Standard Wireline Data Processing

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 29C

Leg: 4

Location: Venezuelan Basin (Caribbean Sea)

Latitude: 35° 21.00' N

Longitude: 67° 31.00' W

Logging date: March 13-14, 1969

Sea floor depth (DSDP Reports): 4284 mbrf (questionable)

Sea floor depth (step in GR log): 4258 mbrf

Total penetration: 248 mbsf (from DSDP Reports, calculated using a 4284 m water depth)

Total core recovered: 1.38 m (7.6 % of cored section)

Oldest sediment cored: Middle Eocene

Lithologies: cherty radiolarite

Data

The logging data was recorded by Schlumberger as analog data only. Two passes were recorded with two logging strings: 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	Pass 1	0	267	219	Neutron, GR
2. GNT	Pass 2	0	269	114	Neutron, GR
3. EL/SP	Pass 1	220	264	219
4. EL/SP	Pass 2	115	260	114

The top of the drill bit was positioned at 50 m above the bottom of the hole during the first pass and at 150 m above the bottom during the second pass. Gamma ray and neutron logs were recorded through bottom hole assemble and drill pipe up to the mudline during both passes.

The depths in the table are for the processed logs (after 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. Typical reasons for depth discrepancies are ship heave, wireline and pipe stretch, tides, and the difficulty of getting an accurate sea floor, in this case, from echo-sounding.

Processing

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 unique definition of the water depth in the DSDP Reports; the values given vary in different sections of the site report and in a logging paper published in the DSDP volume. Therefore, the sea floor depth was determined by the step in gamma ray value at 4258 mbrf observed on the GR/neutron log.

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 or density 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 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