Wireline Standard Data Processing

ODP Logging Contractor: LDEO-BRG

Hole: 979-A

Leg: 161

Location: Alboran Basin (Alboran Sea)

Latitude: 35° 43.427'N

Longitude: 3° 12.353' W

Logging Date: June, 1995

Bottom Felt: 1074 mbrf (used for depth shift to sea floor)

Total Penetration: 580.9 mbsf

Total Core Recovered: 583.1 m (100.4 %)

Logging Runs

Logging String #1: DIT/SDT/HLDT/CNTG/NGT (two sections)

Wireline heave compensator was used to counter ship heave.

Bottom-hole Assembly

The following bottom hole assembly, drill pipe or casing depths are as they appear on the logs after differential depth shift (see DEPTH SHIFT section) and depth shift to the sea floor. As such, there might be a discrepancy with the original depths given by the drillers onboard. Possible reasons for depth discrepancies are ship heave, use of wireline heave compensator, and drill string and/or wireline stretch.

DIT/SDT/HLDT/CNTG/NGT: bottom hole assembly at ~80 mbsf (upper section).

DIT/SDT/HLDT/CNTG/NGT: bottom hole assembly at ~237 mbsf (lower section).

Processing

Depth shift: Only one logging string was run; no differential depth shift required.

All original logs have been depth shifted to the sea floor (- 1074 m).

Gamma-ray processing: Data have been processed to correct for borehole size and type of drilling fluid.

Acoustic data processing: The array sonic tool was operated in standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') and short-spacing (3-5-5-7') logs. At Hole 979A the acoustic data was greatly affected by the large and irregular borehole; because of the lack of a good set of transit times the routine processing could not be performed. Therefore, velocity has been estimated by the best single channel, not borehole-compensated reading. One of the 10'-spacing readings has been edited and used to compute slowness and velocity. In general, however, caution is recommended when using the results quantitatively because the calculated velocity is not borehole-compensated.

High-resolution data: Bulk density and neutron porosity data were recorded at a sampling rate of 2.54 and 5.08 cm respectively. The enhanced bulk density curve is the result of Schlumberger enhanced processing technique performed on the MAXIS system onboard. While in normal processing short-spacing data is smoothed to match the long-spacing one, in enhanced processing this is reversed. In a situation where there is good contact between the HLDT pad and the borehole wall (low density correction) the results are improved, because the short-spacing have better vertical resolution.

Quality Control

null value=-999.25. This value generally appears in discrete core measurement files and also it may replace recorded log values or results which are considered invalid (ex. processed sonic data).

During the processing, quality control of the data is mainly performed by cross-correlation of all logging data. Large (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization (CNTG, HLDT) and a good contact with the borehole wall. At Hole 979A the density data are degraded by the large and irregular hole and are not presented.

Data recorded through bottom hole assembly, such as the gamma ray and neutron porosity data above 80 and 237 mbsf, should be used qualitatively only because of the attenuation on the incoming signal. Invalid data were recorded on the gamma ray log at 64, 72-76, and 243-248 mbsf.

Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI).

Details of standard shore-based processing procedures are found in the "Explanatory Notes" chapter, ODP IR Volume 161. For further information about the logs, please contact:

Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia