Wireline Standard Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 586C
Leg: 89
Location: Ontong-Java Plateau (tropical SW Pacific
Latitude: 00° 29.84' S
Longitude: 158° 29.89'E
Logging date: November 1982
Sea floor depth (drillers' mudline): 2223.1 mbrf
Sea floor depth (step in GR log): 2218 mbrf (used for sea floor depth in DSDP Vol. 89)
Total penetration: 623.1 mbsf
Total core recovered: 2.18 m (23 % of cored section)
Oldest sediment cored: early Miocene
Lithologies: carbonate ooze.
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 January 2004.
| Tool string |
|
|
|
|
Notes |
| 1. DIT/BHC/GR/MCD |
Main
|
0
|
623
|
71
|
|
|
Repeat
|
544
|
622
|
|
|
|
| 2. FDC/CNL/GR |
Main
|
0
|
624
|
73.5
|
Reference run
|
|
Repeat
1
|
553
|
624
|
|
|
|
|
Repeat
2
|
0
|
453
|
72.5
|
|
Both tool strings reached the bottom of the hole and very good 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 FDC/CNL/GR tool string, and were then shifted to the sea floor (-2218 m). The FDC/CNL/GR main pass was chosen as the reference run because it had complete coverage of the hole, and crossed the sea floor. The GR and SFLU logs from the two FDC/CNL/GR repeat passes were already on depth with the reference run. The GR logs had few features, and the values were low, and thus it was not possible to match the DIT/BHC/GR/MCD passes to the reference run. The DIT/BHC/GR/MCD repeat pass is on depth with DIT/BHC/GR/MCD main.
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 2218 mbrf. A mudline core in Hole 586 gave a depth of 2223.1 mbsf, but the shipboard party preferred to use the log-based depth of 2218 mbsf for the sea floor (see the DSDP Initial Reports, Volume 89).
Sonic data: The BHC tool was used in this hole. Only a single slowness, DT, was output, and this was converted to velocity.
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 was in good condition, with the hole diameter varying smoothly from 9 inches wide as the base of the hole to 12 inches wide at the top.
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 89. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia