Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 402A
Leg: 48
Location: Bay of Biscay (NE Atlantic)
Latitude: 47° 52.48 N
Longitude: 8° 50.44' W
Logging date: June 1976
Sea floor depth (bottom felt): 2355.5 mbrf
Sea floor depth (from gamma ray log, see below): 2358 mbrf
Total penetration: 469.5 mbsf
Total core recovered: 179.69 m (48 % of cored section)
Oldest sediment cored: Early Aptian
Lithologies: nannofossil ooze, chalk, limestone, ,silicified mudstone, chert
The logging data was recorded by Schlumberger in LIS format on tape. Due to parity errors, however, the data was digitized by Centerline Data in 2004. 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 |
| BHC/MCD/GR |
Main
|
0
|
438
|
106
|
Reference
|
|
Repeat
|
97
|
188
|
105
|
|
|
| 2. IEL/ISF/SP/GR |
Main
|
103
|
441
|
106
|
|
|
Repeat
|
97
|
444
|
106
|
|
|
| 3. FDC/CNL/GR |
Main
|
98
|
425
|
105
|
|
|
Repeat
|
100
|
421
|
105
|
|
All logging runs reached to within 30-45 m of the bottom of the hole, and good quality logs were obtained.
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 the main pass of the BHC/MCD/GR tool string, and were then shifted to the sea floor (-2358 m). The depth matches were crosschecked with the physical property logs from each tool string. The caliper logs were moved downwards by 2.5 m to match the other logs on the BHC/MCD/GR tool string.
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 small step in gamma ray values at the sediment-water interface in the main pass of the BHC/MCD/GR to be 2358 mbrf. This differs by 2.5 m from the driller's "bottom felt" sea floor depth.
Sonic data: The DT logs were converted to velocity. Some intervals of bad data are present between 162-191 mbsf in both passes of the BHC/MCD/GR.
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). The data recorded at Hole 402A are of good quality.
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). Hole diameter was recorded by the MCD tool (CALI) on the BHC tool string, but the caliper jammed below 190 mbsf on the main pass, and was unable to record diameters greater than 10 inches. The short repeat pass is probably more representative of the actual hole conditions (9-13 inches wide).
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