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Logging While Drilling

State-of-the-art LWD tools will be deployed to 350 mbsf at proposed Sites HR2alt and HR3a and to total depth (TD) at other sites to obtain high-quality porosity and density (ADN) logs, various resistivity measurements (including oriented resistivity images similar to Formation MicroScanner [FMS] images but with 360° coverage of the borehole wall), and gamma radiation (resistivity at the bit [RAB]). The RAB images should be particularly interesting and will be potentially useful for pinpointing gas hydrate–bearing zones. In addition to directly addressing the leg objectives, the information retrieved through logging while drilling will be critical for planning subsequent leg activities, such as locating intervals where the pressure core sampler (PCS) and HYACE tools might be used as well as positioning of stations for VSP experiments.

Wireline Logging

Wireline logging and VSP experiments will be performed at three proposed sites, HR1a, HR2alt, and HR3a. Proposed Site HR1a will be cored and logged to 350 mbsf, and proposed Sites HR2alt and HR3a will be cored and logged to 650 and 700 mbsf, respectively. The standard wireline triple combination (triple combo) and FMS-sonic tool strings will be deployed, as well as the three-component well seismic tool (WST-3) to record the vertical and offset VSP experiments. Acoustic-velocity logs (along with the VSPs) are critical in determining the velocity structure associated with the BSR. Depth-to-seismic ties will also be accomplished by means of synthetic seismograms computed from density and sonic logs. This correlation can be made using standard logs to measure the density, porosity, and compressional velocity of the sediments. Both P-wave and S-wave velocity measurements will be made using the standard dipole sonic imager (DSI) tool. Sediment permeability may be estimated using temperature gradients and heat flow changes by running temperature, porosity, and resistivity (dual induction tool [DIT]) logs. The dual laterolog (DLL) would only need to be used in the unlikely event that the bulk resistivity is extremely high. Previously measured resistivities at nearby Site 889 and Site 890 were <1.0 and <2.5 m, respectively, and therefore the standard dual induction resistivity (DITE) tool should provide adequate results. High-resolution FMS images will compliment the RAB images and may indicate thin beds and fractures in hydrated sediments (e.g., sediment fabric).

The VSPs will be coupled with walkaway shooting. Results from an offset VSP would also show velocity changes below hydrates, particularly if free gas is present. Walkaway shooting will provide a unique understanding of the lateral variations in the acoustic structure (both VP and VS) that are in turn related to the extent, fabric, and distribution of gas hydrates.

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