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doi:10.2204/iodp.sp.314.2007

Introduction

NanTroSEIZE is a multiyear, multistage, and multiplatform effort planned for IODP. The first stage of NanTroSEIZE drilling operations will comprise three individual expeditions by the new riser-capable drilling vessel Chikyu, slated to begin September 2007. As each of the three Stage 1 expeditions have very different drilling targets and objectives, this Scientific Prospectus covers the objectives and operational plan for IODP Expedition 314 (LWD Transect).

The primary objectives of Expedition 314 are to obtain a comprehensive suite of geophysical logs and other downhole measurements at six sites, using state-of-the-art LWD technology. Three of these six sites are slated for continuous core sampling during the two subsequent IODP NanTroSEIZE Stage 1 expeditions scheduled to immediately follow Expedition 314. Coring at the other three sites is planned but currently deferred to a later time. Operations at the three sites (Figs. F1, F2) include sampling of the incoming plate trench sediments, the frontal thrust and toe of the accretionary prism, forearc basin deposits, a major out-of-sequence thrust (OOST) system, or megasplay fault, and highly deformed rocks of the interior of the accretionary prism. These sites are designed to accomplish the principal goals of the NanTroSEIZE Science Plan, including documenting the material inputs to the subduction conveyor (fluid, solids, and heat), the properties of major thrust faults and their wall rocks at depths shallower than ~1.4 km, the geology of the accretionary prism and overlying slope basin sediments, and installing a long-term monitoring system at proposed Site NT3-01 to record pore fluid pressure, temperature, seismic events, geodetic strain, and tilt over time (Tobin and Kinoshita, 2006; Kinoshita et al., 2006).

To begin Stage 1, we will acquire borehole LWD data by drilling dedicated holes with a drill string with the logging instruments incorporated into drill collars just above the drilling bit. A number of previous Ocean Drilling Program (ODP) and IODP expeditions have employed LWD technology.

Plans are subject to change, but at this time we plan to deploy Schlumberger LWD/measurement-while-drilling (MWD) tools including the following:

• geoVISION: records formation resistivity, including 360° borehole resistivity imaging and spectral natural gamma radiation
• adnVISION: mainly records porosity and neutron density
• sonicVISION: provides sonic P-wave velocity
• seismicVISION: records check shot vertical seismic profile (VSP) arrivals during LWD drilling and so provides interval velocity.

Additional measurements primarily intended for monitoring drilling conditions (MWD), such as torque, downhole weight on bit, and so on, as well as borehole pore fluid pressure (annular pressure while drilling [APWD]) will be recorded and also transmitted using MWD uphole telemetry.

Expedition 314 will be entirely dedicated to the LWD effort, and no coring or downhole measurement operations are planned. Some casing may be installed to facilitate deep penetration at several sites. Logging data will be analyzed initially by the Shipboard Scientific Party and will be made available to the scientific parties of the NanTroSEIZE Stage 1 expeditions.

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