Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 481A
Leg: 64
Location: Guaymas Basin (tropical NE Pacific)
Latitude: 27° 15.18' N
Longitude: 111° 30.46' W
Logging date: January 1979
Sea floor depth ("bottom felt"): 2016.5 mbrf
Total penetration: 384 mbsf
Total core recovered: 161.12m (47 % of cored section)
Oldest sediment cored: Late Quaternary
Lithologies: muddy diatom oozes, and turbidites (sediments) and dolerite sills (basement).
The logging data was recorded by Gearhart-Owen in analog form only; it was digitized by Centerline Data in 2004. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in October 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. CDL/GR |
Main
|
92.5
|
374
|
92.5
|
Reference run
|
|
Repeat
|
315
|
474.5
|
|
|
|
| Temperature |
Pass
1
|
0
|
378.5
|
|
Downlog
|
| 2. BHC/CL//GR |
Main
|
94
|
365.5
|
92.5
|
Same GR/CL
as in CDL/GR
|
|
Repeat
|
316.5
|
368
|
|
Same GR/CL
as in CDL/GR
|
|
| 3. LL3/NL/GR |
Main
|
94
|
382
|
94.3
|
|
|
Repeat
|
314
|
382
|
|
Same COND as
in main run
|
|
| 4. IEL/GR |
Main
|
96
|
378
|
94.3
|
Same GR as in
LL3/NL/GR
|
|
Repeat
|
315.5
|
377.5
|
|
Same GR as in LL3/NL/GR Same CILD as in main run |
|
| 5. Temperature |
Pass
2
|
0
|
378.5
|
|
Downlog
|
All tool strings reached close to the bottom of the hole and good quality logs were obtained.
Although the original blueprints showed both main and repeat sections for each of the tool strings, during the processing it was noticed that some of the logs of the repeat sections had been replaced by the corresponding log from the main run. Also, though the logging report mentions the recording of GR on each of the tool strings, the blueprints of the BHC/CL/GR show both GR and CL from the CDL/GR runs. The same happened for the IEL/GR logs. Therefore, since the data was digitized from the blueprints, some of the actual data from the repeat runs is missing. A summary is provided in the above table.
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 RHOB log from the main pass of the CDL/GR tool string, and were then shifted to the sea floor (-2016.5 m). The RHOB, DT, COND, and CILD logs from the other passes were matched to the RHOB log from the reference run. The GR logs were not used for depth matching, as they were not available for all of the tool strings.
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 temperature, the logs did not extend to the sea floor, so the drillers' sea floor depth of 2016.5 mbrf was used to shift the data to the sea floor..
Sonic data: Sonic velocity was calculated from the DT slowness log. The background levels in the sonic velocity logs (1.25-1.45 km/sec) are low compared to the expected values in sediments (>1.5 km/sec), indicating some systematic bias. Most of the velocity logs are also rather flat and featureless, in contrast to the other physical property logs. However, the dolerite sills are generally well defined in the velocity log.
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: the hole is typically between 10 and 12 inches in diameter below 140 mbsf, widening to > 14 inches shallower in 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 64. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia