Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1512A
Expedition: 369
Location: Great Australian Bight (SE Indian Ocean)
Latitude: 34° 1.6406' S
Longitude: 127° 57.7605' E
Logging date: October 13, 2017
Sea floor depth (driller's): 3081.7 m DRF
Sea floor depth (logger's): 3087.5 m WRF
Total penetration: 3782.5 m DRF (700.8 m DSF)
Total core recovered: 632 m (90 % of cored section)
Oldest sediment recovered: Turonian
Lithology: Carbonatic ooze, biosiliceous silty clay, silty claystone, siltstone and sandstone
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 2017.
| Tool string | Pass | Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
1. HRLA/DSI/HLDS/EDTC-B/HNGS
|
Downlog
|
~ 57 |
Caliper closed and invalid HLDS
|
||
Repeat
|
open hole |
||||
Main
|
55.8 |
Reference |
No issues were encountered during the drilling operations. Because gas hydrates were identified in some cores, as a precaution heavier (10 lb/gal) mud weight was pumped in preparation for logging. The tool string was modified to include the acoustic tool (DSI) instead of the porosity (APS) tool. Also, the magnetic susceptibility (MSS) was eliminated and its lowest positon in the tool string was taken by the gamma ray (HNGS) tool: this was aimed at possibly identifying on the logs a boundary that was missed on the cores due to partial recovery. Also included were the density (HLDS), resistivity (HRLA), and gamma ray (EDTC-B) tools. No major problems were encountered during the downlog with the exception of a ledge at about 3580-3590 m WRF (492.5-502.5 m WSF). During the uplog repeat pass it was observed that the caliper did not fully open even to bit size ((9 7/8 in). Initially it was believed that this was due to the presence of heavy clay but later analysis of the deviation curve showed that the deviation was 26 degrees at total depth. The tool orientation must have the caliper on the top side of the hole in order to fully open, a condition that was prevented by the hole deviation. A main pass was carried out with the caliper fully open.
The FMS and VSI runs were canceled due to time constraints.
Average heave was estimated at 1.5 m, with occasional values of 3 m. The motion heave compensator was used whenever the tools were in open hole.
The depths in
the table are for the processed logs (after depth shift to the sea floor and depth matching between passes). 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 a 'bottom felt' depth in soft sediment.
Depth match and depth shift to sea floor. The original logs were first depth-matched to the gamma ray log from the main pass. The main pass was chosen as the reference run because it was the longest pass. 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 depth-matched logs have then been shifted to the sea floor.
High-resolution
data. The bulk density
(HLDS) data were recorded sampling rates of 2.54 cm in addition to the standard sampling rate of 15.24 cm.
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.
Gamma Ray data from the EDTC-B tool were recorded at sampling rates of 5.08 and 15.24
cm.
Acoustic
data. The dipole shear
sonic imager (DSI) was operated in the following modes: P&S monopole
and upper and lower dipole on all three passes. The velocities were computed from the delay times. They are
generally of good quality, though waveform analysis is recommended post-cruise in order to get more accurate results.
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 of 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). As mentioned above, the hole deviation locally affected the qualty of the caliper readings. Wherever fully open, the caliper varied mostly between 10.5 and 11.5 in, with some intervals of 12.5 in. Overall, the hole conditions were excellent, a rare occurrence in IODP.
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 369. 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