Standard Wireline Data Processing

 

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 441-A

Leg: 57

Location: Japan Trench (NW Pacific)

Latitude: 39° 45.05' N

Longitude: 143° 4.59' E

Logging date: November 1977

Sea floor depth ("bottom felt"): 5656 mbrf

Sea floor depth (step in GR log): 5658 mbrf

Total penetration: 662 mbsf

Total core recovered: 19.7 m (14 % of cored section)

Oldest sediment cored: claystone (Late Miocene)

Lithologies: claystone and diatomaceous claystone

 

Data

 

The logging data was recorded by Schlumberger in LIS format. Some data missing from the original tapes was digitized by Centerline Data in 2004. Data were processed at the Borehole Research Group of the Lamont-Doherty  Earth Observatory in March 2005.

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Bit depth (mbsf) Notes
1. FDC/GR
Main
0
571.9
102
Reference
Repeat
281.8
561.1

 

 

Rapidly deteriorating weather allowed only one tool string to be run before the hole was abandoned. Despite poor sea conditions – a heave of 2 m was reported, which interfered with the log recording – the FDC/GR was completed without incidents..

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.

 

Processing

 

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/GR tool string, and were then shifted to the sea floor (- 5658 m). The main pass was chosen as the reference run because it was the longer one and the only run to cross the sea floor. The sea floor depth was determined by the step in gamma ray values at 5658 mbrf. This differs by 2 m from the sea floor depth given by the drillers (see above).

 

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.

 

Quality Control

 

The quality of the data is usually 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). In Hole 441A there is acceptable repeatability between main and repeat pass of the GR log; the density log, however, has a pretty uniform character, with a n unusually low density from 370 to 560 mbsf, just below a higher density interval..

 

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 (FDC). No hole diameter was recorded by the hydraulic caliper on the FDC 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 57 . For further questions about the logs, please contact:

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia