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 C0002Q:

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 C0002Q:

LWD/MWD tools	Run	Top Depth (m LSF)	Bottom Depth (m LSF)	Casing Depth (m LSF)	Notes
Telescope	1	2881	3022	2887.3	Real Time only
arcVision/MicroScope/SonicScope/TeleScope/seismicVision	2	2893	2931	2887.3	No seismic data or caliper acquired. Hole diameter computed from MicroScope and arcVision resistivity.
arcVision/TeleScope	3	-	-	2887.3	No data acquired
arcVision/TeleScope	4	2923	3258	2887.3
arcVision/TeleScope	5	2912	2955	2887.3	No torque and downhhole weight-on-bit recorded.

During Run 2 the drill pipe was jarred multiple times, in order to get the BHA through the casing, but the repeated shocks ultimately damaged some of the tools. Shore-based inspection of the tools revealed that the seismicVision was damaged; regardless, the seismic Vision tool did not pass through the kick-off window, therefore no data was acquired. Also, three out of the eight buttons of the ultrahigh-resolution pad of the MicroScope tool were damaged, which resulted in some missing data. The button resistivity measurements, instead, were not affected, as the sensors were not damaged. One of the receivers of the SonicScope was also damaged, but it did not seem to have affected the data acquisition. No damage was found on the TeleScope. Further jarring damaged the spare TeleScope used during Run 4; this resulted in using another spare without downhole torque and downhole weight-on-bit measurements during Run 5.

In Hole C0002Q the average rate of penetration (ROP) was between 20 and 40 m/hr after the LWD/MWD string passed the casing window during Run 2 and between 2 and 4 m/hr during Run 4.

Processing

Depth shift: The original data have been depth shifted to the sea floor (- 1967.5 m) by the logging scientists aboard the ship using Schlumberger's Techlog software. The computed caliper (e-caliper), the acoustic data processed by Schlumberger, and the sonic waveforms were depth-shifted by personnel at the Borehole Group of the Lamont-Doherty Earth Observatory. The depth shift amount matches the water depth used in the C0002P hole logged during expedition 348. The sea floor depth had been originally determined by the step in gamma ray values at the sediment-water interface.

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. No GR was recorded by the arcVision tool during Run 2, as it was recorded by the MicroScope tool. Because no direct caliper measurement was available at this hole and the bit size was used instead, the correction cannot fully account for the the very large hole diameter above 2950 mLSF, where the computed e-caliper shows a hole in excess (> 11 in) of the bit size, with the most enlarged sections as large as 20 in.

Acoustic data: Most of the data acquired with the SonicScope were inside the casing. Open hole data were obtained from 2887.3 to 2908 mLSF. The data was acquired in three modes: monopole, monopole-low frequency, and quadrupole mode. The data was processed onhore by Schlumberger personnel, which provided both compressional and shear wave outputs.

Sampling rates: The following sampling rates have been used:

Attenuation and Phase Resistivity (arcVision): 15.24 cm

Button Resistivity (MicroScope): 5.08 cm

Computed e-caliper (arcVision): 3.04 cm

Computed e-caliper (Microscope): 3.04 cm

Gamma ray (arcVision): 15.24 cm

Gamma ray (MicroScope): 5.08 cm

Miscellaneous (arcVision, TeleScope): 15.24 cm

Slowness/Velocity (SonicScope): 5.08 cm

Sonic waveforms (SonicScope): 5.08 cm /15.24 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 C0002Q, however, none of the tools deployed provided a direct measurement of the borehole diameter. During Run 2 the borehole size (e-caliper=electromagnetic caliper) was computed onboard from the resistivity measured by the arcVision tool by the Schlumberger engineer. The e-caliper (HD_ARC) is obtained by inverting the arcVision raw amplitude and phase data with various depths of investigation, assuming that the curve separation is due only to the hole size and the type of drilling fluid. Since this estimate is not as accurate a measure of the biorehole diamter as the ultrasonic caliper, it is advised to use the e-caliper qualitatively only. Above 2950 m LSF, the computed e-caliper shows a borehole larger than 11 in (bit size is 8.5 in), with some of most degraded intervals larger than 20 in. The hole conditions are greatly improved below 2960 m LSF. An additional computation of the borehole diameter was obtained from the inversion of the MicroScope resistivity data (HD_MI6). Due to shallow depth of investigation of the MicroScope, however, the estimate is accurate only in the 2907-2916 and 2919-2923 m LSF intervals, where it matches the estimate computed from the arcVision tool.

The quality of the resistivity and the gamma ray logs is greatly affected by the large hole diameter above 2950 m LSF. This is very evident in the discrepancy between the logs recorded at Hole C0002Q and those from Hole C0002P over the same depth interval; only below 2950 m LSF the logs show a good match.

The SonicScope data were acquired in open hole from the bottom of the casing at 2887.3 to 2908 m LSF. Because of the very large hole in this interval, the quality of the data is generally poor. Reliable data, consistent with Hole C0002P data, was obtained only over very short intervals.

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