Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1481A
Expedition: 362
Location: Sumatra Seismogenic Zone (Equatorial SE Indian Ocean)
Latitude: 2° 45.2861' N
Longitude: 91° 45.5771' E
Logging date: October 1, 2016
Sea floor depth (driller's): 4189.8 m DRF
Sea floor depth (logger's): 4189.8 m WRF (MSS/HRLA/HLDS/DSI/EDTC/HNGS Downlog)
Total penetration: 5689.8 m DRF (1500 m DSF)
Total core recovered: 219.8 m (62.7 % of cored section)
Oldest sediment recovered:
Lithologies: Clay with silt containing thick beds of fine sand/sandstone with plant fragments and clasts
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 2016.
| Tool string | Pass |
Top depth (m WMSF) |
Bottomp depth (m WMSF) |
Casing depth (m WMSF) |
Notes |
MSS/HRLA/HLDS/DSI/EDTC-B/HNGS
|
Downlog
|
730 |
No HLDS source. Invalid caliper. Depth reference.
|
||
Repeat |
open hole |
No HLDS source. Caliper only. |
|||
Main |
730 |
No HLDS source. Caliper only. |
In preparation for logging, 350 carrels of heavy mud were pumped in the hole and the bottom of the tool string was raised to about 4251 m DRF (~61 m DSF). Given the concern that the hole might collapse, it was decided to run the HLDS (lithodensity tool) without the nuclear source, along with the spectral gamma ray (HNGS), sonic (DSI), resistivity ((HRLA), and magnetic susceptibility tools (MSS). The Active Heave Compensator (AHC) was used during the entire logging operations. The tool string reached the bottom of the hole without any problem and collected a downlog, main, and repeat pass.
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 shift to sea floor and depth match.
The original logs were first shifted to the sea floor (-
4189.8 m). The sea floor depth
was determined by the step in gamma ray values at 4189.8 m WRF. This depth coincides with the the sea floor depth given by the drillers (see
above). The depth-shifted logs have then been depth-matched to the gamma ray log from the downlog.
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.
Environmental corrections. The HNGS and HRLA data were corrected for hole size during the recording.
High-resolution
data. The 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, upper and lower dipole, and
Stoneley (run/pass). The velocities were computed from the DTCO, DT1, DT2, and DTST logs. They are
generally of good/low 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).
The gamma ray logs
recorded through casing (down to 730 m WMSF) and drill pipe (down to 61 m WMSF) should be used only
qualitatively, because of the attenuation of the incoming signal by the
thick-walled casing and the thinner-walled drill pipe.
The caliper log shows an enlarged (> 14 inches) and rugged borehole down to 1360 m WMSF. Hole conditions improve substantially in the lowermost 140 m, where the hole is less irregular and the diameter is generally less than 12 inches.
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 360. 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