).
Sonic data: The transit time data were processed using an in-house program that compares the slowness derived from the 8 different transmitter-receiver pairs 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 free of the artifacts present in the velocities derived directly from DT and DTL.
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 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 3-arm MCD tool (CALI), but in this hole it appears to have a bad calibration; although the caliper logs from the two passes match each other, the values are too low.
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 96. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia