Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 57
Leg: 6
Location: Caroline Ridge (equatorial NW Pacific)
Latitude: 8° 40.90' N
Longitude: 143° 32.00' E
Logging date: October 1983
Sea floor depth (from DSDP IR): 3310 mbrf
Sea floor depth (step in GR log): logs did not reach seafloor
Total penetration: 335.3 mbsf
Total core recovered: 6.7 m (54.9 % of cored section)
Oldest sediment cored: ooze (upper Oligocene)
Lithologies: coccolith-diatom-radiolarian ooze (sediments) and diabasic basalts (basement)
The logging data were recorded by Schlumberger and Dresser Atlas. Because the original tape contained parity errors, the data was digitized by Centerline Data in 2004 and processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in September 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
|
1. AL/CL
(Dresser-Atlas) |
|
99
|
328
|
95.5
|
|
|
2. EL/SP
(Schlumberger) |
|
98
|
331
|
95.5
|
|
|
3. GNT
(Schlumberger) |
|
110.3
|
337
|
95.5
|
|
|
4. GGD
(Schlumberger) |
|
93
|
337
|
95.5
|
|
Very little information was found about logging operations in the Initial Reports of the DSDP, Leg 6. After drilling, the drill pipe was set at 95.5 mbsf. Four logging runs were completed, but no information could be found on the actual logging sequence.
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: 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 57 a GR measurement was performed with both the GNT and the GGD tool strings. Depth match was performed between these two runs as described above. As to the AL/CL and EL/SP tool string, correlation is poor and no depth matching could be performed. The caliper log, however, was shifted upward 4 m, as the log header indicated that it had been recorded too deep.
All the logs were then shifted to the sea floor by subtracting 3310 m. This amount corresponds to the value given in the DSDP Initial reports and on the log headers. No logs reached the sea floor.
Sonic data: The acoustic slowness was converted into velocity. The acoustic data seem to be of very poor quality; the show no correlation with any of the other logs.
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).
At hole 57 the different logs recorded show poor correlation, with the exception of the GR recorded with the GGD and GNT tool strings. There is no report available suggesting the cause of such poor quality. The caliper tool (CL), however, ndicates a very large hole (the tool saturated at 18 inches) in the upper part, down to 175 mbsf; below, the hole gradually becomes smaller and smoother, with readings of 14 inches or less from 200 mbsf to the bottom of the hole.
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 6. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia