Standard Wireline Data Processing
IODP logging
contractor: USIO/LDEO
Hole: U1438D
Expedition:
Location: Amami-Sankaku Basin (Philipine Sea)
Latitude: 27 ° 23.0218' N
Longitude: 134° 19.1023' E
Logging date:
Sea floor
depth (driller's):
4711 m DRF
Sea floor
depth (logger's):
4708.5 m WRF (HRLA/APS/HLDS/EDTC-B/MSS/HNGS downlog)
Total
penetration: 5608.8 m DRF (897.8 m DSF)
Total core
recovered: 77
% of cored section (upper 257 m cored in U1438B, with 88% recovery)
Oldest
sediment recovered: Eocene
Lithologies:
Mud with ash, turbidites, conglomerates, breccia, sandstone
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 June 2014.
Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
1. HRLA/APS/HLDS/EDTC-B/MSS/HNGS
|
Downlog
|
95 |
Closed caliper. Invalid APS/HLDS. Reference Run. |
||
Uplog
|
94.5 |
After drilling completion, the hole was conditioned with sepiolite-based mud. Because drilling conditions were good, similar conditions were expected for the logging operaions. The pipe was pulled to 300 m DSF before dropping a free fall funnel. Afterwards, the ship moved 20 m to the north and the drill bit was released. Re-entering the hole was difficult but it finally succeeded.
The HRLA/APS/HLDS/EDTC-B/MSS/HNGS tool string was lowered without probems to a depth of 5021 m WRF, where it encounterd a bridge. After several attempts to pass the bridge it was decided to start logging up. Because the bridge depth corresponded to the depth of the drill pipe during the deployment of the free fall funnel, it was believed that the drill pipe itself caused the bridge. The pipe was then lowered to 5039 DRF (19 m below the obstruction) and the drill string was lowered again. However, it encountered a new obstruction at 5080 m WRF, at which point it was deemed safe to stop logging and retrieve the tool string.
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 (- 4708.8 m). The sea floor depth was determined by the step in gamma ray values on the downlog at 4708.5 m WRF. This differs by 2 .5 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 downlog.
Note that before depth-matching there was a mismatch of about 6-7 m between the two passes.
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; since the caliper was closed during the downlog, the bit size was used instead.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 SGT tool were recorded at sampling rates of 5.08 and 15.24
cm.
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 and porosity 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). The hole is enlarged and more irregular down to about 165 m WMSF, smoother but enlarged (mostly above 16 in) from 160 to 270 m WMSF and sometimes below bit size in the bottom 20 m interval. Overall, the hole size was not ideal for logging and the quality of the density and porosity logs was affected by the lack of contact with the borehole wall.
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 reports,
Proceedings of the Integrated Drilling Program, Expedition 351.
For further questions about the logs, please contact:
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia