Standard Wireline Data Processing

DSDP operator and logging contractor: Scripps Institution of Oceanography

Hole: 530A

Leg: 75

Location: Walvis Ridge (tropical SE Atlantic)

Latitude: 19° 11.26' S

Latitude: 9° 23.15' E

Logging date: August 1980

Sea floor depth ("bottom felt"): 4645 mbrf

Sea floor depth (step in GR log): none observed

Total penetration: 1121 mbsf

Total core recovered: 614.46 m (62.2 % of cored section)

Oldest sediment cored: Brown clay (Late Albian)

Lithologies: Diatom nannofossil ooze, clay, mudstone, sandstone, claystone, and limestone (sediments). Basalt (basement).

Data

The logging data was recorded by Gearhart-Owen in GO format. The data was digitized by Centerline Data in 2004 and processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory.

Logging Runs

Tool string	Pass	Top depth (mbsf)	Bottom depth (mbsf)	Bit depth (mbsf)	Notes
1. NL/GR	Main	0	1053		Thru pipe and BHA
	Repeat	905	1052
2. CDL/GR/TEMP		0	855	625
3. BHC/CL/GR	Main	608	942	630	Reference
	Repeat	614	732.8
4. IEL/GR	Main	614	903	630
	Repeat	626	751.9
5. LL3/NL/GR	Main	608.1	889.9	630
	Repeat	608.1	695.1
6. TEMP	Downlog	0	855

Logging at Hole 530A was generally successful, with the exception of the CDL/GR/TEMP tool string, which could not be lowered past a bridge in the hole. The TEMP tool was run separately at the end of the logging program.

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 BHC/CL/GR tool string, and were then shifted to the sea floor (- 4645 m).

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.

Correlation at Hole 530A is generally difficult, due to poor repeatability; as a consequence, the NL/GR logs (main and repeat) could not be matched to the reference run. Also, it is unclear if the two high resistivity-velocity intervals at 791 and 801 mbsf should be matched to the low GR intervals at 795 and 805 mbsf. The sea floor depth corresponds to the "bottom felt" depth given by the drillers, as no step was observed on either the GR or the NL, which were recorded through bottom hole assembly and drill pipe well above the sea floor.

Sonic data: The delay time from main and repeat pass was converted into velocity.

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).

The comparison between logs from the different tool strings does not show a high degree of correlation; some major lithologic features are recognized on all logs, but for the majority of the hole any correlation, even between the GR logs that were recorded on each tool string, is uncertain. The log header from the BHC/CL/GR bears the remark that the GR is expressed as count rates per second (cps); the GR from the IEL/GR string, however, does not bear any remark, though the value range is the same as that of the GR from the sonic log. GR from the NL/GR tool string run through pipe also seems to be expressed as cps, though it show much higher values, as does the GR from the LL3/NL/GR string. In conclusion, it remains unclear which GR are expressed as cps, and they should all be used qualitatively only.

The laterolog LL3 did not work properly, indicating a generally flat trend and conductivities that are too low; the conductivity from the IEL should be used instead.

A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (NL). Hole diameter was recorded by the hydraulic caliper on the BHC/CL/GR tool string (CALI); the hole looks generally smooth and in gauge down to about 830 mbsf. Locally large washouts can be observed below that depth.

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

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia