Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1554E
Expedition: 395C
Location: Bjorn Drift (Central North Atlantic Ocean)
Latitude: 60° 7.5235' N
Longitude: 26° 42.1324' W
Logging date: June 27-28, 2021
Sea floor depth (driller's): 1880.8 m DRF
Sea floor depth (logger's): 1880.8 m WRF
Total penetration: 2528.5 m DRF (647.7 m DSF)
Total core recovered: 541.07 m (93.1 % of cored section)
Oldest sediment recovered: 12.7 Ma
Lithology: Clay and silt
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 2021.
| Tool string |
Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
|
1. MSS/HRLA/APS/HLDS/HNGS
|
Downlog
|
0
|
647
|
~ 70.7
|
Caliper closed. Invalid APS and HLDS. Reference run for depth matching.
|
|
Repeat
|
527
|
647
|
Recorded open hole
|
||
|
Main
|
0
|
647
|
~ 62.8
|
||
|
2.FMS/DSI/GPIT/HNGS
|
Downlog
|
0
|
647
|
~ 72
|
Caliper closed. Invalid FMS.
|
|
Repeat
|
122
|
647
|
Recorded open hole
|
||
|
Main
|
0
|
647
|
~62
|
After completion of coring operations, sepiolite and barite were added to the hole to keep it from collapsing during logging. The MSS/HRLA/APS/HLDS/HNGS tool string was deployed first. A downlog was run with the caliper closed and the APS radioactive source turned off. A short repeat pass and a main pass followed (each with the radioactive source turned on), the latter from total depth to the sea floor. Some difficulty was experienced getting the tool string inside the drill pipe, which resulted in increased radioactive activation of the formation and therefore higher gamma ray readings. The drill pipe was moved about 10 m higher than in the downlog: The MSS, HRLA and HLDS recorded this change, while the HNGS did not. The FMS/DSI/GPIT/HNGS tool string was run next and collected a downlog (with the caliper closed), a long repeat, and a main pass. Again the tool had difficulty entering the drill pipe, though it was easier than during the first pass.
The Wireline Heave Compensator (WHC) was used in the open hole intervals to counter the ship weave. Sea conditions were good, with ship's heave in the 0.5-1 m range.
The depths in the table are for the processed logs (after depth matching between passes and depth shift to the sea floor). 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 downlog pass of the MSS/HRLA/APS/HLDS/HNGS tool string. The downlog was chosen as the reference run because it showed the sea floor at the same depth as the drillers'. 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.
The depth-matched logs have then been shifted to the sea floor. The sea floor depth was determined by the step in gamma ray values at 1880.8 m WRF observed on the MSS/HRLA/APS/HLDS/HNGS downlog pass. This value matches the sea floor depth given by the drillers (see above).
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-B tool were recorded at sampling rates of 5.08 and 15.24 cm. The HRLA is also acquired every 5.08 cm; in the database itis resampled at 15.24 cm, for ease of comparison with the other logs.
Acoustic data. The dipole shear sonic imager (DSI) was operated in the following modes: P&S monopole and upper and lower dipole. The velocities were computed from the slownesses. They locally correlate well with the resistivity logs; shear velocities are not reliable between 75 and 220 mbsf. Processing from the sonic waveforms is recommended to improve the results, though this interval below the drill pipe was washed out, as indicated by the caliper readings (see summary plot).
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). Repeatability between logging runs is generally good in Hole U1554E.
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) and by the FMS tool (C1 and C2). The hole was very irregular and locally enlarged up to the maximum opening of the caliper tool.
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 395C.
For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.