Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1562B
Expedition: 395C
Location: Bjorn Drift (Central North Atlantic Ocean)
Latitude: 60° 6.2993' N
Longitude: 26° 30.1026' W
Logging date: July 19-20, 2021
Sea floor depth (driller's): 2014.5 m DRF
Sea floor depth (logger's): 2015 m WRF
Total penetration: 2576 m DRF (561.5 m DSF)
Total core recovered: 153.4 m (47.6 % of cored section)
Oldest sediment recovered: 13.9 Ma
Lithology: Bioturbated sediments with low carbonate content (upper 150 m), increasing in the lower interval. Basalts with various degrees of alteration.
The logging data were recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group of the Lamont-Doherty Earth Observatory in July 2021.
| Tool string | Pass |
Top depth (m WMSF) |
Bottom depth (m WMSF) |
Drill pipe depth (m WMSF) |
Notes |
| 1. MSS/HRLA/APS/HLDS/HNGS | Downlog | 0 | 559 | 86 | Caliper closed. Invalid APS and HLDS. |
| Repeat | 136 | 559 | Recorded open hole | ||
| Main | 0 | 560 | 86 | Reference run for depth matching. | |
| 2. FMS/DSI/GPIT/HNGS | Downlog | 0 | 485 | 86 | Caliper closed. Invalid FMS. |
| Repeat | 419 | 558 | Recorded open hole | ||
| Main | 0 | 558 | 86 | ||
| 3.VSI/HNGS | 81 | 113 | 86 |
Pass depth-tied to main pass of MSS/HRLA/APS/HLDS/HNGS |
|
| Recorded open hole |
Seismic data acquired at 4 stations (420, 428.5, 459.4, 556.1 m DSF) |
||||
| 4. UBI/GPIT/HNGS | Pass 1 | 408 | 553 | Recorded open hole | No UBI images. |
| Pass 2 | 410 | 553 | Recorded open hole | ||
| Pass 3 | 0 | 407 | 86 |
Following coring operations at Hole U1562B, logging started by rigging up the MSS/HRLA/APS/HLDS/HNGS tool string. A downlog was conducted first, with the caliper closed and the radioactive source turned off, followed by a repeat and and a main pass uphole. The caliper recorded on both passes was used to correct the the resistivity and gamma ray data for hole size. The FMS/DSI/GPIT/HNGS tool string was run next and it acquired a downlog (no FMS images, as the caliper arms were closed) and a main and repeat pass uphole. The VSIT/HNGS was run next. After a brief pass uplog to depth-tie any upcoming shots to the MSS/HRLA/APS/HLDS/HNGS main pass, 5 stations were acquired at 2571.1, 2474, 2443.5, and 2435 m DRF (556.5, 459.4, 428.5 and 420 m WMSF). The logging operations were completed by finally running the UBI/GPIT/HGNS tool string, which acquired three passes.
The wireline heave compensator (WHC) was used in the open hole intervals to counter the 0,25-0.5 m ship heave.
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 depth-matched to the gamma ray log from the main pass of the MSS/HRLA/APS/HLDS/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 the features in the equivalent logs from the other runs are matched to it in turn. This matching is performed manually. The depth adjustments 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 were shifted to the sea floor. The sea floor depth was determined by the step in gamma ray values at 2015 m WRF observed on the MSS/HRLA/APS/HLDS/HNGS main pass. This value differs for 0.5 m from 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 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 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 HRLA resistivity was also acquired every 5.08 cm; in the database it was resampled at 15.24 cm, for ease of comparison with the other logs.
Acoustic data. The dipole shear sonic imager (DSI) was operated in P&S monopole and upper and lower dipole. The sonic velocities were computed from the delay times (slownesses). Processing of the sonic waveforms is recommeneded to improve the quality of the 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). Repeatability between logging runs is generally good.
The quality of the velocities from the monopole transmitter is acceptable in the basement interval while it is quite poor in the slow sediments above. The dipole shear velocities are of better quality in the sediment section.
Gamma ray logs recorded through bottom hole assembly (BHA) and drill pipe should be used only qualitatively because of their attenuation of the incoming signal. The gamma ray from both the FMS down and main pass showed a sudden drop at about 14 m above the sea floor in the original data; this was an artifact that resulted from the formation being radioactively actived during the main pass of the MSS/HRLA/APS/HLDS/HNGS tool string, when the APS radiactive source was turned off only after entering the drill pipe, at about 14 m above the sea floor. Since this sudden drop in the gamma ray readings was above the sea floor it is not included in the processed dataset, only in the original data.
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 generally large, with readings mostly near 17 in. above the basalts, while it seems to be mostly in gauge in the basement interval.
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.