Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 731C

Leg: 117

Location: Owen Ridge (tropical NW Indian Ocean)

Latitude: 16°28.229 ' N

Longitude: 59°42.149' E

Logging date: October, 1987

Bottom felt: 2370.9 mbrf (used for depth shift to sea floor)

Total penetration: 994.2 mbsf

Total core recovered: 59.8 m (39 %)

Logging Runs

Logging string 1: DIT /BHC/GR

Logging string 2: GST/CNTG/GPIT/NGT (upper, lower, and middle section)

Because of good sea conditions, it was decided not to use the wireline heave compensator.

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 and drill string and/or wireline stretch.

DIT /BHC /GR: Bottom-hole assembly at ~63.5 mbsf

GST/CNTG/GPIT/NGT: Drill pipe at ~64 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to GR from DIT/BHC/GR run and to the sea floor (- 2370.9 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 gamma ray curve (GR or SGR) from the GR/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 from GST/CNTG/GPIT/NGT have been processed to correct for borehole size and type of drilling fluid.

Acoustic data processing: The sonic tool used was the BHC (3'-5'-3'-5' spacing). The sonic log was of good quality and so no acoustic data processing other than converting delay time to velocity was done.

Geochemical processing: The elemental yields recorded by the GST tool represent the relative contribution of only some of the rock-forming elements (iron, calcium, chlorine, silicon, sulfur, hydrogen, gadolinium, and titanium - the last two ones computed during geochemical processing) to the total spectrum. Because other rock-forming elements are present in the formation (such as aluminum, potassium, etc.), caution is recommended in using the yields to infer lithologic changes. Instead, ratios (see acronyms.doc) are more appropriate to determine changes in the macroscopic properties of the formation.

Processing of the geochemical data of Hole 731C was carried out in 1988; revision of the results and their inclusion in the database will be completed at a later date.

Quality Control

null value=-999.25. This may replace invalid log values or results.

Data recorded through bottom-hole assembly or pipe, such as the NGT data above 64 mbsf should be used qualitatively only because of the attenuation on the incoming signal. The gamma ray measurement from the NGT tool shows evenly spaced spikes corresponding to pipe collars above 64 mbsf.

Gamma ray (NGT) and neutron porosity data from the upper, middle, and lower sections have been merged as follows:

gamma ray merged at 885 and 755 mbsf

neutron porosity merged at 890 and 770 mbsf.

Because of the limited overlap between the middle and lower section, the gamma ray data in the 875-885 mbsf interval and particularly the spike at 885 mbsf must be considered invalid.

No caliper measurement was performed at Hole 731C.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 117. For further questions about the logs, please contact:

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