Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 474A
Leg: 64
Location: Baja California (tropical NE Pacific)
Latitude: 22° 57.56' N
Longitude: 108° 58.68' W
Logging date: December 1978
Sea floor depth ("bottom felt"): 3043 mbrf
Total penetration: 626 mbsf
Total core recovered: 283.8m (61% of cored section)
Oldest sediment cored: Late Pliocene
Lithologies: claystones and siltstones (sediments); dolerite sills, and basalt flows (basement).
The logging data was recorded by Gearhart-Owen in their own format and subsequently translated to LIS format by WHOI. Data were processed in October 2004 at the Borehole Research Group of the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. CDL/GR |
Main
|
136
|
537
|
136
|
Reference run
|
|
Repeat
1
|
505
|
543
|
|
|
|
|
Repeat
2
|
302
|
357
|
|
|
|
| Temperature |
Pass
1 upper
|
30
|
132
|
|
Downlog
|
|
Pass
1 lower
|
145
|
562
|
|
Downlog
|
|
| 2. BHC/CL/GR |
Main
|
133.5
|
542
|
137
|
|
|
Repeat
1
|
477
|
546
|
|
|
|
|
Repeat
2
|
465
|
536
|
|
|
|
|
Repeat
3
|
453
|
543
|
|
|
|
| 3. LL3/NL/GR |
Pass
1
|
137.5
|
574
|
|
|
|
Pass
2
|
153
|
624
|
|
|
|
|
Repeat
1
|
510
|
567
|
|
|
|
|
Repeat
2
|
501
|
559
|
|
|
|
| 4. Temperature |
Pass
2
|
11
|
634
|
|
Uplog
|
A full suite of good quality logs was obtained in Hole 474A. The LL3/NL/GR tool string reached to the bottom of the hole, but the other tool strings were not able to pass into the basalt flows below about 543 mbsf. The logs are affected by ship heave.
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 CDL/GR tool string, and were then shifted to the sea floor (-3043 m). The CDL/GR main pass was chosen as the reference run because it was one of the longest runs, seemed to be least affected by ship heave, and the depths of the logs matched the depth in the original Gearhart-Owen paper log plots (the other tool strings were offset by about 4.5 m upwards). The GR logs from the other passes were matched to the GR log from the reference run. Logs within a single tool string may be locally offset from each other by up to 1 m due to ship heave.
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.
With the exception of the temperature logs, the other logs did not extend to the sea floor, so the drillerŐs sea floor depth of 3043 mbrf was used to depth shift the data to the sea floor.
Sonic data: Sonic velocity was calculated from the DT slowness logs. The data contain some cycle-skips to higher or lower velocities, and the quality increases towards the bottom of the hole where the velocities are higher. The BHC main and repeat 2 passes appear to have the better data than repeat passes 1 and 3.
Conductivity data: The LL3 conductivity logs have two different scales: LL3 main 2 and repeat 2 have units of millimhos/m (values ranging from 0-700); LL3 main 1 and repeat 1 logs have values a factor of 20 smaller.
Temperature data: The temperature unit is degrees Fahrenheit.
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).
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (CDL, NL). Hole diameter was recorded by the hydraulic caliper on the CDL tool and in the BHC tool string (CALI): the hole diameter is highly variable, varying between 10 and 14 inches on m-scale. There are several thin washed-out zones where the hole exceeds 17-inches-wide, which cause anomalous reductions in the density values, for example at 431 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 64. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia