Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1508C
Expedition: 371
Location: Tasman Sea
Latitude: 34° 26.8905' S
Longitude: 171° 20.2558' E
Logging date: August 31, 2017
Sea floor depth (driller's): 1619.7 m DRF
Sea floor depth (logger's): 1617 m WRF (MSS/HRLA/DSI/HLDS/HNGS/EDTC-B downlog)
Total penetration: 2324 m DRF (704.3 m DSF)
Total core recovered: 163.12 m (66.5 % of cored section)
Oldest sediment recovered:
Lithology: Gray nannofossil chalk with forminifers
The logging data
were recorded by Schlumberger in DLIS format. Data were processed at the
Borehole Research Group of the Lamont-Doherty Earth Observatory in August 2017.
Tool string |
Run
|
Top depth (m WMSF) |
Bottom depth (m WMSF) |
Drill Pipe (m WMSF) |
Notes |
MSS/HRLA/DSI/HLDS/HNGS/EDTC-B
|
Downlog
|
0 |
660 |
87.5 |
Reference run. Caliper closed and invalid HLDS |
Repeat |
580 |
672 |
recorded open hole |
Invalid HLDS (no Radoactive source) |
|
Main |
271 |
672 |
recorded open hole |
Invalid HLDS (no Radoactive source) |
The MSS/HRLA/DSI/HLDS/HNGS/EDTC-B tool string was rigged up in the usual manner using an air tugger. The nuclear source on the HLDS tool was not used for this run due to standing concerns about hole stability, especially in the sandy upper part. After tool assembly was complete, the string was powered up, tool zero was set, and the pre-survey calibrations were conducted, the tools were lowered into the hole. Just above sea floor and still inside the drill pipe, the up-going tension was checked and then a downlog was started. This pass reached total drilling depth without incident while recording all sensors but the caliper, which remains closed on downward passes.
Once the tool reached bottom, the caliper was opened and the repeat pass was conducted up to a depth of 2196 m DRF. At that point, the caliper was closed and the tool was returned to TD for the main pass. The main pass alsowent fairly smoothly until a depth of 1890 m DRF. At that point, high tension was noted on both the surface and head tension sensors, indicating that the tool was encountering some resistance. A few attempts were made to free the tool, but it was quickly found that the tools were firmly stuck and would not budge in either direction. After only a few tries, there was a notable pickup in head tension while the winch was static, indicating that something had fallen in on top of the tool. After that, any further attempts to pull resulted in a change in surface tension, but not in head tension. This indicated that the cable had also become trapped, so the only option was to terminate logging, cut the cable at surface, and conduct a “cut and thread” type fishing operation using the Schlumberger spear and overshot adaptors kept on board for that purpose. The tools were rigged down normally after a safe and professional fishing effort from the drill crew.
The sea state was high with 2 m peak-to-peak heave, so the Active Heave Compensator (AHC) was switched on during the logging operation.
Depth match and depth shift to sea floor. 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 equivalent logs from the other runs are matched to it. 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 downlog pass was chosen as the reference run for depth matching because it was the only pass that reached the sea floor. The depth-matched logs were then shifted to the sea floor (-1617 m). This amount differs 2,7 m from the driller's sea floor (see above).
Environmental
corrections. The HNGS and HRLA data were corrected for hole size during the recording.
High-resolution
data.
Acoustic data. The dipole shear sonic imager (DSI) was operated in the following modes: P&S monopole, upper, and lower dipole for all three passes. The velocities were computed from the DTCO (compressional), DT1 and DT2 (shear) delay times. They are generally of good quality. Post-cruise processing is however recommended to obtain accurate shear velocities from the dipole data.
Quality Control
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).
The gamma ray logs recorded through the drill pipe should be used only qualitatively because of the attenuation of the incoming signal.
The hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL). A wide (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (HLDS). The hole was in gauge from TD to 270 m WSF. Accordingly, the log data over this interval were of good quality. The hole conditions above 270 m WSF were poor and, thus logs in this interval should be used with caution.
A null value of
-999.25 may replace invalid log values.
Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition report, Proceedings of the International Ocean Discovery Program, Expedition 371. For further questions about the logs, if the hole is still under moratorium please contact the staff scientist of the expedition.
After the moratorium period you may direct your questions to:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu