Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 401
Leg: 48
Location: Rockall Plateau (central N Atlantic)
Latitude: 47° 25.65' N
Longitude: 8° 48.62' W
Logging date: June 1976
Sea floor depth (bottom felt): 2555.5 mbsf
Sea floor depth (used for shifting logs, see below): 2550 mbrf
Total penetration: 341.0 mbsf
Total core recovered: 103.22 m (38.7 % of cored section)
Oldest sediment cored: Late Jurassic
Lithologies: clay, nannofossil ooze, chalk, limestone.
The logging data was recorded by Schlumberger in LIS format on tape. However, the tape was not readable, and the data had to be retrieved from the blueprints made during logging operations. The available blueprints (the blueprint of the BHC/GR/MCD string was missing) were digitized by Centerline Data. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in July 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 2. FDC/CNL/GR |
Main
|
110
|
290
|
113
|
|
|
Repeat
|
115
|
294
|
113
|
|
|
| 3. IEL/ISF/GR |
Main
|
104
|
301
|
114
|
|
|
Repeat
|
185
|
269.5
|
|
|
Good quality logs were obtained at Hole 401. Data from the first run (BHC/GR/MCD) were unavailable at the time of reprocessing (blueprints were missing). The ISF tool failed fduring the main pass of the IEL/ISF/GR toll string, but revived for the repeat pass.
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 (-2550 m). Depth matching between the passes was not necessary, as the logs were already well matched. Small discrepancies exist, but matching the GR did not lead to improvements in the RHOB/IL match.
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 was determined by the step in gamma ray values at the sediment-water interface as plotted in Figure 11 of the Site 401 Chapter in the DSDP reports volume.
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). The hole was mostly in good condition, about 9 inches in diameter, according to the caliper log plotted in Figure 11 of the Site 401 Chapter in the DSDP reports volume.
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