Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1456C
Expedition: 355
Location: Laxmi Basin (Arabian Sea)
Latitude: 16°37.293959' N
Longitude: 68°50.336604' E
Logging date: 17-18 April, 2015
Sea floor depth (driller's): 3649.2 m DRF
Sea floor depth (logger's): 3646 m WRF (FMS/DSI/GPIT/EDTC/HNGS Main run)
Total penetration: 4114.4 DRF (465.2 m DSF)
Total core recovered: 215 m (78.1 % of cored section)
Oldest sediment recovered:
Lithology: Nannofossil ooze, silt, clay
Data
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 April 2015.
Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
1.MSS/HRLA/HLDS/EDTC/HNGS
|
Downlog
|
81 |
No nuclear source and closed caliper. Invalid HLDS. |
||
Repeat
|
recorded open hole |
No nuclear source. Invalid HLDS.
|
|||
Main
|
81 |
No nuclear source. Invalid HLDS. |
|||
2.FMS/DSI/GPIT/EDTC/HNGS
|
Downlog
|
Closed caliper. Invalid FMS. |
|||
Repeat
|
recorded open hole |
||||
Main
|
80 |
||||
3.APS/HLDS/EDTC/HNGS
|
Downlog
|
80 |
Closed caliper. Invalid HLDS. |
||
Pass 1
|
recorded open hole |
||||
Pass 2
|
recorded open hole |
||||
Pass 3
|
80 |
In preparation for logging, the hole was displaced with 171 barrels of 10.5 lb/gal heavy mud.
The wireline heave compensator (WHC) was utilized on all logging runs: on average, the ship's heave was around 0.5-0.75 m peak-to-peak.
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 (-
3646 m). The sea floor depth
was determined by the step in gamma ray values obcserved on the FMS/DSI/GPIT/EDTC/HNGS main run at 3646 m WRF. This differs by 3.2 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 FMS/DSI/GPIT/EDTC/HNGS tool string. This pass was chosen as the reference run because it was the
longest recorded pass to cross the sea floor.
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 at 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 are 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 SGT 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 Stoneley (all passes), cross-dipole (main run), and upper and lower dipole (downlog and repeat run). The velocities were computed from the delay times. They are
generally of good quality, with the exception of the shear velocities acquired in the main run, which therefore have not been included in the shore-based database. Processing of the sonic waveforms is recommended in order 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). The overall quality of the data from Hole U1456C is good.
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.
The density and porosity data were acquired in thee separate passes, but none of them covered the entoire open hole interval. The first pass ranged from about 120 m WMSF to the bottom of the hole and is deemed to be the best one, as in subsequent passes the recording was affected by the irradiation of the formation on passes 2 and 3. As an example, though pass 3 is the only pass that recorded data up to the mudline, the porosity data are clearly degraded, showing, as expected, values that are much higher than on pass 1. In conclusion, the composite plots included online only show the density and porosity data from 120 m downward.
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) and by the FMS tool (C1 and C2). The hole is irregular throughout most of the drilled interval; it's enlarged way above 15" in the uppermost 204 m but it drops to 13-14" values in the lower part.
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 355. 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