Standard Wireline Data Processing
IODP-USIO logging contractor: LDEO-BRG
Hole: U1325C
Expedition: 311
Location: Cascadia Margin (NE Pacific)
Latitude: 48° 38.7006' N
Longitude: 126° 59.0069' W
Logging date: October 22-23, 2005
Sea floor depth (drillers'): 2206.2 mbrf
Sea floor depth (loggers'): 2205 mbrf
Total penetration: 304.3 mbsf
Total core recovered: 62.49 m (54.1 % of cored section)
Oldest sediment cored: Presumably Pleistocene – no diatoms found in the lower part of the hole
Lithologies: Clay and silty clay, with some sandy intervals
The logging data was recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory.
| Tool string | Pass | Top depth (mbsf) | Bottom depth (mbsf) | Bit depth (mbsf) | Notes |
| 1. DITE/HNGS | Uplog |
0
|
259
|
60
|
Reference
|
| 2. DSI/SGT/TAP | Pass 1 |
73
|
185
|
|
|
| Pass 2 |
23
|
183
|
64
|
|
Because of high ship heave (5-7 m) no tools with caliper arms were used. The DITE/HNGS tool reached to within 40 m of the base of the hole, and the DSI/SGT/TAP was blocked from passing below 186 mbsf. The TAP tool was successfully run with the DSI/SGT. The Schlumberger wireline heave compensator (WHC) was used on all passes, because the heave was too large for the LDEO WHC. The match between passes was very good in the challenging heave conditions.
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 and tides.
Depth match and depth shift to sea floor: The original logs were depth-matched to the HSGR log from the DITE/HNGS tool string, and were then shifted to the sea floor (-2205 m). The main pass was chosen as the reference run because it was the only run to cross the sea floor. The ECGR logs from the other passes were matched to the HSGR 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 2205 mbrf. This differs by only 1.2 m from the sea floor depth given by the drillers (see above).
Acoustic data: The dipole shear sonic imager (DSI) was run in P&S monopole, upper and lower dipole, and Stoneley modes on both passes. Because of the slow formation, the automatic picking of wave arrivals in the sonic waveforms did not provide reliable results. Reprocessing of the original waveforms was required to extract meaningful compressional and shear velocities. The most reliable shear velocity is the one derived from the upper dipole (VS2), where the lower source frequency used generated more coherent waveforms.
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 sonic velocity 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 (APS, HLDS). No caliper measurements were taken in U1325C, but the repeatability between the logs suggests the hole was in reasonably good shape.
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 Expedition Reports volume 311. For further questions about the logs, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia
Gilles Guerin
Phone: 845-365-8671
Fax: 845-365-3182
E-mail: Gilles Guerin