Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1466B
Expedition: 359
Location: Maldives (Central Equatorial N Indian Ocean)
Latitude: 4° 55.988' N
Longitude: 73°1.6894 ' E
Logging date: October 25, 2015
Sea floor depth (driller's): 528 m DRF
Sea floor depth (logger's): 528 m WRF (downlog), 527 mWRF (uplog)
Total penetration: 1346 m DRF (808 m DSF)
Total core recovered: 88.54 m (17.9 % of cored section)
Oldest sediment recovered:
Lithology: Lithified/partially lithified dolomitic grainstone with bioturbation
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 2015.
| Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
1. MSS/HRLA/HLDS/EDTC-B/HNGS
|
Downlog
|
102 |
No nuclear source and closed caliper. Invalid HLDS. Reference.
|
||
Uplog
|
102 |
No nuclear source and closed caliper. Invalid HLDS. |
In preparation for logging, Hole U1467C was swept with heavy mud. The tool string, consisting of the MSS/HRLA/HLDS/EDTC-B/HNGS tools, was rigged up without nuclear source due to concerns about hole stability and the potential risk of losing a source in Maldivian territorial waters. The downlog encountered a hole obstruction at about 894 m WRF (366 m WSF), which the tool string was unable to pass. Data was collected upward and then the hole was abandoned.
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 (-
528 m). The sea floor depth
was determined by the step in gamma ray values at 528 m WRF. This corresponds to 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 thedownlog of the MSS/HRLA/HLDS/EDTC-B/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.
High-resolution
data. Gamma Ray data from the EDTC-B 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 log). In Hole U1466B repeatability between the two passes is poor, as a result of sudden variations in wireline tension during both passes. Both gamma ray and resistivity data must be used with caution.
The 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.
Hole diameter could not be recorded because the hydraulic caliper on the HLDS
tool (LCAL) was not opened due to the poor hole conditions and the risk of getting the tool string stuck downhole.
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