Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1568A
Expedition: 396
Location: Mogdunn (North Eastern Atlantic Ocean)
Latitude: 65° 21.9542' N
Longitude: 3° 3.1091' E
Logging date: August 25, 2021
Sea floor depth (driller's): 1715.6 m DRF
Sea floor depth (logger's): 1715 m WRF
Total penetration: 1915.6 m DRF (200 m DSF)
Total core recovered:156.61 m (86.3 % of cored section)
Oldest sediment recovered:
Lithology: Clay with sand and silt.
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 August 2021.
| Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
1. MSS/HRLA/HLDS/HNGS
|
|
83 |
Caliper closed. Invalid HLDS.
|
||
recorded open hole |
|||||
82 |
Depth reference, |
||||
2. FMS/DSI/GPIT/HNGS
|
|
84 |
Caliper closed. Invalid FMS. |
||
recorded open hole |
|||||
82 |
The logging operations at Hole U1568A began with the MSS/HRLA/HLDS/HNGS tool string. At about 10 m inside the drill pipe the string got hung up and after several unsuccessful attempts to free it the run was aborted and the the string was pulled out for inspection. Since the tool string was not damaged, the hole was circulated and a core barrel was lowered to check that the blockage was no longer there. The MSS/HRLA/HLDS/HNGS tool string was lowered again and this time it reached a maximum depth of 1903 m WRF (188 m WSF). A short repeat and a main pass were acquired uphole. The FMS/DSI/GPIT/HNGS was rigged up next and lowered to a total depth of 187 m WSF. It successfully acquired a repeat and a main pass.
The Wireline Heave Compensator (WHC) 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 of the MSS/HRLA/HLDS/HNGS tool string. 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.
Environmental
corrections. The HNGS
and HRLA data were corrected for hole size during the recording. The
HLDS data were corrected for hole size respectively during the
recording.
High-resolution data. Bulk density (HLDS) data were recorded at asampling 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. The HRLA is acquired every 5.08 cm; in the database it is resampled at 15.24 cm, for ease of comparison with the other logs.
Acoustic
data. The dipole shear
sonic imager (DSI) was operated in the following modes: P&S monopole, upper and lower dipole, and
Stoneley mode (all passes). The velocities were computed from the delay times. Shear velocities are
generally of good quality.
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). Hole diameter was recorded by the hydraulic caliper on the HLDS
tool (LCAL) and by the FMS tool (C1 and C2). The borehole is in acceptable condition, with increased diameter and rugosity below 113 m WMSF.
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 396. For further questions about the logs, if the hole is still under moratorium please contact the staff scientist of the expedition. 396
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