Standard Wireline Data Processing

 

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 416A

Leg: 50

Location: Moroccan Basin (NE Atlantic)

Latitude: 32° 50.18' N

Longitude: 10° 48.06' W

Logging date: November 1976

Sea floor depth (Òbottom feltÓ): 4203 mbrf

Total penetration: 1624 mbsf

Total core recovered: 296.2m (47.3 % of cored section)

Oldest sediment cored: Sandstone, calcarenite, and mudstone (Kimmeridgian)

Lithologies: Nannofossile ooze, marl, chalk, mudstone, siltstone, claystone, marlstone.

 

Data

 

The logging data was recorded by Schlumberger in LIS format. The original data tape was incomplete; temperature data were digitized by Centerline Data in 2004. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory.

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Bit depth (mbsf) Notes
1. BHC/GR
Main
93.4
688.9
103
GR missing. Repeat missing.
2. FDC/CNL/GR
Main
93.3
689
103
Reference. Repeat missing. Invalid CNL.
3. DIL/LL8/GR
Main
91.5
688.1
101
Repeat missing
4. HRT
Downlog
0
562

 

 

The caliper tool was removed from the BHC/GR tool string, after the first attempt to lower it through the pipe failed. The FDC/CNL/GR tool string was run without caliper, as its maximum opening was only 8 inches. The HRT tool string would not pass an obstruction at 587 mbsf.

 

All repeat sections were missing from the original data tape.

 

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 and density logs from the main pass of the FDC/CNL/GR tool string, and were then shifted to the sea floor (-4203 m). The sea floor depth used to shift the data was the Òbottom feltÓ depth given by the drillers, which corresponds to the small step in temperature observed on the HRT log.

 

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.

 

In the absence of a GR curve, the acoustic slowness was matched to the density log, as both are affected by the same formation property. The acoustic log, however, was very noisy and correlation was very approximate; the results should be used with caution.

 

Sonic data: Velocity has been computed from delay time.

 

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). The acoustic log was recorded without centralizers, to minimize the risk of the tool getting stuck in the drill pipe; as a result, it is often noisy, with large negative and positive spikes. Correlation with other logs such as resistivity and density is very approximate. Similarly, the FDC and CNL were recorded without eccentralizer; with the tools not properly pressed against the side of the hole, the data is often inconsistent and not quantitatively reliable. The CNL is not included in the online database.

 

No caliper measurement was taken in Hole 416A.

 

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

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia