Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 976E

Leg: 161

Location: Alboran Basin (Alboran Sea)

Latitude: 36° 12.323 ' N

Longitude: 4° 18.744 ' W

Logging date: June, 1995

Bottom Felt: 1119 mbrf (used for depth shift to sea floor)

Total Penetration: 736.3 mbsf

Total Core Recovered: 64.85 m (33.7 %)

Logging Runs

Logging String #1: DIT/SDT/HLDT/CNTG/NGT

Wireline heave compensator was used to counter ship heave.

Bottom-hole Assembly

The following bottom hole assembly depths are as they appear on the logs after 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~569.5 mbsf.

Processing

Depth shift: No differential depth shift necessary as only one tool string was run.

All original logs have been depth shifted to the sea floor (- 1119 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 two modes: linear array mode, with the 8-receivers providing full waveform analysis (compressional and shear) and standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') and short-spacing (3-5-5-7') logs. In the sedimentary section down to 650 mbsf the long-spacing logs showed a slightly better quality than the short-spacing ones and have been processed in order to eliminate some of the noise and cycle skipping experienced during the recording. Using two sets of the four transit time measurements and proper depth justification, four independent measurements over a -2ft interval centered on the depth of interest are determined, each based on the difference between a pair of transmitters and receivers. The program discards any transit time that is negative or falls outside a range of meaningful values selected by the processor. No processing was attempted in the basement section where the logs are too affected by the very poor hole conditions.

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. In hole 976A the quality of both density and neutron logs was degraded by the extremely large (caliper often saturated at 18.5") and irregular hole. Also, due to lack of centralization and to a 5-7o deviation which causes the tool to lean on one side of the hole, the sonic data are of poor quality. While reprocessing of the long-spacing logs yielded decent results in the sedimentary section, the logs recorded in linear array mode provided better readings in the basement section.

Data recorded through bottom hole assembly, such as the neutron and gamma ray data above 569.5 mbsf should be used qualitatively only because of the attenuation on the incoming signal.

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