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doi:10.2204/iodp.sp.319.2009

Logging/downhole measurements strategy

Proposed Site NT2-11B

During drilling of proposed Site NT2-11B, MWD tools will be run between 700–1600 mbsf to monitor drilling conditions and define major lithologic changes in real time. The MWD tool suite will include annular pressure while drilling (APWD), weight on bit (WOB), torque, hole inclination, and gamma ray. The Power-V tool will also be run during drilling to maintain hole inclination at <3°. In addition, if budget permits, the geoVISION LWD tool will be added to this suite to obtain LWD azimuthal resistivity data and borehole resistivity images, in order to further define stratigraphic boundaries and to characterize bedding, minor faults, and any breakouts or tensile fractures induced by drilling (Fig. F10).

Planned wireline logging operations at proposed Site NT2-11B include three runs between 700 and 1600 mbsf prior to installing 13⅜ inch casing (Fig. F8). The first logging run will include resistivity measurement using the Formation MicroImaging (FMI) tool, sonic velocity measurements with the Sonic Scanner (a first for scientific drilling), a six-arm caliper, and mud temperature measurement with the Environmental Measurement Sonde (EMS) tools (Fig. F10). The goals of this logging run are to obtain information about (1) lithology, facies boundaries defined by log characteristics, and bedding dips; (2) structural geology and any borehole breakouts or tensile fractures that may provide information on in situ stress conditions; (3) rock physical properties and data for core-log-seismic integration from sonic velocity logs; and (4) hole conditions from caliper measurement. The second run will include the new-generation triple combination Platform Express and EMS tools to measure density, porosity, gamma ray, resistivity, and caliper (Fig. F10).

If hole conditions and time permit, a third logging run will employ the Modular Formation Dynamics Tester (MDT) dual packer tool (Fig. F10) to measure in situ minimum stress magnitude and pore pressure. The measurement depths and test interval thickness will be defined on board based on results from MWD (and possibly LWD) measurements and the previous wireline logging runs.

Vertical seismic profiling

If time permits, a VSP experiment will be conducted at proposed Site NT2-11B using a Schlumberger Versatile Seismic Imager (VSI) tool, after installation and cementing of the 13⅜ inch casing (Fig. F7, F11). The VSI tool consists of an array of separate sensor shuttles, a VSI cartridge, and a telemetry module. Each sensor shuttle consists of triaxial (three component) geophone accelerometers, a hydrophone, and a locking arm to connect the sensors to provide mechanical coupling to the casing interior. The planned VSP experiment includes two components: a zero-offset experiment, and a two-ship offset ("walkaway") experiment.

The zero-offset VSP experiment will provide interval velocities between the seafloor and TD, which will be used in combination with wireline logging data to gain information about rock physical properties in the upper 1600 m and to tie the borehole data to the 3-D seismic data. The two-ship experiment will be coordinated with a separate shooting vessel to collect data at relatively large offsets in the range up to 40 km, in lines both parallel and perpendicular to the dip of subducting plate.

The offset VSP experiment will allow sampling of reflected seismic waves from beneath the borehole, including those from the plate boundary fault(s), the sediment/basalt interface, and possibly the Mohorovicic discontinuity of the subducting oceanic plate. A key advantage of the VSP experiment compared with existing high-quality 3-D seismic reflection data is that the geometry of the experiment results in significantly less attenuation and reduces the effects from anisotropy from the overlying sediment and ~2000 m of water. Proposed Site NT2-11B is located ~10 km directly above the updip edge of the inferred locked portion of the plate boundary, and the wide range of planned shooting offsets will provide high-resolution data to define the detailed seismic characteristics and physical properties of the rock volume below the borehole containing the plate boundary fault zone.

The VSP data will be treated as a shipboard data set, which will be fully accessible and shared among the Expedition 319 shipboard party and the third party VSP principal investigator (PI) group that is coordinating the experiment. Similarly, we expect that the third party VSP PI group will be granted access to relevant shipboard data, upon request. We anticipate close collaboration between the shipboard party and the VSP group, with the overall goal of being as inclusive as possible and maximizing the scientific return from data sets that are generated by the experiment.

Proposed Site NT2-01J

We plan to conduct LWD and MWD measurements from 36 mbsf (base of 20 inch casing) to TD at 525 mbsf (Fig. F8). At proposed Site NT2-01J, the primary scientific goals of downhole logging will be to (1) define the target interval for casing screens for pore pressure and temperature monitoring and (2) collect data on bedding and fault orientations, fault and fracture density, borehole breakouts if they occur, formation resistivity, gamma ray response, and drilling parameters, complementing data from the previously drilled Site C0004 (located 3.5 km southwest along strike from proposed Site NT2-01J) that also penetrated the shallow megasplay. Because proposed Site NT2-01J is in close proximity to Site C0004 (Kinoshita, et al., 2008; Kimura et al., 2008), we do not plan to core or conduct detailed wireline logging operations. However, we anticipate that the exact depths of the target intervals and the fault zone architecture will differ between the two sites. As at proposed Site NT2-11B, the LWD tool suite will include azimuthal resistivity and gamma ray (geoVISION) and the MWD tool string will include APWD, WOB, torque, and hole inclination (Fig. F10).

Sensor dummy run test

After the casing operations at proposed Site NT2-1J, a test will be conducted to simulate the procedure of installing borehole sensors in the hole as part of the preparation for fabricating long-term borehole observatories for installation during future NanTroSEIZE expeditions. This test has two main objectives: (1) to evaluate environmental conditions, such as shock acceleration and vibration, which borehole sensors will encounter during installation to the bottom of the hole; and (2) to confirm sensor installation operational procedures, such as onboard assembly of the sensor tree, ship maneuvers to reenter the sensor tree, and lowering the sensor into the hole.

The sensor tree for the test is illustrated in Figure F12. The tree will be attached to the end of the drill pipe and reenter the riserless hole. The tree consists, from the bottom up, of 3½ inch tubing; a borehole strainmeter; a multi-instrument carrier that houses a self-recording accelerometer, tiltmeter, and other sensors; and 3½ inch tubing to be attached to the drill pipe. The tree is similar to the structure of the bottom section of the planned observatories, with the exception that no cables or hydraulic lines are attached, and we will not use the same total length of tubing and circulation obviation retrofit kit (CORK) head that will hang down from the casing hanger. Vibration and shock acceleration will be recorded by the self-recording accelerometer and tiltmeter. The strainmeter and seismometer package will be inspected before and after the dummy run to evaluate their performance.

Instrumented retrievable casing packer (temporary monitoring system)

We plan to suspend the hole at proposed Site NT2-01J by installing a mechanically set retrievable packer (Baker A3 Lok-Set) inside the 10⅝ inch casing string, modified to attach a small instrument package (Fig. F13). The packer will be set above the screened casing joints, at a planned depth of ~390 mbsf. The instrument package will be threaded to the bottom of the bridge plug and will include a self-contained temperature sensor and data logger, as well as a pressure gauge and data logger package. These instruments will be in hydrologic communication with the fault zone at the screened interval and will monitor formation pore pressure and temperature from the time the bridge plug is set until they are retrieved at the beginning of permanent riserless observatory installation operations.

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