Data

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.

Logging Runs

Tool string	Pass	Top depth (m WMSF)	Bottom depth (m WMSF)	Pipe depth (m WMSF)	Notes
1. MSS/HRLA/APS/HLDS/HNGS	Downlog	0	371	85	Reference Run. Caliper closed. Invalid APS and HLDS
	Uplog	220	372	-	Tool got stuck before reaching bottom of the drill pipe.

The original logging plan for Hole U1555I included a fuill suite of tools, including the resistivity (HRLA), density (HLDS), porosity (APS), magnetic susceptibility (MSS0, spectral gamma ray (HNGS), borehole imaging (FMS and UBI), and borehole seismics (VSI).

However, after a smooth run downhole with the MSS/HRLA/APS/HLDS/HNGS tool string, which reached total depth, on the way up there was a large pull at 1775 mbsf. The caliper was immediately closed and the radiocative source of the porosity sonde was turned off. After several hours and various unfruitful attempts to log past 1775 mbsf, fishing operations started. The tool was finally recovered, but the rest of the logging program for Hole U1555I was cancelled.

The depths in the table are for the processed logs (after depth matching between passes and depth shift to the sea floor). 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.

Processing

Depth match and depth shift to sea floor. The original logs were first depth-matched to the resistivity log from the downlog of the MSS/HRLA/APS/HLDS/HNGS tool string. The downlog was chosen as the reference run because it was the only run to cross the sea floor. The gamma ray signature was very weak, less than 10 API units, therefore the resistivity log was chosen for the depth match between runs. 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 are 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. Since there was no clear step in the logs at the sea floor, the sea floor depth given by the drillers (see above) was used to depth shift the logs to the sea floor..

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 it is 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).

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. The gamma ray logs show very low values and poor repeability between runs.

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) during the uplog. The hole ranges in diameter between 10 and 18 in. with incerasing variability as the tool approached the depth where the tool string got stuck.

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.