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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

  1. Standard logging toolstring (triple combination [triple combo] toolstring), including the tools for measurements of gamma ray activity, density, porosity, resistivity, and temperature; and
  2. FormationMicroScanner/sonic toolstring, including the tools for measurement of elastic properties and high-resolution resistivity images of the borehole wall.
The triple combo toolstring is a combination of five tools, beginning with the hostile-environment gamma ray sonde (HNGS) on top. This tool measures the natural radioactivity of a formation including the measurement for K, Th, and U contents. It is highly applicable for determining the formation's mineralogy and geochemistry, especially for the detection of ash layers and clay intervals, as well as for different lithostratigraphic units and their boundaries. The accelerator porosity sonde (APS) measures the total rock porosity of a formation and is able to define differences in the crustal structure. In combination with the hostile-environment lithodensity sonde (HLDS), which measures the formation's density, this tool will yield information about the drilled lithology, especially where core information is missing. In combination with the measurements for seismic velocity, density measurements are necessary for creating a synthetic seismogram. The HLDS also measures the photoelectric effect (PEF), which gives addition information about the matrix composition. Either the dual induction tool (DIT) or the dual laterolog (DLL) tool can be used to measure rock resistivity. The DIT provides an indirect measurement of the resistivity and the spontaneous rock potential (SP), as well as the conductivity of the formation in three invasion depths, whereas the DLL measures the direct resistivity in two invasion depths. The tools also differ in their response range, which is 0.2-2000 m for the DIT and 0.2-40000 m for the DLL. Based on core recovered during Leg 203, and contingent on the availability of funding, the decision about which tool to use may be made during Leg 203. The last tool of the triple combo toolstring is the Lamont-Doherty Earth Observatory temperature/acceleration/pressure (TAP) tool, which records the borehole temperature, acceleration, and pressure.

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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