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doi:10.2204/iodp.sp.311.2005
LOGGING/DOWNHOLE MEASUREMENTS PLAN
LWD/MWD tools will be deployed at all proposed sites, including oriented resistivity-at-the-bit (RAB) images with 360° coverage of the borehole wall. The MWD tools will allow us to monitor drilling parameters and to transfer LWD data in real time. Because LWD sensors are located immediately above the drill bit, the measurements are made before gas hydrate dissociates under drilling disturbances, and these data are critical to identify gas hydrate distribution in the formation. In addition to directly addressing the expedition objectives, RAB images will be potentially useful for identifying high-resistivity features, helping to locate intervals where the pressure coring tools might be used to recover gas hydrate samples. To optimize scheduling and drilling success, all LWD/MWD operations will be completed at the beginning of the cruise, prior to coring operations.
Because of the unstable nature of gas hydrate, wireline logging is critical to measure in situ properties of hydrate-bearing sediments. The standard triple combo (density, porosity, and resistivity) and FMS-sonic tool strings will be deployed at all sites. In addition, the three-component Well Seismic Tool (WST-3) will be used to record a VSP at proposed Site CAS-01B. Acoustic logs (along with the VSP) are critical in determining the acoustic velocity structure associated with the BSR. Both P-wave and S-wave velocity measurements will be made using the Dipole Sonic Imager (DSI) tool. Depth-to-seismic ties will also be accomplished by means of synthetic seismograms computed from the wireline density and sonic logs. High-resolution FMS electrical images will compliment the RAB images and may indicate features such as thin beds, veins, and fractures in hydrate-bearing sediments.
A zero-offset VSP is planned for proposed Site CAS-01B. The VSP will be conducted by repeatedly shooting a generator injection (GI) gun (105 in3 generator/45 in3 injector) hung from the drillship 2 m below the surface for each geophone depth in the borehole. The geophone will be placed at about 10 regular intervals throughout the entire 350 m deep borehole. At each recording depth, the GI gun will be fired several times (up to 10 times) to stack the seismic signals and enhance the signal-to-noise ratio.