Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 534A
Leg: 76
Location: Blake-Bahama Basin (tropical NW Atlantic)
Latitude: 28° 20.6' N
Longitude: 75° 22.9' W
Logging date: December 1980
Sea floor depth ("bottom felt"): 4976 mbrf
Sea floor depth (step in temperature log): 4970 mbrf
Total penetration: 1666.5 mbsf
Total core recovered: 629.8 m (56 % of cored section)
Oldest sediment cored: Reddish brown shale (middle Callovian)
Lithologies: clay, chert, porcellanite, shale, claystone, limestone, (sediments); basalt and basaltic breccia (basement)
The logging data was recorded by Gearhart-Owen in GO format. The data was digitized by Centerline Data in 2004 and processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Casing/ Pipe depth (mbsf) | Notes |
| 1. CDL/GR/TEMP/ |
Downlog
|
0
|
1424
|
538.5
(csg) 962.5 (pipe)
|
Only
temperature recorded
|
|
Middle
|
964
|
1411
|
|
|
|
|
Upper
|
530
|
768
|
538.5
|
Reference
|
|
| 2. BHC/GR |
Main
|
529.3
|
763
|
538.5
|
|
|
Repeat
|
563.9
|
722.3
|
|
|
|
| 3. CDL/GR |
Lower
|
1461
|
1514
|
|
Reference
|
|
Repeat
|
1259.1
|
1408.1
|
|
|
|
| 4. BHC/GR |
|
|
|
|
Failed
|
Hole 534A was logged in three non-overlapping sections, due to the presence of bridges that prevented the tool strings from reaching total depth and providing a continuous recording of the cored interval. The CDL/GR/TEMP tool string was deployed first: a continuous temperature log was recorded downhole from the mudline to about 1425 mbsf. Then density and GR data were acquired uphole in the middle and upper section of the hole. The BHC/GR tool string was lowered next but the GR component malfunctioned and could only record acoustic data in the upper part of the hole. The bottom part of the hole (only down to ~ 1515 mbsf) could be logged with the CDL/GR string only, due to the presence of impassable bridges during the lowering of the BHC/GR tool string. Because of the poor hole conditions encountered during the first log attempts, it was decided not to run the neutron and resistivity logs.
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 BHC log was depth matched to the RHOB log from the upper pass of the CDL/GR tool string, and was then shifted to the sea floor (- 4970 m). The repeat pass of the CDL/GR log was depth matched to the lower pass recorded with the same tool string.
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 –in Hole 534A the GR could not be used because it was recorded only on with the CDL/GR tool string), 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 temperature observed at 4970 mbrf. This differs by only 6 m from the sea floor depth given by the drillers (see above).
Sonic data: Velocity was calculated from the delay time.
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 density log should show similar features to the sonic velocity log). The quality of the logs recorded at Hole 534A is generally poor, due to the bad hole conditions. Correlation between logs measuring the same physical property is generally difficult, with the exception of few intervals where the hole is in good conditions (for example from 650 and 690 mbsf).
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.
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (CDL). Hole diameter was recorded by the hydraulic caliper on the FDC tool (CALI); it shows a very irregular hole with values often as high as the maximum opening of the tool.
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 76. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia
Trevor Williams
Phone: 845-365-8626
Fax: 845-365-3182
E-mail: Trevor Williams