Data

The logging data was recorded by Schlumberger in DLIS format. Data were processed by the European Petrophysics Consortium.

Logging Runs

Tool string	Pass	Top depth (mbsf)	Bottom depth (mbsf)	Pipe depth (mbsf)	Notes
FMS/BHC/GPIT/NGT/SGT	1	45.2	217	66.5
FMS/BHC/GPIT/NGT/SGT	2	0	220	66.5	Reference

A complete list of tool and log acronyms is available at http://brg.ldeo.columbia.edu/data/iodp-eso/exp302/exp_documents/iodp-eso-302-acronyms.html.

After completion of the coring at Hole M0004B, the pipe was pulled to ~66 mbsf and the rig floor prepared for logging.

No major problems were encountered while logging Hole M0004B. All measurements were performed under open borehole conditions (no casing) from the bottom of the borehole at 220 m upwards to the seafloor, providing data through units 1/3 to 1/6. The FMS-Sonic toolstring (FMS/BHC/GPIT/NGT/SGT) was deployed with two passes successfully completed, with the entire logging operation achieved in just over 9 hours. The choice of tool strings was such that it could be run as a straight-through tool string without the need for articulated and eccentralized subs. The logs recorded were of good quality and the use of a guar gum mud combined with a primarily fine-grained formation resulted in a good-quality borehole wall. The short time between cessation of drilling and commencement of logging also favoured good borehole conditions. No heave compensation was used during the operation due to the damping effect of sea ice on ocean surface movement.

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 logs were first bulk shifted to sea floor (-1291 m). The sea floor depth was determined by identifying the step in gamma ray values at the sediment-water interface from Pass 2. The logging sea-floor depth is within 1.3 m of the 'bottom felt' depth given by the drillers. The shifted logs were depth-matched to the caliper log from Pass 2 of the FMS/BHC/GPIT/NGT/SGT tool string. It is standard procedure to use the gamma ray log for depth matching. In this instance, however, the caliper logs provided greater detail than the gamma ray logs and so a more accurate depth match was obtained by using the caliper logs.

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, but in this case teh caliper log), 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 pass.

The sea floor depth was determined by the step in gamma ray values at 1291 mbrf. This differs by 1.3 m from the sea floor depth given by the drillers (see above).

Environmental corrections: The NGT and SGT data were corrected for hole size during the recording.

High-resolution data: Gamma Ray data from the SGT tool were recorded at sampling rates of 5.08 and 15.24 cm.

Acoustic data: The borehole compensated sonic tool (BHC) was run in both pass 1 and pass 2 and records compressional wave velocities only. The BHC applies the 'depth-derived' borehole compensation principle using two transmitter (1x) receiver (2x) groups, one group being inverted. Hole size compensation is achieved by averaging the two compressional wave delay time (ΔT) readings measured across the same interval. The velocity logs are generally of good quality and show no indication for cycle skipping so no corrections were applied.

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 (APS, HLDS). Hole diameter was recorded by the FMS tool (C1 and C2). The hole was under guage (< 9.5 in) for the most part and narrows significantly between 75 and 90 mbsf, at 155 mbsf and again between 180-184 mbsf. The caliper logs indicate that nowhere was the borehole washed out to the degree where it would adversely affect the tool response. This is supported by a favourable comparison of parameter magnitudes between passes as well as a good depth match over much of the logged interval (< +- 1 m). There is an apparant bridge in the borehole at around 150-160 m, clearly observed in the caliper logs. This feature has affected the movement of the tools and the logs in this interval cannot thus be depth matched precisely. Thus, the depths of logs in this around 10 m interval should be treated with caution, with a discrepancy of 3-4 m between pass 1 and pass 2. Above and below this interval the depth matches are good.

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 section of the Site Chapter in IODP Proceedings of Expedition 302. For further questions about the data, please contact:

Jennifer Inwood

Leicester University

Phone: 011-44-116-252-3327

Fax: 011-44--116-252-3918

E-mail: iodp@le.ac.uk

For any web site-related problem please contact:

E-mail: logdb@ldeo.columbia.edu