Logging Tools

The downhole logs were recorded using the LWD/MWD (Logging-While-Drilling/Measurement-While-Drilling) system, which employs instruments that are part of the drill string itself. The advantages of this technique include being able to log in formations that would not provide a stable hole for wireline logging (e.g. the upper section of sedimentary formations) and logging a hole immediately during drilling, so that it is in good condition and largely free of wash-outs. The data is recorded in the tool's memory and downloaded when the drill string returns to the rig floor. A small subset of the data is transmitted up to the ship during drilling. The following data was processed by the logging specialists aboard the Chikyu. In order to meet the format constraints of the LogDB online database, some additional data reduction has been performed by personnel at the Borehole Group of the Lamont-Doherty Earth Observatory.

The following LWD/MWD services were employed in Hole C0002R:

LWD

arcVision ((phase and attenuation resistivity, gamma ray, annulus temperature and pressure, equivalent circulation density)

MWD

TeleScope (torque, drilling weight on bit)

Multiple runs were carried out at Hole C0002R:

LWD/MWD tools	Run	Top Depth (m LSF)	Bottom Depth (m LSF)	Casing Depth (m LSF)	Notes
Telescope	1	2795	2964	2848.5	Real Time only
Telescope	2	2994	3084	2848.5	Real Time only
arcVision/TeleScope	3	2853	2908	2848.5
arcVision/TeleScope	4	2795	2866	2848.5

No problem was encountered in hole C0002R, with the exception of a gas occurrence between 2893 and 2899 mLSF (as indicated by a drop in the equivalent circulation density,), which dissipated after a period of monitoring. In Hole C0002R, the rate of penetration ranged from 3 to about 400 m/hr in the upper part of the hole (Run 4, 2800-2874 mLSF) and from 2 to 4 m/hr in the lower part of the hole (Run 3), with locally higher values of up to 20 m/hr.

Processing

Depth shift: The original have been depth shifted to the sea floor (- 1967.5 m) by the logging scientists aboard the ship using Schlumberger's Techlog software. This value matches the water depth used in the C0002P hole drilled during expedition 348 and in the C0002Q hole drilled during expedition 358. The sea floor depth was determined by the step in gamma ray and resistivity values at the sediment-water interface. The sea floor depth was originally determined by the step in gamma ray and resistivity values at the sediment-water interface. The computed eCaliper was depth shifted at Lamont.

Gamma Ray data processing: The Gamma Ray logs have been environmentally corrected for bit size (8.5 in) and mud properties by the Schlumberger engineer aboard the ship.

Sampling rates: The data was acquired with the following sampling rates:

Attenuation and Phase Resistivity (arcVision): 15.24 cm

Computed e-caliper (arcVision): 15.24 cm

Drilling parameters (arcVision): 15.24 cm

Gamma ray (arcVision): 15.24 cm

Miscellaneous (arcVision, TeleScope): 15.24 cm

Temperature / Pressure (arcVision): 15.24 cm

IMPORTANT: Because Hole R is a sidetracked hole, there is a difference between the measured depth (MD) and the true vertical depth (TVD). Because the difference is small (~1-2 m in total in expedition 358 Holes C0002Q and C0002S), the scientists decided to use the the measured depth instead of the true vertical depth for all the measurements reported in the IODP Proceedings. Accordingly, we have used the measured depth in the files included in the LogDB online database.

Quality Control

During the processing, quality control of the data is performed by inter-comparison of all logging data to ensure that reasonable values are returned for expected lithology types and that features on the logs reflect true formation characteristics and are not artifacts.

A good data quality indicator is usually given by the caliper measurement of the borehole diameter. In Hole C0002R, however, none of the tools deployed provided a direct measurement of the borehole diameter. A computation of the borehole size from the arcVISION phase and attenuation resistivities was carried out onboard for both runs 3 and 4 using Techlog. An additional, more accurate computation was performed by the Schlumberger engineer for run 4. The data show a very enlarged hole above 2865 mLSF, a reflection of the separation between the resistivities at different depths of investigation. Comparison with the data of hole C0002P in the same depth interval suggests that both the variable resistivity values and the lower gamma ray values are caused by the enlarged borehole rather than by lithologic changes. In summary, the only reliable data acquired in hole C0002R come from the 2866-2894 and below 2904 mLSF (both in run 3).

Additional information about the drilling and logging operations can be found in the Site Expedition Report and in the Methods section, Proceedings of the International Ocean Discovery Program, Expedition 358.

For questions about the logging operations and the processing performed onboard, please contact:

Yukari Kido

E-mail: ykido@jamstec.go.jp

For database-related questions you may contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia

Tanzhuo Liu

Phone: 845-365-8630

Fax: 845-365-3182

E-mail: Tanzhuo Liu