Standard Wireline Data Processing

 

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 548A

Leg: 80

Location: Goban Spur (central N Atlantic)

Latitude: 48° 54.93' N

Longitude: 12° 09.87' W

Logging date: June 1981

Sea floor depth (piston core mudline): 1256 mbrf

Sea floor depth (step in GR log): 1256 mbrf

Total penetration: 551.5 mbsf

Total core recovered: 249.33 m (72 % of cored section)

Oldest sediment cored: Devonian quartzite (basement)

Lithologies: nannofossil ooze and chalk (to late Cretaceous), quartzite (basement, 532 mbsf).

 

Data

 

The logging data was recorded by Schlumberger in LIS format. Data were processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in March 2004.

 

Logging Runs

 

Tool string Pass Top depth (mbsf) Bottom depth (mbsf) Bit depth (mbsf) Notes
1. DIT/LSS/GR/MCD
main
150
536
171
2. FDC/CNL/GR
main
0
530
172
reference
repeat
441.5
492

 

 

Both tool strings reached close to the bottom of the hole, and the first penetrated the basement. The LSS sonic tool was run on a single transmitter. A second sonic tool was rigged up but a surface equipment failure aborted the run. There was a constant value offset (+ 155 API units) in the gamma ray log from the FDC/CNL/GR run. The hole was flushed with 50 barrels of mud prior to logging.

 

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/CNL/GR tool string, and were then shifted to the sea floor (-1256 m). The FDC/CNL/GR main pass was chosen as the reference run because it had the longest hole coverage, crossed the sea floor, and because the cable speed was held relatively constant. The GR logs from the other passes were matched to the GR log from the reference run.

 

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 sea floor at 1256 mbrf. This is the same as the sea floor depth determined by the drillers from a piston core of the mudline.

 

Sonic data: Typically, LSS sonic data is processed in the following way. The transit time data were processed using an in-house program that compares the slowness derived from the 8 different transmitter-receiver combinations at each depth, and discards those times that are significantly different from the majority as bad data. The 'points' column in the LSS data files is a measure of confidence:  it records the number of transmitter-receiver pairs retained - a value of 8 means that no data was discarded.  This processing leads to improved compressional wave velocity logs that are free of the artifacts present in the velocities derived directly from DT and DTL.

 

In the case of the LSS in Hole 548A only one of the two transmitters was used (according to the paper plot made at the time of logging). However, the transit times, TT1, TT2, TT3, TT4, are present in the original Schlumberger LIS file, representing transmitter-receiver spacings of 12, 10, 10, and 8 feet respectively. This is in contrast to the normal transmitter-receiver spacings of 10, 8, 12, and 10 feet respectively, and impossible to achieve with only one transmitter. It is unclear to us exactly how all the transit time data were generated. We therefore present only the transit time data, with no velocity calculation, with the caution that the data does not follow the normal LSS pattern.

 

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 hydraulic caliper on the FDC tool (CALI) and by the 3-arm MCD tool (CALI). The hole varies between 9-13 inches in diameter according to the MCD caliper, while the FDC caliper read between 10-11 inches, which is less realistic than the MCD caliper data.

 

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

 

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia