Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 556
Leg: 82
Location: Mid-Atlantic Ridge (central N Atlantic)
Latitude: 38° 56.38' N
Longitude: 34° 41.12' W
Logging date: September 1981
Sea floor depth (drillers' mudline): 3690 mbrf
Sea floor depth (step in GR log): 3689
Total penetration: 639 mbsf
Total core recovered: 85.21 m (46 % of cored section)
Oldest sediment cored: early Oligocene
Lithologies: basalt limestone breccia (sediments), basalt, breccia, and gabbro (basement).
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.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. LSS/GR/MCD |
pass
1 main
|
89
|
627
|
|
|
|
pass
1 repeat 1
|
75
|
127
|
|
|
|
|
pass
1 repeat 2
|
449
|
516
|
|
|
|
|
pass
2 repeat
|
580.5
|
627
|
|
SWF recorded
|
|
|
pass
2 lower
|
525.5
|
628
|
|
SWF recorded
|
|
|
pass
2 upper
|
407
|
530
|
|
SWF recorded
|
|
| 2. DLL/GR |
main
|
91
|
628
|
|
|
|
repeat
|
403
|
496
|
|
|
|
| 3. FDC/CNL/GR |
main
|
0
|
632
|
|
reference
|
|
repeat
|
512
|
628
|
|
|
|
| 4. HRT |
down
+ uplog
|
0
|
483
|
|
|
Logging was successful at Hole 556, with all tool strings except the HRT temperature probe reaching to within 10 m of the base of the hole. The sediment-basement contact is at 460 mbsf.
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 and NPHI logs from the main pass of the FDC/CNL/GR tool string, and were then shifted to the sea floor (- 3689 m). The FDC/CNL/GR main pass was chosen as the reference run because it was the only run to cross the sea floor. The LLS logs from the DLL/GR tool string were matched to the NPHI reference log because the GR logs had low values and were noisy. The LSS/GR/MCD runs were matched to the GR log from the reference run.
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 3689 mbrf. This differs by 1 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 combinations 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 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 hydraulic caliper on the FDC tool (CALI) and by the 3-arm MCD tool (CALI). The hole is approximatel 10 inches in diameter in the basement section, and washed ut to 16 inches or wider in the sediments.
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 82. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia