Standard Wireline Data Processing

 

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 439

Leg: 57

Location: Japan Trench (NW Pacific)

Latitude: 40° 37.61' N

Longitude: 143° 18.63' W

Logging date: November 1977

Sea floor depth ("bottom felt"): 1666 mbrf

Sea floor depth (Gamma Ray log): 1567.5 mbrf

Total penetration: 1157.5 mbsf

Total core recovered: 163.1 m (48 % of cored section)

Oldest sediment cored: Late Cretaceous

Lithologies: clay and claystone with sands, silts, volcanic ash.

 

Data

 

The logging data was recorded by Schlumberger in LIS format. The original data tape was partly corrupted, therefore only partial data could be recovered. The remainder of the data was digitized from the original blueprints in 2004. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in March 2005

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Bit depth (mbsf) Notes
1. BHC/GR
main
96
1000
94
Reference
repeat
96
958
Bad DT
2. FDC/CNL/GR
main
87
958.4
94
repeat
822.7
947.2
Bad CNL
3. DLL/SP/GR
main
96
955.6
94
repeat
821.3
953.3
4. HRT
0
958.9

 

 

Logging at Hole 439 was successful, with all tool strings reaching close to total depth.

 

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 from the BHC/GR main run; this log was chosen as the reference because was the longest one and reached the mudline. The logs were shifted to the sea floor based on the GR step observed at 1666 mbrf; this value coincides with the Òbottom feltÓ depth given by the drillers.

 

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.

 

Sonic data: The DT slowness data have been converted to velocity. The data contain many anomalous positive and negative spikes, especially in the lower part of the hole, below 850 m; however, the general trend can be considered reliable. The data on tape was truncated at 1000 mbsf; the lower 100 m, however, are of very poor quality, and were not digitized.

 

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). Good correlation exists between most logs; an exception is represented by the CNL data from the repeat run, which are not included in the online database.

 

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). No caliper log was acquired at this hole due to malfunction of the device (it would open to only 8Ó inches. The FDC density log, however, appears to be of good quality, so it is likely that the hole was in good condition.

 

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