Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1571A
Expedition: 396
Location: Skoll High (North Eastern Atlantic Ocean)
Latitude: 67° 18.4017' N
Longitude: 3° 44.2496' E
Logging date:
Sea floor
depth (driller's):
1217.1 m DRF
Sea floor
depth (logger's):
1216 m WRF
Total
penetration: 1464.7 m DRF (247.6 m DSF)
Total core
recovered: 116.55 m (47.1%)
Oldest sediment recovered: Late early Eocene
Lithologies: Clay with silt and sand; volcaniclastic sediments; aphyric basalts
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 September 2021 .
| Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
1. MSS/HRLA/HLDS/HNGS
|
|
||||
Recorded open hole
|
|||||
Depth reference. |
|||||
2. FMS/DSI/GPIT/HNGS
|
|
86.5 |
Caliper closed. Invalid FMS. |
||
Recorded open hole |
|||||
86 |
|||||
2. UBI/GPIT/HNGS
|
|
||||
Recorded open hole
|
|||||
In preparation for logging the hole was swept by pumping 50 barrels of high viscosity mud and thenit was displaced with 75 barrels of 10.5 ppg mud. The MSS/HRLA/APS/HLDS/HNGS tool string was deployed first and reached total depth without incidents. A downlog, a short repeat and a main pass to the mudline were acquired. The FMS/DSI/GPIT/HNGS tool string followed; data were acquired downhole and then uphole during a repeat and main pass. During the recording it was observed that the hole was severely under gauge from total depth (1464 m WRF, 248 m WSF) to about 1450 m WRF (234 m WSF), indicating that either the hole was starting to collapse or some debris had accumulated at the bottom of the hole. After the FMS tool string was rigged down, preparations for the UBI/GPIT/HNGS started, by first displacing the heavy mud in the hole with sea water. Since the velocity of sea water is well known, the downlog was conducted in low-resolution mode instead of Fluid Properties Mode. This allowed to acquire some image data should any further pass become compromised due to deteriorating hole conditions or technical problems.
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/APS/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.
High-resolution
data. Bulk density
(HLDS) data were recorded every 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.
Acoustic
data. The dipole shear
sonic imager (DSI) was operated in the following modes: P&S monopole, upper and lower dipole), and
Stoneley (all passes). The velocities were computed from the delay times. They are generally of acceptable quality but the results could be improved by processing the original 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 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).
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.
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