Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 793B

Leg: 126

Location: Izu-Bonin Forearc (NW Pacific)

Latitude: 31° 6.33' N

Longitude: 140° 53.27' E

Logging date: June, 1989

Bottom felt: 2975.3 mbrf (used for depth shift to sea floor)

Total penetration: 1682 mbsf

Total core recovered: 697.9 m (63.7 %)

Logging Runs

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

Logging string 2: FMS/GPIT/NGT (upper and lower sections)

Logging string 3: ACT/GST/NGT (upper and lower sections)

Logging string 4: WST

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

Bottom-hole Assembly/Pipe/Casing

The following bottom-hole assembly, pipe, and casing 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: Casing at ~565 mbsf (pass1, upper section)

DIT/SDT/NGT: Bottom-hole assembly at ~771 mbsf (pass 2, middle section)

DIT/SDT/NGT: Bottom-hole assembly at ~1033.5 mbsf (pass 3, upper and lower sections)

FMS/GPIT/NGT: Bottom-hole assembly at ~771 mbsf (lower section)

FMS/GPIT/NGT: Recorded open-hole (upper section)

ACT/GST/NGT: Bottom-hole assembly at ~771 mbsf (lower section)

ACT/GST/NGT: Recorded through drill pipe (upper section).

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/SDT/NGT pass 3 and to the sea floor (- 2975.3 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: NGT data from the DIT/SDT/NGT and ACT/GST/NGT runs have been processed to correct for borehole size and type of drilling fluid.

Acoustic data processing: The long spacing sonic logs from the three sections 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 invalid recorded values or results.

During the processing, quality control of the data is mainly performed by cross-correlation of all logging data. Large (>12") and/or irregular borehole and hole deviation affect most recordings; the FMS, for example, is not designed to be run in holes deviated more than 10 degrees, as the tool weight might cause the caliper to close (in hole 792E the deviation of the hole from the vertical was about 2°).

During the recording, the spherically focused resistivity (SFLU) was found to be un-calibrated; for this reason it has not been included in the database.

Data recorded through bottom-hole assembly or pipe should be used qualitatively only because of the attenuation on the incoming signal. Invalid gamma ray spikes were recorded during the DIT/SDT/NGT runs at 756-763, 988-990, 995-997.5, and 1023-1025 mbsf.

Hole diameter was recorded by the caliper on the FMS string (C1 and C2).

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 126. For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia