Standard Wireline Data Processing
The logging data was recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in October 2004.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. DIT/APS/HLDS/GPIT/HNGS/TAP | Downlog | 15 |
212 |
96.5 |
|
| Main | 0 |
255 |
96.5 |
Reference |
|
| Repeat | 109 |
194 |
The DIT/APS/HLDS/GPIT/HNGS/TAP tool string reached to within approximately 29 m of the cored depth. At the end of the repeat pass, the caliper arm on the HLDS tool would not close and the tool string was pulled into the drill pipe with difficulty. Owing to increasing heave conditions (typically 3 m, reaching 4-5 m), no further logging was carried out at the site. The LDEO wireline heave compensator was used during all passes.
The depths in the table are for the processed logs (after depth matching between passes and depth shift to the sea floor). Generally, discrepancies may exist between the sea floor depths determined from the downhole logs and those determined by the drillers from the pipe length. Typical reasons for depth discrepancies are ship heave, wireline and pipe stretch, tides, and the difficulty of getting an accurate sea floor from the 'bottom felt' depth in soft sediment.
Depth match and depth shift to sea floor: The original logs were depth-matched to the GR log from the main pass of the DIT/APS/HLDS/GPIT/HNGS/TAP tool string, and were then shifted to the sea floor (-3471 m). The DIT/APS/HLDS/GPIT/HNGS/TAP main pass was chosen as the reference run because it was the only run to cross the sea floor. The GR logs from the other passes were matched to the GR log from the reference run.
Depth matching is typically done in the following way. One log is chosen as reference (base) log (usually the total gamma ray log from the run with the greatest vertical extent and no sudden changes in cable speed), and then the features in the equivalent logs from the other runs are matched to it in turn. This matching is performed manually. The depth adjustments that were required to bring the match log in line with the base log are then applied to all the other logs from the same tool string.
The sea floor depth was determined by the step in gamma ray values at 3471 mbrf. This differs by 1.3 m from the sea floor depth given by the drillers (see above).
High-resolution data: Bulk density and neutron porosity data were recorded at a sampling rate of 2.54 and 5.08 cm, respectively. The enhanced bulk density curve is the result of Schlumberger enhanced processing technique performed on the MAXIS system onboard. While in normal processing short-spacing data is smoothed to match the long-spacing one, in enhanced processing this is reversed. In a situation where there is good contact between the HLDS pad and the borehole wall (low-density correction) the results are improved, because the short spacing has better vertical resolution.
The quality of the data is assessed by checking against reasonable values for the logged lithologies, by repeatability between different passes of the same tool, and by correspondence between logs affected by the same formation property (e.g. the resistivity log should show similar features to the density log).
Gamma ray logs recorded through bottom hole assembly (BHA) and drill pipe should be used only qualitatively, because of the attenuation on the incoming signal. The thick-walled BHA attenuates the signal more than the thinner-walled drill pipe.
A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (HLDS, APS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL). The hole diameter varies from 16 to >18 inches.
A null value of -999.25 may replace invalid log values.
Additional information about the drilling and logging operation can be found in the Operations section of the Site Chapter in IODP Initial Reports volume 303. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia