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 and the Schlumberger engineer aboard the Chikyu, with additional processing performed onshore by Schlumberger Information Solution. 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 C0024A:

LWD

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

MicroScope (gamma ray, resistivity, resistivity images)

SonicScope (velocity)

seismicVision (seismic data)

MWD

TeleScope (torque, drilling weight on bit)

Multiple runs were carried out at Hole C0024A:

LWD/MWD tools	Run	Top Depth (m LSF)	Bottom Depth (m LSF)	Notes
arcVision/MicroScope/SonicScope/TeleScope/seismicVision	Main	20	868	34 seismic stations with about 10 shots each.
arcVision/MicroScope/TeleScope	Repeat 1	258	309
arcVision/MicroScope/TeleScope	Repeat 2	0	14

A full suite of LWD logging data was acquired during a main pass at Hole C0024A, followed by two short repeats aimed at obtaining a second set of electrical images in an interval where the orientation of wellbore breakouts appeared to be changing (repeat pass 1) and to confirm the seafloor depth (repeat pass 2). At the beginning of drilling, no electric power was supplied by the TeleScope, due to low flow rate during jet-in. Therefore the first gamma ray and resistivity data from the arcVision started at 20 mLSF, while the MicroScope resistivity and resistivity-at-bit images start at 24 and 77 mLSF, respectively.

In Hole C0024A, the rate of penetration was stable around 20 m/hr below 70 mLSF with a constant roration rate of about 110 rpm. Drilling conditions gradually deteriorated towards the bottom of the hole, especially below 790 mLSF, as indicated by the decrease in rate of penetration and the increase in circulating density.

Processing

Depth shift: The original data have been depth shifted to the sea floor (- 3870 m) by the logging scientists aboard the ship using Schlumberger's Techlog software. The sonic waveforms were depth-shifted by personnel at the Borehole Group of the Lamont-Doherty Earth Observatory. The sea floor depth had been originally determined by the step in gamma ray values at the sediment-water interface observed in the main and repeat run 2.

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.

Acoustic data: The data was processed onhore by Schlumberger personnel at Schlumberger Information Solution, who provided both compressional and shear wave outputs. The data was acquired in three modes: monopole, monopole-low frequency, and quadrupole mode. The compressional data were generally of good quality; the strong signal displayed by the waveforms indicates that the SonicScope was not adversely affected by the variability in hole size. The quadrupole mode yielded usable shear velocity values over most of the logged intervalbelow 410 mLSF. No reliable shear velocities could be obtained above 339 mLSF, thus the online data should be used with caution over this interval.

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

Attenuation and Phase Resistivity (arcVision): 15.24 cm

Button Resistivity (MicroScope): 3.04 cm

Computed e-caliper (Microscope): 3.04 cm

Miscellaneous (arcVision, TeleScope): 15.24 cm

Gamma ray (arcVision): 15.24 cm

Slowness/Velocity (SonicScope): 5.08 cm

Sonic waveforms (SonicScope): 5.08 cm

Temperature / Pressure (arcVision): 15.24 cm

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. The best data are acquired in a circular borehole. A good data quality indicator is usually given by the caliper measurement of the borehole diameter. In Hole C0024A, however, none of the tools deployed provided a direct measurement of the borehole diameter. The borehole size (e-caliper, HD_MI6) was computed by Schlumberger from the MicroScope data using an inversion technique. The computed e-caliper ranges from close to 8.5 inches below 438 mLSF to about 11.5 inches in the upper half of the hole. A second e-caliper was calculated onboard from the arcVision resistivities (CALE_ARC) using Techlog. This e-caliper shows a significantly larger hole and much higher variations in hole size than the MicroScope-computed e-caliper.

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