Wireline Standard Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 555
Leg: 81
Location: Rockall Plateau (central N Atlantic)
Latitude: 56° 33.7' N
Longitude: 20° 46.93' W
Logging date: September 1981
Sea floor depth (drillers' mudline): 1669 mbrf
Sea floor depth (step in GR log): logs did not cross the sea floor.
Total penetration: 964 mbsf
Total core recovered: 505.25 m (54 % of cored section)
Oldest sediment cored: late Paleocene
Lithologies: ooze, chalk. tuffaceous mudstone (sediments), basalt (basement, at 927 mbsf).
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
|
151
|
675
|
197
|
|
|
Pass
2 rep
|
874.5
|
927
|
683
|
|
|
|
Pass
2 main
|
643
|
926
|
683
|
|
|
| 2. FDC/CNL/GR |
|
246
|
911
|
255
|
reference
|
| 3. DLL/GR |
|
246
|
880
|
254
|
|
| 4. LSS/GR/MCD |
Pass
3
|
251
|
865
|
255
|
|
Logging at Site 555 was made difficult by both hole conditions and weather conditions. The first pass of the LSS/GR/MCD tool string could not pass below 675 mbsf because of a bridge. After this pass, operations were suspended because of heavy swells and winds gusting to 40 knots. After cleaning the hole with a bit run and circulating 100 barrels of gel mud, and 30 hours after the first pass, and with the pipe set deep in the hole, a second pass of the LSS/GR/MCD tool string was made successfully. The pipe was raised to 255 mbsf, and three tool strings were run without difficulty. The logs from this hole are generally of good quality.
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 FDC/CNL/GR run, and were then shifted to the sea floor (- 1669 m). The FDC/CNL/GR run was chosen as the reference run because it had the longest hole coverage, and because the cable speed was held relatively constant. The GR logs from the other passes 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 could not be determined by the step in gamma ray values at the sea floor, as normal, because none of the logs crossed the sea floor. Hence the drillers' sea floor depth, 1669 mbsf, determined in the mudline core, was used to shift the log depths.
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 varies between 9-13 inches in diameter according to the FDC caliper. The first pass of the MCD caliper indicates similar diameters, but also a narrowed section (to 5-7 inches between 248-272 mbsf). MCD pass 2 appears to be reading too narrow, and MCD pass 3 gave invalid (negative) values.
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 81. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia