ODP logging contractor: LDEO-BRG

Well name: 768C

Leg: 124

Location: Sulu Basin (Sulu Sea)

Latitude: 8° 00.04' N

Longitude: 121° 13.18' E

Logging date: December, 1988

Bottom felt: 4395.5 mbrf

Total penetration: 1271 mbsf

Total core recovered: 525.7 m (57 %)

Logging Runs

Logging string 1: BHTV

Logging string 2: DIT/SDT/NGT (upper and lower sections)

Logging string 3: LDT/CNTG/GPIT/NGT (upper and lower sections)

No information available could be found about use of wireline heave compensator to counter ship heave.

Bottom-hole Assembly/Pipe

The following bottom-hole assembly and drill 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 and drill string and/or wireline stretch.

DIT/SDT/NGT: Bottom-hole assembly at ~112 mbsf (upper section)

DIT/SDT/NGT: Drill Pipe at ~35 mbsf (upper section)

LDT/CNTG/GPIT/NGT: Bottom-hole assembly at ~112 mbsf (upper section)

LDT/CNTG/GPIT/NGT: Drill Pipe at ~35 mbsf (upper section)

Lower sections recorded open-hole.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/NGT upper and lower sections and to the sea floor (- 4402.5 m). This value corresponds to the water depth as seen on the logs and differs 7 meters from the "bottom felt" depth (see above) given by the drillers. The LDT/CNTG/GPIT/NGT lower section from the proprietary tape was about 50 m higher than the actual logged section and has been block-shifted accordingly before performing any depth match with the reference runs. The program used is an interactive, graphical depth-match program which allows to visually correlate logs and to define appropriate shifts. The reference and match channels are displayed on the screen, with vectors connecting old (reference curve) and new (match curve) shift depths. The total gamma ray curve (SGR) from the NGT tool run on each logging string is used to correlate the logging runs most often. In general, the reference curve is chosen on the basis of constant, low cable tension and high cable speed (tools run at faster speeds are less likely to stick and are less susceptible to data degradation caused by ship heave). Other factors, however, such as the length of the logged interval, the presence of drill pipe, and the statistical quality of the collected data (better statistics is obtained at lower logging speeds) are also considered in the selection. A list of the amount of differential depth shifts applied at this hole is available upon request.

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

Acoustic data processing: The SDT logs 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.

Quality Control

null value=-999.25. This value may replace recorded log values or results which are considered invalid (ex. processed sonic data).

Data recorded through bottom-hole assembly should be used qualitatively only because of the attenuation on the incoming signal.

No measurement of hole diameter was collected at Hole 768C

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 124. For further questions about the logs, please contact:

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