Wireline Standard Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 558
Leg: 82
Location: Mid-Atlantic Ridge (central N Atlantic)
Latitude: 37° 46.2' N
Longitude: 37° 20.61' W
Logging date: October 1981
Sea floor depth (drillers' mudline): 3766 mbrf (used for shift to sea floor; see processing)
Sea floor depth (step in GR log): 3775 mbrf
Total penetration: 561 mbsf
Total core recovered: 239.16 m (59 % of cored section)
Oldest sediment cored: early Oligocene
Lithologies: nannofossil chalk (sediments), basalt and serpentinized 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 February 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. LSS/GR/MCD |
pass
1 main
|
83
|
546
|
95
|
|
|
pass
1 repeat
|
439
|
504
|
|
|
|
|
pass
2 main
|
383.5
|
476
|
|
SWF recorded
|
|
|
pass
2 repeat
|
398
|
447
|
|
SWF recorded
|
|
| 2. HRT |
See
below
|
0
|
200
|
|
|
| 3. DLL/GR |
main
|
80
|
448
|
|
|
|
repeat
|
377
|
450
|
|
|
|
| 4. FDC/CNL/GR |
main
|
0
|
448
|
|
|
|
repeat
|
385.5
|
445
|
|
|
|
| 5. HRT |
See
below
|
0
|
560
|
|
|
Logging of Hole 558 was made difficult the bad hole conditions. The hole was constricted in places, and some of the tool strings became jammed, which slightly distorts the depth scales at that depth. Therefore, no depth matching between runs was carried out. The caliper arm was broken off the FDC tool in an attempt to pass through a constricted section of hole. There is a clear sea floor signature in the gamma radiation log at 3775 mbrf, but the drillers' sea floor depth (3766 mbrf) was used in the processing (see below).
The two HRT runs each comprise a downlog (subdivided into upper and lower sections) and an uplog.
The depths in the table are for the processed logs (after 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 not depth-matched to each other due to lack of clearly repeatable features in either the GR logs or the porosity-dependant logs, compounded by the slight depth distortions due to the non-uniform tool speed. The logs were shifted to the sea floor (-3766 m). This sea floor depth was determined by the drillers, although no mudline core was taken (sediments were washed down to 158 mbsf). A piston core mud line depth of 3777 mbrf was determined in Hole 558A, and the step in gamma radiation is as 3775 mbrf, and therefore this is likely nearer the true depth to sea floor. Despite this, we used 3766 mbrf for the sea floor in order to better match the depth to the sediment-basement contact (406 mbsf) and downhole logs as described and plotted in the DSDP initial reports volume.
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 generally free of the artifacts present in the velocities derived directly from DT and DTL. For this hole, probably the more reliable velocities are from LSS/GR/MCD pass 1, because they have a closer match to the LLD and LLS induction logs than the velocities from LSS/GR pass 2.
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.)
The FDC and CNL logs are very poor shallower than 151 mbsf, probably due to the FDC caliper arm being ripped off the tool near this depth.
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 3-arm MCD tool (CALI). The basement section reads between 6-10 inches wide, and from 12 to greater than 15 inches in the sediment section. The LSS/GR/MCD main caliper indicates some bridges, e.g. at 229 and 458 mbsf.
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