Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG
Hole: 801C
Leg: 185
Location: Pigafetta Basin (tropical NW Pacific)
Latitude: 18° 38.5380' N
Longitude: 156° 21.5880' E
Logging date: May, 1999
Bottom felt: 5685 mbrf (assumed)
Total penetration: 926 mbsf
Total core recovered: 160.42 m (47.28%)

Logging Runs

Logging string 1: DLL/APS/HLDS/HNGS
Logging string 2: FMS/DSI/GPIT/NGT (1 pass, invalid DSI)
Logging string 3: FMS/SDT/GPIT/NGT (1 pass)

The wireline heave compensator was used on the second and third runs, but not the first. The DSI (Dipole Sonic Imager) failed down hole, hence after one pass the tool string was brought to the surface and the DSI replaced with the SDT (Sonic Digital Tool) for the final run.

Casing

Hole 801C was cased on a previous leg; the bottom of casing was determined to be at 481 mbsf (6166 mbrf) by the drillers, though the mudline was not well determined: these same values were used by the drillers on Leg 185. From Leg 144 logging of Hole 801C, the bottom of casing was found at 6175 mbrf. It was decided then that the value of 481 mbsf at the base of casing should be used as the reference level for the Leg 144 logs (implying a water depth of 5694 mbrf), so that the core and log depth would be comparable. Possible reasons for depth discrepancies are ship heave, tides, use of wireline heave compensator, and drill string and/or wireline stretch. For the Leg 185 logging data we have also used 481 mbsf for the base of casing (the Leg 185 logs did not extend to the mudline).

Processing

Depth shift: The original logs have been interactively depth shifted in three steps. First, the HNGS from the DLL/APS/HLDS/HNGS main pass was matched to the reference run from the Leg 144 log data (so that both data sets may be directly compared). Then this resulting HNGS log was used as the reference for the two FMS-sonic runs. Finally, all the logs were shifted to meters below sea floor, using a value of 5694 mbrf for the sea floor.

An interactive, graphical depth-match program was used to visually correlate the 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 or HSGR) from the NGT/HNGS 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 are 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 FMS/DSI/GPIT/NGT and FMS/SDT/GPIT/NGT runs have been processed to correct for borehole size and type of drilling fluid. HNGS data were corrected for hole size during the recording.

Acoustic data processing: The long-spacing (8’-10’-10’-12') sonic data were generally of good quality, apart from the interval from 510 to 535 mbsf, which resulted from working the tool string up and down to get it past a tight spot. This bad data is included in the velocities calculated from the DTLN and DTLF delay times; a complete velocity log has also been generated by splicing the good velocity log acquired on Leg 144 to the Leg 185 log at 535 mbsf. A handful of small anomalous spikes elsewhere in the hole was also removed.

High-resolution data: Neutron porosity data were recorded at a 5.08 cm sampling rate.

Quality Control

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

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 arms to close.

The small amount of NGT/HNGS data recorded through the casing (above 481mbsf) should not be used for lithologic interpretation, because of variable attenuation of the incoming signal.

Hole diameter was recorded by the calipers on the HLDS tool (LCAL) and FMS string (C1 and C2). The hole is generally smooth and in-gauge (~11 inches) apart from 2 intervals containing generally wider and more rugose borehole walls — these occur between 510 and 530 mbsf, and between 626 and 713 mbsf.. In these intervals, APS and HLDS data are less reliable, as indicated by anomalously high porosity readings, high APS standoff (STOF), and high density correction values (DRH).

SDT Sonic data between 510 and 535 mbsf is very poor, due to the tool becoming stuck in this interval (see Acoustic Data section above).

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 185. For further questions about the logs, please contact:

Cristina Broglia
phone: 845-365-8343
fax: 845-365-3182
email: Cristina Broglia