Standard Wireline Data Processing

 

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 406

Leg: 48

Location: Rockall Plateau (central N Atlantic)

Latitude: 55° 15.5' N

Longitude: 22° 5.41' W

Logging date: July 1976

Sea floor depth (echo sounding): 2907 mbsl

Sea floor depth (used for shifting logs, see below): 2894 mbrf

Total penetration: 831.5 mbsf

Total core recovered: 189.42 m (38.69 % of cored section)

Oldest sediment cored: Middle Eocene

Lithologies: clay, nannofossil ooze, chalk, diatomaceous chalk, limestone.

 

Data

 

The logging data was recorded by Schlumberger as analog data only: the data was digitized by Centerline Data in 2004. The data was processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in July 2004.

 

Logging Runs

 

Tool string

Pass

Top depth (mbsf)

Bottom depth (mbsf)

Bit depth (mbsf)

Notes
1. BHC/MCD
Pass 1
140.5
811
Pass 2
125.5
810.5
2. FDC/CNL/GR
106
814
127
Reference log
3. IEL
Pass 1
129
819.5
129
Pass 2
129
 819.5
129

 

 

All logging runs reached close to the bottom of the hole, and good quality logs were obtained. The gamma ray tool on the BHC logging string 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 density (RHOB) log from the FDC/CNL/GR tool string, and were then shifted to the sea floor (-2894 m). The acoustic log (DT) from the BHC/MCD tool string and the resistivity logs (RIL) from the IEL tool string were matched to the density log from the reference run (the usual log used for depth matching, GR, was only available for one tool string).

 

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 is usually determined by the step in gamma ray values at the sediment-water interface. However, the sea floor was not crossed by the logs, so the sea floor depth that led to a match with the logs as plotted in the DSDP volume was used (2894 mbrf). Various other estimates are given for the sea floor depth in the DSDP volume: 2907 mbsl (echo sounding), 2923 mbrf (bottom felt), 2911 mbrf (on figure 20).

 

Sonic data: The DT logs were converted to velocity.

 

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). All logs collected at Hole 406 were recorded open hole.

 

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). The hole is mostly in good condition (varying fbetween 9-11 inches in diameter), though there are several thin washouts in the 220-343 mbsf intercal.

 

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