Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 786B

Leg: 125

Location: Izu-Bonin Forearc (NW Pacific)

Latitude: 31° 52.45' N

Longitude: 141° 13.59' E

Logging date: April, 1989

Bottom felt: 3082.4 mbrf

Total penetration: 828.6 mbsf

Total core recovered: 190 m (28.5 %)

Logging Runs

Logging string 1: DIT/SDT/NGT (upper section)

Logging string 2: HLDT/CNTG/NGT (upper section)

Logging string 3: DIT/SDT/HLDT/NGT (lower section)

Logging string 4: ACT/GST/GPIT/NGT

Logging string 5: BHTV

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

Bottom-hole Assembly/Pipe

The following bottom-hole assembly/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 ~120 mbsf

HLDT/CNTG/NGT: Bottom-hole assembly at ~120 mbsf

DIT/SDT/HLDT/CNTG/NGT: Bottom-hole assembly at ~467 mbsf

ACT/GST/GPIT/NGT: Bottom-hole assembly at ~467 mbsf

ACT/GST/GPIT/NGT: Drill pipe at ~380.5 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from ACT/GST/GPIT/NGT run and to the sea floor (- 3076.5 m). The amount of depth shift to the sea floor corresponds to the mudline as seen on the logs and differs 5.9 m from the "bottom felt" depth (see above). 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 DIT/SDT/NGT upper and lower section s have been processed to correct for borehole size and type of drilling fluid.

Acoustic data processing: The array sonic tool was operated standard depth-derived borehole compensated mode including long-spacing (8-10-10-12' logs. Because of the extremely noisy character of the logs recorded in the upper part of the hole, only the sonic logs from the lower section and the 350-440 mbsf interval in the upper part have been processed, in an attempt 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 invalid recorded log values or results 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, HLDT) and a good contact with the borehole wall.

Invalid medium resistivity values (IMPH) were recorded in the lower part of the hole; they are not included in the database.

Data recorded through bottom-hole assembly or drill pipe should be used qualitatively only because of the attenuation on the incoming signal. Drill pipe collars, evenly spaced every 10 m, are clearly shown as sharp decreases in gamma ray values above 380 mbsf.

Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 125. 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