Standard Wireline Data Processing

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 53

Leg: 6

Location: Iwo Jima Ridge (tropical NW Pacific)

Latitude: 18° 2.00' N

Longitude: 141° 11.50' E

Logging date: July 1969

Sea floor depth (from DSDP IR): 4640 mbrf

Sea floor depth (step in GR log): 4630 mbrf.

Total penetration: 200.6 mbsf

Total core recovered: 12.1 m (34.6 % of cored section)

Oldest sediment cored: Miocene

Lithologies: ashes of andesitic and basaltic composition (sediments), basaltic flows and intercalated limestones.

Data

The logging data were recorded by Schlumberger and Dresser Atlas in analog form only. The data was digitized by Centerline Data in 2004 and processed at the Borehole Research Group at the Lamont-Doherty Earth Observatory in September 2004.

Logging Runs

Tool string	Pass	Top depth (mbsf)	Bottom depth (mbsf)	Bit depth (mbsf)	Notes
1. AL/CL (Dresser-Atlas)	main	94	171	87	AL of very bad quality; not digitized.
	repeat	94	176	87	AL of very bad quality; not digitized
2. EL/SP (Sclumberger)	main	88	181	87
	repeat	88	181	87
3. GNT (Schlumberger)	main	0	188	87

Very little information was found about logging operations in the Initial Reports of the DSDP, Leg 6. Hole 53 was drilled and then cased down to a depth of 87 mbsf. Three logging runs were completed, but no information could be found on the actual logging sequence. The acoustic log (AL) was very erratic, due to low velocity formations and signal attenuation; for this reason it was not digitized.

The depths in the table are for the processed logs (after depth matching 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: 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.

In the case of Hole 53 no depth match could be performed between passes, as there are no common curves. The log header from the acoustic log, however, indicated that the caliper had been recorded 4 meters too deep. The calipers from the main and repeat from the acoustic log show a good match.

All the logs were then shifted to the sea floor by subtracting 4640 m. This amount corresponds to the value given in the DSDP Initial reports and on the log headers; a close look at the GR log, however, shows a clear step in gamma ray values at 4630 mbrf, 10 m higher than the drillers' water depth. Because we assume that the cores sub-bottom depths were obtained using the value of 4640 m, for consistency sake we shifted the logs by the same amount (also see figure on page 1325).

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 GNT porosity can sometimes be used qualitatively through the BHA and pipe, but most of the other logs will not give usable data.)

Hole diameter was recorded by the by the 3-arm CL tool.

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 6. 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