ODP logging contractor: LDEO-BRG

Well name: 765C

Leg: 123

Location: Argo Abyssal Plain (SE Indian Ocean)

Latitude: 15° 58.54' S

Longitude: 117° 34.49' E

Logging date: September, 1988

Bottom felt: 5728.2 mbrf

Total penetration: 963.9 mbsf (hole deviated)

Total core recovered: 373.4 m (60 %)

Logging Runs

Logging string 1: DIT/SDT/NGT (3 passes)

Logging string 2: LDT/CNT/NGT (2 passes)

Wireline heave compensator was used to counter ship heave.

Hole 765C was drilled with the following deviation from the vertical:

4 ° at 400 m (deviated)

6.6 ° at 580 m (deviated)

10 ° at 777 m (deviated).

No correction has been performed for the deviation.

Bottom-hole Assembly/Pipe

The following bottom-hole assembly and 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.

DIT/SDT/NGT: Bottom-hole assembly at ~ 185 mbsf (upper section)

DIT/SDT/NGT: Pipe at ~ 97.5 mbsf (upper section)

DIT/SDT/NGT: Bottom-hole assembly at ~ 524 mbsf (middle section)

DIT/SDT/NGT: Pipe at ~ 439 mbsf (middle section)

DIT/SDT/NGT: Bottom-hole assembly at ~ 660 mbsf (lower section)

LDT/CNT/NGT: Bottom-hole assembly at ~ 156 mbsf.

Processing

Depth shift: Original logs were depth shifted to the sea floor onboard (according to the log header, the amount was 5728 mbsf). Subsequently, they have been interactively depth shifted with reference to NGT from LDT/CNTG/NGT. Correlation between runs was generally difficult and very few tie points could be used. The lowest DIT/SDT/NGT pass did not overlap with any of the other passes and could not be depth shifted. 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: According to the logging data report, the addition of 5 % KCl and barite to the mud in order to stabilize the hole during DIT/SDT/NGT first pass (upper section) greatly affected the NGT response. A real-time correction was applied during logging, but according to the logging report it overcorrected the data and yielded negative potassium values. Subsequently, the hole was flushed with sea water and the effect was therefore greatly reduced during the LDT/CNT/NGT run. At this hole, only the upper and lower sections of the DIT/SDT/NGT tool string and LDT/CNTG/NGT tool string have been processed and yield good results Even though it does not cover the entire hole length, the NGT from the last pass is to be considered the best measurement of gamma ray.

Acoustic data processing: The array sonic tool was operated in standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') logs. The sonic 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 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, LDT) and a good contact with the borehole wall.

The resistivity spherically focused log (SFLU) suffered from intermittent failure during the recording. The data is therefore not included in this database.

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

Hole diameter was recorded by the 3-arm mechanical caliper device. The caliper tool measured 1.4" too high, according to calibration performed on board in casing; the value has been corrected prior to using the caliper for borehole size corrections.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 123. For further questions about the logs, please contact:

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