Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Hole: 856H

Leg: 169

Location: Middle Valley-Juan de Fuca Ridge (NE Pacific Ocean)

Latitude: 48° 26.020' N

Longitude: 128° 40.859' W

Logging date: September, 1996

Bottom felt: 2434.5 mbrf (used for depth shift to sea floor)

Total penetration: 500 mbsf

Total core recovered: 49.1m (12.1%)

Logging Runs

Logging string 1: DIT/APS/HLDS/HNGS

Logging string 2: FMS/GPIT/SDT/NGT (1 downlog and upper, lower, and middle sections). Good data collected by FMS tool only at the beginning of the recording. Good SDT data collected downhole and in 3 uphole sections.

Logging string 3: FMS/GPIT/NGT (upper and lower sections).

Wireline heave compensator was initially used to counter ship heave. Wireline heave compensator was turned off during the first deployment of the FMS tool string.

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/APS/HLDS/HNGS: Bottom-hole assembly at ~ 85 mbsf

FMS/GPIT/SDT/NGT: Bottom-hole assembly at ~ 89 mbsf (pass 1 downlog)

FMS/GPIT/SDT/NGT: Bottom-hole assembly at ~ 85 mbsf

FMS/GPIT/SDT/NGT: recorded in open hole (pass 1 lower section)

FMS/GPIT/SDT/NGT: recorded in open hole (pass 1 middle section)

FMS/GPIT/SDT/NGT: recorded in open hole at mbsf (pass 1 upper section)

FMS/GPIT/NGT: recorded in open hole (pass 2 lower section)

FMS/GPIT/NGT: recorded in open hole (pass 2 upper section).

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from DIT/APS/HLDS/HNGS run and to the sea floor (- 2434.5 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 curves (HSGR and SGR) from the HNGS and NGT tool run on each logging string are 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 have been processed to correct for borehole size and type of drilling fluid. HNGS data are corrected in real time during the recording.

Acoustic data processing: The array sonic tool was operated in standard depth-derived borehole compensated mode, including long-spacing (8-10-10-12') and short-spacing (3-5-5-7') logs. Acoustic data were recorded downhole and uphole in three separate sections. The short-spacing transit times from both downhole and uphole 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. The transit times from the four sections recorded have been merged as follows:

90.5-115.5 mbsf: downlog

115.5-215.5 mbsf: uplog, upper section

215.5-288 mbsf: downlog

288-478 mbsf: uplog, lower section.

High-resolution data: Neutron porosity data were recorded at a sampling rate of 5.08 cm during the repeat section of the DIT/APS/HLDS/HNGS run. No high resolution data recording was performed during the main pass as logging speed was kept very high at ~ 600 m/hr, in an attempt to minimize the exposure to the possibly high temperatures. The tool internal temperature readings, however, revealed that the temperature was less than 100 °C and therefore a repeat pass was recorded at ~ 300 m/hr. Due to a software problem, no HLDS data could be recorded in high resolution mode.

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 (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 (APS, HLDS) 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. The hole at 856H is generally smooth, with most readings below 12 inches. The standoff measured by the APS tool ranges from 0 to about 2 inches, with most readings below 1 inch.

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

The resistivity data in the 85-210 mbsf interval corresponding to a sulfide-rich zone are of very poor quality and are not included in the database.

Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and on the FMS string (C1 and C2, upper and lower section during second pass ).

Details of standard shore-based processing procedures are found in the "Explanatory Notes" , ODP IR Volume 169. For further information about the logs, please contact:

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