Wireline Standard Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 620
Leg: 96
Location: Central channel of the Mississippi Fan (Gulf of Mexico)
Latitude: 26° 50.12' N
Longitude: 88° 22.25' W
Logging date: October 1983
Sea floor depth (mud line punch core): 2612.4 mbrf
Sea floor depth (step in GR log): 2609.0 mbrf
Total penetration: 422.7 mbsf
Total core recovered: 197.95 m (47% of cored section)
Oldest sediment cored: Pleistocene
Lithologies: Clay, silt
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. DIT/LSS/GR |
main
|
70
|
292
|
76
|
Reference run
|
|
repeat
|
0
|
133
|
After a first attempt at logging, which failed, the MCD module was removed from the tool string because of the concern that it would become jammed with the sticky clay of the formation.
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 (-2609 m). The DIT/LSS/GR main pass was chosen as the reference run because it was the longest pass 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 2609 mbrf. This differs by 3.4 m from the sea-floor depth given by the drillers (see above).
Sonic data: The transit time data from Hole 620 were nearly free from artifacts, and good velocity logs could be obtained directly from the DT and DTL slowness logs, without the need for reprocessing.
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 not recorded in this hole.
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 any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.