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 July 2024.

Logging Runs

Tool string	Pass	Top depth (m WMSF)	Bottom depth (m WMSF)	Pipe depth (m WMSF)	Notes
1. MSS/HRLA/HLDS/APS/HNGS/EDTC-B	Downlog	0	580.0	69	Caliper closed. Invalid HLDS, APS, and MSS.
	Repeat	483.3	584.3	Recorded open hole	Invalid MSS.
	Main	0	579.0	68	Invalid APS and MSS.

After the completion of coring operations, the MSS/HRLA/HLDS/APS/HNGS/EDTC-B tool string was deployed for logging. A downlog was run with the caliper closed and the HLDS radioactive source turned off. Both repeat and main logs were then run with the caliper opened and the radioactive source turned on. Due to some technical issues with the tool string during the logging operations, no porosity data from APS during the main pass and no valid susceptibility data from MSS were acquired for this hole.

The average heave was estimated at 0.3 m peak-to-peak and the heave compensator was utilized during the logging operation.

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 first depth-matched to the gamma ray log from the downlog pass of the MSS/HRLA/HLDS/APS/HNGS/EDTC-B 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 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 shifted to the sea floor. The sea floor depth was determined by the step in gamma ray values at 1597.0 m WRF observed on the downlog. This value differs by 0.5 m from the sea floor depth given by the drillers (see above).

Environmental corrections. The HRLA and HLDS logs were corrected for hole size and the APS logs were corrected for standoff 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 EDTC-B tool were recorded at sampling rates of 5.08 and 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).

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. For U1620D, the caliper was 12" or less for most of the logged hole interval below 350 m WMSF. The hole size increased to 13-17" above this depth level with numerous tight spots and washouts. As a result, the log data from the upper portion of the logged hole section (70-350 m WMSF) should be used with caution.

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 403.

For any question about the data or about the LogDB database, please contact LogDB support: logdb@ldeo.columbia.edu.