Standard Wireline Data Processing
DSDP operator and logging contractor: Scripps Institution of Oceanography
Hole: 71A
Leg: 8
Location: Clipperton Fracture Zone (tropical central S Pacific)
Latitude: 4° 28.28' N
Longitude: 140° 18.91' W
Logging date: November 1969
Sea floor depth: 4419 mbrf
Sea floor depth (step in electrical log log): logs did not reach the mudline
Total penetration: 558 mbsf
Total core recovered: 5.7 m (14.1 % of cored section)
Oldest sediment cored: Late Eocene
Lithologies: calcareous and siliceous oozes, chalk, limestone, and chert.
The logging data was recorded by Welex in analog format only. According to the DSDP Reports volume, logging was carried out at the first hole (Hole 70), which could not be drilled deeper than 475 mbsf. Logs, however, were recorded down to 561 mbsf, therefore they must have been recorded in the second hole (71A). The data was digitized by Centerline Data in 2004 and processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in July 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. EL/SP |
uplog
|
192.6
|
561.4
|
Recorded open hole
|
|
The depths in the table are for the processed logs (after 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 shift to sea floor: The original logs were depth shifted to the sea floor. Because the logs did not reach the mudline, they were shifted by the amount used to shift the cores (4419 m). This gives a total logging depth (561 mbsf) that is consistent with the total penetration depth for the hole (558 mbsf).
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 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 8. 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@ldeo.columbia.edu