Standard Wireline Data Processing
Science operator: Texas A&M University
Hole: U1572A
Expedition: 396
Location: Skoll High (North Eastern Atalantic Ocean)
Latitude: 67° 19.8477' N
Longitude: 3° 37.162' E
Logging date: September 2021
Sea floor depth (driller's): 1221.8 m DRF
Sea floor depth (logger's): 1216 m WRF
Total penetration: 1552.3 m DRF (330.5 m DSF)
Total core recovered: 139.53 m (42.2 % of cored section)
Oldest sediment recovered: Late early Eocene
Lithology: Clay with sand and silt; diatom ooze; volcaniclastic sediments; basaltic andesite and aphyric basalts.
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 September 2021.
| Tool string |
Pass
|
Top depth (m WMSF) | Bottom depth (m WMSF) | Pipe depth (m WMSF) | Notes |
|
1. MSS/HRLA/HLDS/HNGS
|
Downlog
|
0
|
250
|
87
|
Depth reference
|
|
Repeat
|
124
|
250
|
Recorded open hole
|
||
|
Main
|
0
|
250
|
87
|
After coring was completed, in preparation for logging, a mud sweep was conducted with high viscosity mud, followed by hole displacement with 110 barrels of 10.5 ppg mud. The MSS/HRLA/HLDS/HNGS was lowered first; as it reached the sea floor it was noted that the gamma ray log showed the mudline almost 6 m above the driller's sea floor. An explanation is that the hole was displaced both at the top and at the bottom, possibly leaveing a largenr than npormal cutting pile that made the GR-based mudline depth pick questionable. The tool string hung up at about 1474 m WRF (258 m WSF) and despite few attempts to set it free and move past it, it was not possible to reach total depth. Therefore, a repeat and a main pass were acquired from that depth without further incidents, except for some difficulty in closing completely experienced by the caliper tool after the repeat pass. The problem resolved at the end of the main pass but at rig down it was noted that there was some sediment debri s firmly stuck under the caliper arm. Furthermore, the caliper readings were well under gauge at 1325 m WRF (6 inches instead of the nominal bit size of 9 7/8 inches), about 22 m below the drill pipe. This was a warning that the hole conditions were deteriorating and it might be too risky to run further tools. No additional logging was performed.
The ship's heave was quite high; the Wireline Heave Comensator 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 downlog of the MSS/HRLA/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 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 1216 m WRF. This differs by 5.8 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 HLDS data were corrected for hole size during the recording.
High-resolution data. Bulk density (HLDS) were recorded with asampling rate of 2.54 cm, 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 was acquired every 5.08 cm; in the database itis resampled at 15.24 cm, for ease of comparison with the other logs.
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).
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 (HLDS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL). The hole is in good condition, wioth calipe readings in the 12-15 inch range. Only above 120 m WWSF the caliper reading fall below the nominal bit range (9 7/8 in), indicating that the sediment section of the hole may be rapidly deterirating and at risk of collapse.
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 396.
For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.