Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 403
Leg: 48
Location: Rockall Plateau (central N Atlantic)
Latitude: 56° 8.31' N
Longitude: 23° 17.64' W
Logging date: July 1976
Sea floor depth (bottom felt): 2317 mbrf
Sea floor depth (from gamma ray logs, see below): 2313 mbrf
Total penetration: 489 mbsf
Total core recovered: 160.8 m (32.9 % of cored section)
Oldest sediment cored: Early Eocene or late Paleocene
Lithologies: nannofossil ooze and chalk, volcaniclastics, mudstones, sands, and tuffaceous conglomerate.
The logging data was recorded by Schlumberger in LIS format. The dataset, however, was incomplete, so the blueprints were digitized by Centerline Data. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in November 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. BHC/MCD/GR |
Main
|
126
|
427
|
114
|
|
|
Repeat
|
0
|
462
|
114
|
|
Good quality logs were obtained from both passes. Further tool strings were not run because of deteriorating weather conditions.
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 (-2313 m).
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. In the case of Hole 403, the two passes were already on depth with each other. The caliper data were originally offset 2.5 m too high; they have been moved down to their correct position with respect to the other logs.
The sea floor depth was determined by the step in gamma ray values at the sediment-water interface in the repeat pass of the BHC/MCD/GR tool string to be 2313 mbrf; this is the same depth shift value used in the DSDP reports volume to shift the cores.
Sonic data: The DT logs were converted to velocity. The data are of good quality.
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 3-arm MCD tool. The hole is mostly in good condition, between 10-11 inches in diameter.
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 48. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia