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Downhole logging will be an important complement to coring operations during Expeditions 309 and 312 and will greatly assist in the attainment of the scientific objectives by providing continuous, in situ geophysical measurements on the drilled basalts, dikes, and gabbros. The logging program will refine the volcanic stratigraphy, eruptive morphology, and variations in seawater-basalt alteration as a function of depth as well as allow direct correlation of wireline measurements with discrete laboratory measurements on recovered core (e.g., Fig. F7). In addition, downhole measurements will be used in conjunction with core images from the DMT Core Scanner to reorient veins, fractures, and other features back into the geographic reference frame. Core recovery during drilling of igneous basement is often incomplete and biased, with weaker rock types preferentially lost. In contrast, wireline logging provides continuous data across all intervals including those with low recovery.
Five logging tool strings will be deployed in Hole 1256D during Expeditions 309 and 312, and the characteristics of these five tool strings are briefly described below. We anticipate that we will log Hole 1256D toward the end of Expedition 309 once significant penetration has been achieved, and logging will be the final act on site during Expedition 312. In addition, we have set aside time in the operations schedule for preliminary logging of Hole 1256D during Expedition 309, enabling an equilibrium temperature profile to be measured using the Water Sampling Temperature Probe (WSTP) and a deep basement water sample to be taken. The borehole will also be checked for breakouts and variations in hole diameter using the triple combo tool string.
The UBI measures the amplitude and transit time of an acoustic wave propagated into the formation. It provides high-resolution images with 100% borehole wall coverage, allowing detection of small-scale fractures. The GPIT is deployed with the UBI and enables borehole images, fractures, and other structural features to be oriented. This can provide important information on the local stress field and borehole geometry even within casing. An SGT is included in this tool string to allow correlation with other logging runs for establishing consistent depth estimates.
The Göttingen Borehole Magnetometer (GBM) that was successfully deployed during basement drilling during ODP Leg 197 and IODP Expedition 305 has been requested as a third-party tool for Expeditions 309 and 312. This tool has three fluxgate sensors that measure three orthogonal components of the magnetic field. The tool includes a gyroscope, which measures the tool rotation during data acquisition and allows the orientation of the tool to be determined. The data from the magnetometer will be used to monitor changes in the magnetic properties of the oceanic lithosphere as well as changes in paleomagnetic direction, primary objectives of the Site 1256 experiment. Although deployment of the GBM is contingent on availability of the tool and funds, we expect that the Joint Oceanographic Institute (JOI) logging contractors will support an experiment that is a named priority in the IODP Initial Science Plan.
The three-component Well Seismic Tool (WST-3) records acoustic waves generated by an air gun located near the sea surface. It provides a complete checkshot survey, a depth-traveltime plot, and accurate estimates of drilling depth. A generator-injector air gun positioned over the side of the JOIDES Resolution will be used as the seismic source.