DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 549

Leg: 80

Location: Goban Spur (central N Atlantic)

Latitude: 49° 05.28' N

Longitude: 13° 05.88' W

Logging date: June 1981

Sea floor depth (punch core mudline): 2533 mbrf

Sea floor depth (step in GR log): 2535 mbrf.

Total penetration: 1001.5 mbsf

Total core recovered: 369.7 m (45.5 % of cored section)

Oldest sediment cored: Devonian micaceous sandstone (basement)

Lithologies: Ooze, chalk, limestone, and mudstone (to early Cretaceous), quartzite (basement, 960 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	0	991	102.5	reference
	repeat	937	990

The hole was filled with fresh water bentonite mud prior to logging. Logging commenced under high heave and pitch conditions. The tool string reached within 10 m of the bottom of the hole, after passing three ledges or bridges, and recorded good logs. At the end of the logging run when the tool string arrived at the rig floor, the tool string was not stopped before it hit the circulating head, the cable’s weak point broke, and the tool string fell back down the pipe. After fishing attempts, the tool string became wedged in the BHA and was eventually recovered, damaged, with the BHA. No further logging runs were attempted.

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 repeat DIT/LSS/GR/MCD logs were depth-matched to the GR log from the main pass of the DIT/LSS/GR/MCD tool string, and were then shifted to the sea floor (-2535 m). The DIT/LSS/GR/MCD 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 log from the repeat pass was 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 2535 mbrf. This differs 2 m from 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.

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.

A wide (>12") and/or irregular borehole affects most recordings. Hole diameter was recorded by the hydraulic caliper on the 3-arm MCD tool (CALI). The hole has three bridges to less than 6 inch diameter at 285-290, 340-350, and 710-713 mbsf, and two intervals where the diameter varies over short distances between 9-16 inches at 650-715 and 850-920 mbsf. In the remaining hole intervals, the hole diameter varies smoothly, generally between 9-12 inches, widening towards the top of the hole.

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:

Trevor Williams

Phone: 845-365-8626

Fax: 845-365-3182

E-mail: trevor@ldeo.columbia.edu

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: chris@ldeo.columbia.edu