Standard Wireline Data Processing

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 28

Leg: 4

Location: Outer Ridge (Caribbean Sea)

Latitude: 20° 35.19' N

Longitude: 65° 37.33' W

Logging date: March 3, 1969

Sea floor depth (DSDP Reports): 5521 mbrf, 5534 mbrf

Sea floor depth (step in GR log): 5518 mbrf

Total penetration: 404 mbsf (from DSDP Reports, calculated using a 5534 m water depth)

Total core recovered: 13.9 m (4 % of cored section)

Oldest sediment cored: Late Cretaceous

Lithologies: Clays and chalks.

Data

Neutron and gamma ray data (GNT) were 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 June 2004.

Logging Runs

Tool string	Pass	Top depth (mbsf)	Bottom depth (mbsf)	Bit depth (mbsf)	Notes
1. GNT	Uplog	0	418	405	Neutron/GR (thru pipe)

The data was recorded through 5 1/2"-thick pipe and 8 1/4"-thick collars. The drill pipe collars are clearly visible on the neutron log as large spikes every 9.5 meters.

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 gamma ray log was depth-matched to the neutron log, because according to the log header it had been recorded 2.72 m too deep. There is no clear definition in the DSDP Reports as to the correct water depth. Two values are given: 2521 and 2534 mbrf (or 2520 mbsf, 14 m being the distance between sea level and rig floor). The gamma ray log, however, shows the mudline at 2518 mbrf. Therefore, logs were depth shifted this amount.

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 and neutron 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. Drill pipe collars are clearly visible as spikes.

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

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia