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doi:10.2204/iodp.sp.324.2009 Logging/downhole measurementsThe main objectives of the downhole measurement program are to document crustal physical properties, define structural and lithologic boundaries as a function of depth, and reconstruct the volcanic stratigraphy of Shatsky Rise, which could be used for assessing the evolution of this plateau and testing the hypotheses of a plume head versus plate-controlled origin of volcanism. In addition, wireline logging data can be compared to results of laboratory analyses of discrete samples and should help delineate alteration patterns, fracture densities, and structural orientations and determine how these correlate with current and paleostress environments. These measurements will complement core measurements by determining the thickness of lithologic units in intervals where core recovery is poor. Wireline loggingA series of wireline tool string deployments are planned for all sites drilled during this expedition. These tool strings will provide measurements including natural gamma ray, density, magnetic susceptibility, sonic velocities, microresistivity, and borehole images. The operational time estimates for all deployments are given in Table T1. Detailed descriptions of all wireline tools and applications are provided at iodp.ldeo.columbia.edu/TOOLS_LABS/index.html. The first tool string deployment in each hole will consist of at least total and spectral gamma ray (HNGS), density, photoelectric effect factor, caliper, and potentially, magnetic susceptibility measurements. These measurements will be used for characterization of stratigraphic sequences, assessment of alteration, and reconstruction of the volcanic stratigraphy. Depending on borehole conditions, subsequent deployments could consist of a Formation MicroScanner (FMS), Dipole Sonic Imager (DSI), HNGS, and if tools are commercially available, an ultrasonic borehole imager (UBI). The FMS and UBI will provide high-resolution borehole images of lithostratigraphic sequences and boundaries, oriented fracture patterns, and structural features that could be related to current and paleostress environments. These images can also be used for reconstruction of the volcanic stratigraphy and reorientation of core pieces. The DSI measurements will include a full set of compressional and shear wave forms and cross-dipole shear wave velocities measured at different azimuths. These types of measurements can be used to determine preferred mineral and/or fracture orientations, fracture densities, and paleostress directions. The velocity and density downhole measurements can also be used for constructing synthetic seismograms and tying the core-log measurements to regional seismic lines. |
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