Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1471E
Expedition: 359
Location: Maldive Archipelago (central equatorial N Indian Ocean)
Latitude: 4° 45.9829' N
Longitude: 73° 8.0929' E
Logging date: November 25, 2015
Sea floor depth (driller's): 430.3 m DRF
Sea floor depth (logger's): 430.5 m WRF (HRLA/MSS/APS/HLDS/EDTC/HNGS Main Run)
Total penetration: 1434 m DRF (1003.7 m DSF)
Total core recovered: 171.54 m (42.1 % of cored section)
Oldest sediment recovered:
Lithology: Lithified foraminifer-rich packstone and wackestone.
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 November 2015.
| Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) |
Notes |
1.HRLA/MSS/APS/HLDS/EDTC/HNGS
|
Downlog
|
96.5 |
|
||
Repeat
|
Recorded open hole |
||||
Main
|
96.5 |
Reference |
|||
2. VSI/EDTC |
96.5 |
5 stations, 30 shots |
|||
3. DSI/EDTC
|
Downlog
|
96.5 |
|||
Uplog
|
96.5 |
Three logging runs were planned for deployment in Hole U1471E. The HRLA/MSS/APS/HLDS/EDTC/HNGS tool string was rigged up first: it reached the total depth of 1433.32 m WRF and acquired a downlog, a repeat and a main pass. In anticipation of the porosity recording during the main pass, the APS nuclear source was switched off during the downlog and the repeat pass to prevent activation of the formation. The string reached the surface without incidents but it was clear from the caliper readings that most of the hole was washed out and that in several spots, where it was smaller than the bit size, it was starting to collapse. The VSI/EDTC string was run next. It hung up at about 1042 m WRF,which corresponded to one of the narrow readings on the caliper log from the previous run. Since all attempts to pass the bridge failed, 30 shots in 5 stations were acquired above that depth. Proper anchoring of the tool was made difficult by the large hole diameter and by the rapidly degrading hole conditions. Because it was deemed too risky to run the FMS, a modified string was deployed last, which included only the DSI and the EDTC tools. The downlog would not pass below 884 m WRF, thus an uplog was acquired from that depth. The tools returned to the surface in good conditions.
The sea state was calm, therefore no Wireline Heave Compensator (WHC) was used.
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 (- 430.5 m). The sea floor depth was determined by the step in gamma ray values at 430.5 m WRF on the HRLA/MSS/APS/HLDS/EDTC/HNGS main pass. This differs by 0.2 m from 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 main run/pass of the HRLA/MSS/APS/HLDS/EDTC/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.
Environmental
corrections. The HNGS
and HRLA data were corrected for hole size during the recording. The APS and
HLDS data were corrected for standoff and hole size respectively during the
recording.
High-resolution
data. Bulk density
(HLDS) and neutron porosity (APS) data were recorded sampling rates of 2.54 and
5.08 cm, respectively, 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 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 dipole, and Stonely mode in standard frequency, and
lower dipole mode in lower frequency. The velocities were computed from the delay times. They are
generally of acceptable quality, though waveform processing is recommended to get better 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 (APS, HLDS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL). It indicates a washed out hole with few spots where it narrows to values lower that the bit size, indicating a collapsing borehole.
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 359. 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