Downhole logging at proposed Site OSN-2 will provide in situ information about a wide range of petrophysical properties. The planned logging program includes the deployment of two toolstrings (for further information, see http://www.ldeo.columbia.edu/BRG):
The main components of the second toolstring (FMS/sonic) are an FMS and a sonic tool, which for Leg 203 will be the dipole shear sonic imager (DSI). The special design of the latter tool makes it possible to measure a full waveform of the formation, including compressional wave (P-wave), shear wave (S-wave), and Stoneley wave (St-wave). Applications are mainly identifications of structural characteristics, estimation of fracture porosity, and the creation of a seismic impedance log, the basic measurement needed for a synthetic seismogram. Furthermore, the dipole technique of the tool is capable of measuring in "soft" as well as in "hard" formations, thus contributing to the scientific prospectus of similar depth penetrations in sediments and basement rocks. The FMS tool obtains a high-resolution microresistivity picture of the borehole wall, mainly leading to the identification of lithologic units and tectonic features (e.g., presence of fractures and faults, their orientations, and their degree of alteration). Comparison of fractures detected from these log images could provide information on the lateral extension of the fracture system beyond the borehole and the significance of borehole-induced features vs. natural fractures. The FMS caliper log can also be used for hole size estimation.
Given the tight time constraints, it is unlikely that additional logging runs will be made, but use of two specialty tools, the well seismic tool (WST) and the ultrasonic borehole imager (UBI), are under consideration. The WST provides a complete check-shot survey, a depth-traveltime plot, and accurate estimates of the drilling depth. The UBI measures the amplitude and the transit time of an acoustic wave propagated into the formation. It provides high-resolution images with 100% borehole coverage, which aid in detecting small-scale fractures. Evaluation and orientation of fractures can provide information about local stress field and borehole geometry even within the casing. Use of the UBI, besides depending on scientific need and operation time, is contingent on availability of funds, which will be determined prior to Leg 203
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