Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 718C

Leg: 116

Location: Bengal Fan (central tropical S Indian Ocean)

Latitude: 1° 1.252' S

Longitude: 81° 24.065' E

Logging date: July, 1987

Bottom felt: 4741.6 mbrf (used for depth shift to sea floor)

Total penetration: 935 mbsf

Total core recovered: 274 m (29.6 %)

Logging Runs

Logging string 1: DIT/SDT/NGT

Logging string 2: ACT/GST/LDT/NGT

No information available about use of wireline heave compensator.

Bottom-hole Assembly

The following bottom-hole assembly 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 ~96 mbsf

ACT/GST/LDT/NGT: bottom hole assembly at ~96 mbsf.

Processing

Depth shift: as four passes of DIT/SDT/NGT and two passes of ACT/GST/LDT/NGT were run, and none one them covered the entire logging section, different passes were used according to the following sequence:

1. Main ACT/GST/LDT/NGT was used as reference for the lower, middle and first of the two upper sections of the DIT/SDT//NGT string.

2. The shifted upper section of the DIT/SDT//NGT run has been used as reference for the second upper section of the DIT/SDT/NGT string and for the repeat of the ACT/GST/LDT/NGT string.

All original logs have then been depth shifted to the sea floor (- 4741.6 m). 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: 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') 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.

Geochemical data: Due to the low quality of the results, no processed geochemical data from this hole has been included in the online database.

Quality Control

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

During the processing, quality control of the data is mainly performed by cross-correlation of all logging data.

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

No valid caliper measurements were performed.

Due to telemetry problems during the recording, the ACT/GST data are of very questionable quality through the entire logged section. The entire data set is invalid above 230 mbsf, whereas the capture cross section (CSIG) and aluminum (ALUM) data are invalid through the entire hole.

The data from the DIT/SDT/NGT passes have been merged as follows:

gamma ray data

551 to 283 mbsf (lower section)

283 to 183.5 mbsf (middle section)

183.5 to 80 mbsf (upper section #1)

80 to mudline(upper section #2)

acoustic data:

543 to 283.5 mbsf (lower section)

283.5 to 176.8 mbsf (middle section)

176.8 to 98.5 mbsf (upper section)

resistivity data:

557 to 283 mbsf (lower section)

283 to 172 mbsf (middle section)

172 to 98 mbsf (upper section)

The density data from the ACT/GST/LDT/NGT passes have been merged as follows:

density data:

552 to 134 mbsf (lower section)

134 to 96 mbsf (upper section)

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 116. For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia

Trevor Williams

Phone: 845-365-8626

Fax: 845-365-3182

E-mail: Trevor Williams