Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 948C

Leg: 156

Location: N Barbados Ridge (tropical NW Atlantic)

Latitude: 15° 31.568' N

Longitude: 58° 43.935' W

Logging date: June, 1994

Bottom felt: 4943 mbrf (used for depth shift to sea floor)

Total penetration: 592 mbsf

Total core recovered: 171.5 m (94.9%, in 420592 mbsf cored interval)

Logging Runs

Logging string 1: SDT/HLDT/NGT

Wireline heave compensator was used to counter ship heave.

Bottom-hole Assembly/Pipe

The following bottom-hole assembly/pipe 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.

SDT/HLDT/NGT: Bottom-hole assembly at ~84.5 mbsf

SDT/HLDT/NGT: Pipe at ~22.5 mbsf.

Processing

Depth shift: As only one tool string was run, no depth shift to match runs is necessary. Original logs have been depth shifted to the sea floor (- 4937.5 m). This amount corresponds to the depth of the sea floor as observed on the logs and differs 5.5 m from the drillers' "bottom felt" depth.

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

Acoustic data processing: The SDT was operated in DDBHC long-spacing mode. The 4 transit times have been processed 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.

High-resolution data: Bulk density data were recorded at a sampling rate of 2.54 m. 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 has 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 (HLDT) and a good contact with the borehole wall. Invalid density spikes were recorded at 394, 416, 437.5, 462-467, and 520-525 mbsf.

Data recorded through bottom-hole assembly, such as the gamma ray recorded above 84 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); the caliper was restricted from about 525 to 330 mbsf due to severe collapse of the hole. Caliper was closed at 102.5 mbsf before entering the bottom hole assembly; consequently, the density logs could not be corrected in real time from 102.5 to 89 mbsf. The hole irregularities greatly affect the response of the logging tools; while the sonic log seems to yield a reliable response, the density log should be used with extreme caution. Its use for the calculation of an impedance curve for synthetic seismogram is not recommended.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 156. For further questions about the logs, please contact:

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