Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1528D
Expedition: 376
Location: Brothers Volcano (SW Pacific Ocean)
Latitude: 34° 52.9219' S
Longitude: 179° 4.1164 ' E
Logging date: June 8, 2018
Sea floor depth (driller's): 1239 m DRF
Sea floor depth (logger's): 1238 m WRF (HLDS/EDTC-B/HNGS downlog)
Total penetration: 1598.3 m DRF (359.3 m DSF)
Total core recovered: 87.23 m (29.3% of cored section)
Oldest sediment recovered:
Lithology: volcaniclastic rocks (lapillistone, lapilli-tuff and tuff-breccia)
The logging data
was recorded by Schlumberger in DLIS format. Data was processed at the
Borehole Research Group of the Lamont-Doherty Earth Observatory in June, 2018.
| Tool string | Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Casing depth (m WMSF) | Notes |
1. HLDS/EDTC-B/HNGS
|
|
49 |
60.3
|
Caliper closed. Invalid HLDS |
||
Recorded open hole |
60.3
|
|||||
49 |
60.3
|
A 13 3/8 inch casing was installed with a re-entry system from sea floor down to 60.3 m DSF. The drill pipe was placed at 50.3 m DSF (shown at 49 m WSF on the GR logs). In preparation for logging the hole was flushed with fresh water. Because of the high temperature expected in the hole, the HNGS tool was pre-cooled to -20 degC for several hours prior to logging. The tool string did not reach below 1571 m WRF (i.e., about 25 m above total depth). The mud temperature profile shows a maximum value of 222 degC during the downlog, 225 degC during the repeat run, and 247 degC during the main run.
The average ship heave was 0.5-1 m during the logging operation. The heave compensator was used whenever the tools were in open hole.
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 match and depth shift to sea floor. The original logs were first depth-matched to the gamma ray log from the downlo. 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 were then applied to all the other logs from the same tool string. The logs show excellent repeatability and minimal depth shifts were required during the matching process.
The depth-matched logs were then shifted to the sea floor.
High-resolution data. The Gamma Ray data from the EDTC-B tool and the bulk density (HLDS) data were recorded at sampling rates of 5.08 and 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.
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 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 inches) and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall (HLDS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL). While in general there was good replication between runs, the caliper readings were significantly different between 1500 m and 1571 m WRF. Several washouts, corresponding to caliper readings of 17 inches were observed over this interval. The borehole roughness also affected the quality of the density readings, especially where the tool could not make a proper contact with the borehole wall.
Also, it was found onboard that the temperature sudden change from 237 m WSF to the bottom of the logged interval impacted adversely the RHOM density readings due to the way the short -spaced high voltage detectors worked. Apparently the long-spaced detectors did not have any temperature-related issues and the RHL (Long-Spaced Density) is, in this case, a better estimate of the density of the formation in the lower part of the hole. Both RHOM (Corrected Bulk Density) and RHL are included in the database for comparison.
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 376. 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