Standard Wireline Data Processing

Science operator: Texas A&M University

Hole: U1557D

Expedition: 390

Location: Mid-Atlantic Ridge (S Atlantic Ocean)

Latitude: 30° 56.4651' S

Longitude: 26° 37.7892' W

Logging date: May 18, 2022

Sea floor depth (driller's): 5021.9 m DRF

Sea floor depth (logger's): 5020.0 m WRF

Total penetration: 5706.6 m DRF (684.7 m DSF)

Total core recovered: 784.2 m (76.5% of cored section)

Oldest sediment recovered: ~61 Ma

Lithology: Alternating silty clays/nannofossil ooze, nannofossil/calcareous chalk, basement basalt and sedimentary breccia

Data

The logging data were recorded by Schlumberger in DLIS format. Data were processed at the Borehole Research Group of Lamont-Doherty Earth Observatory in May, 2022.

Logging Runs

Tool string	Pass	Top depth (m WMSF)	Bottom depth (m WMSF)	Pipe depth (m WMSF)	Notes
MSS/HLDS/HRLA/HNGS/EDTC-B	Downlog	0	658	29	Caliper closed. Invalid HLDS
	Repeat	566	691	Recorded open hole
	Main	0	690	29

After the completion of coring operations, the MSS/HLDS/HRLA/HNGS/EDTC-B tool string was deployed for logging, resuting in three good passes (downlog, repeat amd main) without difficulties.

The Wireline Heave Compensator (WHC) was used in the open hole interval to counter the ship heave while logging. Sea condition was high with ship heave in the 2 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 the 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.

Processing

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/HLDS/HRLA/HNGS/EDTC-B tool string (reference run). 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 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 then depth-shifted to the sea floor. The sea floor depth was determined by the step in gamma ray values at 5020.0 m WRF observed on the reference run. This differs by 1.9 m from the sea floor depth given by the drillers (see above).

Environmental corrections. The HRLA and HLDS logs were corrected for hole size during the recording.

High-resolution data. Bulk density (HLDS) data were recorded at a sampling 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 are 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 HNGS tool were recorded at a sampling rate of 15.24 cm. The HRLA data were also acquired every 5.08 cm; in the database they were resampled at 15.24 cm, for ease of comparison with the other logs.

Quality Control

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). For hole U1557D, repeatability between logging passes is generally good.

Gamma ray logs recorded through bottom hole assembly (BHA), drill pipe, and cased hole section (29-571 m WMSF) 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. Similarly, the cased borehole wall attenuates the signal more than the open borehole wall.

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. For hole U1557D, the caliper was near bit size of 9 7/8" for most of the logged open hole interval below the casing depth (571 m WMSF), and the log data acquired are of good quality. The 5 m long rathole below casing (570-575 m WMSF) was drilled at 14.5" and its large size may affect the readings of gamma ray, resistivity, and magnetic susceptibility logs.

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 390. 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:

Tanzhuo Liu

Phone: 845-365-8630

Fax: 845-365-3182

E-mail: Tanzhuo Liu

Gilles Guerin

Phone: 845-365-8671

Fax: 845-365-3182

E-mail: Gilles Guerin