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 August 2020.

NOTE: Some of the data posted online were erroneusly labelled at the time of acquisition. These include the original DLIS and LDF format data. The label reads Site U1555F instead of Hole U1555G.

Logging Runs

Tool string	Pass	Top Depth (mWMSF)	Bottom Depth (mWMSF)	Drill Pipe Depth (mWMSF)	Notes
1. MSS/HRLA/APS/HLDS/HNGS	Downlog	0	258	203	Caliper closed. Invalid HLDS.
	Repeat	254.5	308	recorded open hole
	Main	0	297	203
2. FMS/DSI/GPIT/HNGS	Downlog	0	297	203	Calipers closed. Invalid FMS.
	Repeat	251	297	recorded open hole	Reference run.
	Main	0	297	203
3. VSI/HNGS		191	226	recorded open hole	10 shots at two depths

Overall, logging operations at Hole U1555G were uneventful. The first run with the MSS/HRLA/HLDS/APS/HNGS consisted of a downlog, followed by a short repeat section and a main uplog. The following FMS/DSI/GPIT/HNGS also consisted of a downlog, a repeat, and a main pass. Some tight spots were encountered during the downlog and the second pass but in both cases the tool string got free without much effort. The last run, consisting of the VSI/HNGS tool string, collected five good shots at two depths, 1767.1 and 1827.6 m WRF.

The WHC was used whenever the tools were in open hole. The average ship's heave was 0.5-1 m.

The depths in the table are for the processed logs (after depth matching between passes and depth shifting 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 have first been depth-matched to the gamma ray log from the main pass of the FMS/DSI/GPIT/HNGS tool string. This pass has been chosen as the reference run because it shows the best gamma ray curve, despite a general very low signature (less than 10 GAPI), which often makes depth matching difficult. Because of the low gamma ray signature, the depth match values for the repeat section of the MSS/HRLA/HLDS/APS/HNGS have been obtained by comparing the resistivity to that of the depth-matched main section.

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.

Finally, the depth-matched logs have then been shifted to the sea floor. The sea floor depth has been determined by the step in gamma ray values at 1532 WRF. This differs by 2.6 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. No APS or HLDS were acquired during the downlog.

High-resolution data. Bulk density (HLDS) and neutron porosity (APS) data were recorded with 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 is acquired every 5.08 cm; in the database it is resampled at 15.24 cm, for ease of comparison with the other logs.

Acoustic data. The dipole shear sonic imager (DSI) was operated in the following modes: P&S monopole (standard frequency), upper dipole (standard frequency) and lower dipole (low frequency). The velocities from all passes were computed from the corresponding slowness values. They are generally of acceptable quality.

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). Comparison between runs performed with the MSS/HRLA/APS/HLDS/HNGS tool string show a good repeatability, with the exception of the gamma ray, which shows much higher counts in the lower part of the hole during the main run, when the formation was still affected by the radioactive activation of the APS tool occurring during the preceding run. During the first run, a long pull was encountered at about 305 m WSF, which lasted until 301 m WSF and affected the different tools in the string at different depths. The effect of this pull can be seen at 301-304 m WFS (MSS tool), 296-291.5 m WSF (HRLA), 285-280 m WSF (HLDS), and 278-274 m WSF (HNGS).

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 (APS, HLDS). Hole diameter was recorded by the hydraulic caliper on the HLDS tool (LCAL) and by the FMS tool (C1 and C2). The calipers indicate a very nice hole, ranging from bit size values to about 11-12 inches. A tight spot of a bout 9 inches is shown at about 248 m WMSF.

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 384.

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