Standard Wireline Data Processing

 

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 405

Leg: 48

Location: Rockall Plateau (central N Atlantic)

Latitude: 55° 20.18' N

Longitude: 22° 3.49' W

Logging date: July 1976

Sea floor depth (bottom felt): 2974 mbrf

Sea floor depth (from gamma ray logs, see below): 2983 mbrf

Total penetration: 407 mbsf

Total core recovered: 172.05 m (42.7 % of cored section)

Oldest sediment cored: Early Eocene

Lithologies: nannofossil ooze, siliceous/calcareous mudstone, silicified mudstone, and chert.

 

Data

 

The logging data was recorded by Schlumberger in LIS format on tape. However, the tape was not readable, and the data had to be retrieved from the paper plots made during the logging operation. The plots were digitized by Centerline Data. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in August 2004.

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Bit depth (mbsf) Notes
1. BGC/MCD/GR
Pass 1
0
376
104
Pass 2
0
373.5
105
2. FDC/CNL/GR
102
378
105
Reference log
3. IEL/GR
Pass 1
103
376
105
Pass 2
103
377
105

 

 

All logging runs reached to within ~30m of the bottom of the hole, and good quality logs were obtained. The gamma ray tool on the BHC logging run failed.

 

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 FDC/CNL/GR tool string, and were then shifted to the sea floor (-2983 m).

 

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.

 

The sea floor depth was determined by the step in gamma ray values at the sediment-water interface in pass 1 of the BHC/MCD/GR to be close to 2983 mbrf, the depth shift value used in the DSDP reports volume.

 

Sonic data: The DT logs were converted to velocity. Some intervals of bad data are present in pass 1 between 239-254 mbsf.

 

Quality Control

 

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).

 

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. (The CNL porosity can sometimes be used qualitatively through the BHA and pipe, but most of the other logs will not give usable data.)

 

A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (FDC, CNL). Hole diameter was recorded by the 3-arm MCD tool (CALI). Below 300 mbsf, the hole is mostly in good condition, with a diameter of about 10 inches; between 105 and 300 mbsf, however, the hole is wider and shows more rapid variations in diameter.

 

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 48. For further questions about the logs, please contact:

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia