Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 844B

Leg: 138

Location: Guatemala Basin (tropical NE Pacific)

Latitude: 7° 55.279' N

Longitude: 90° 28.846' W

Logging date: May, 1991

Bottom felt: 3425 mbrf (used for depth shift to sea floor)

Total penetration: 290.9 mbsf

Total core recovered: 289.9 m (99.7 %)

Logging Runs

Logging string 1: DIT/SDT/HLDT/NGT

Logging string 2: ACT/GST/NGT

Logging string 3: FMS/GPIT/NGT (2 passes)

The wireline heave compensator broke down at the bottom of the hole and could not be fixed; sea, however, were calm, and the lock of WHC did not significantly affect logging quality.

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, use of wireline heave compensator, and drill string and/or wireline stretch.

DIT/SDT/HLDT/NGT: Recorded open-hole

ACT/GST/NGT: Bottom-hole assembly at ~42.5 mbsf

FMS/GPIT/NGT: Recorded open-hole.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from FMS/GPIT/NGT pass 1 and to the sea floor (- 3425 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 DIT/SDT/HLDT/NGT and ACT/GST/NGT runs 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. Because of the good quality of the sonic data, no processing has been necessary. Velocity has been computed from the delay times.

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

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 (HLDT) and a good contact with the borehole wall. Hole deviation can also affect the data negatively; 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.

Data recorded through bottom-hole assembly should be used qualitatively only because of the attenuation on the incoming signal. An invalid gamma ray spike was recorded by the ACT/GST/NGT tool string at 20 mbsf.

Hole diameter was recorded by the hydraulic caliper on the HLDT tool (CALI) and 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 138. For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia