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doi:10.14379/iodp.sp.352.2013 Downhole measurements strategyDownhole loggingThe plan for downhole measurements will help meet the scientific objectives of Expedition 352 through the provision of a continuous, in situ data set of physical, structural, and chemical properties. This data set will aid in the characterization of the sites drilled, with the main focus being on the basement intervals. Logging measurements will complement the core data and allow core-log integration, as well as provide the only stratigraphic data for any intervals where core is not recovered. Combinations of three wireline logging tool strings will be deployed during Expedition 352:
The first run will be the triple combo tool string, which logs total and spectral natural gamma radiation (NGR), porosity, density, borehole diameter, and resistivity. The triple combo also has the option for the deep reading magnetic susceptibility sonde (MSS-B) to be included in the tool string. The combination of these measurements will enable lithologic changes and variations in alteration to be assessed. In addition, the borehole diameter measurements will give a provisional indicator of hole conditions, which is essential for assessing log quality in this and successive tool deployments. The second run will be the FMS-sonic tool string, providing oriented, high-resolution electrical images of the borehole wall and acoustic velocity data. In addition, total NGR, borehole diameter, and General Purpose Inclinometry Tool magnetometer measurements will also be collected during the run. Synthetic seismograms will be generated from a combination of the density and sonic velocity data sets, providing an essential link between the borehole stratigraphy and the seismic sections. The UBI tool provides oriented, high-resolution acoustic amplitude images with 360° borehole wall coverage. Use of the FMS and UBI images in tandem can help detect small-scale structures and lithologic variations and aid in understanding the fracture, and hence fluid flow, history in the fore-arc crust. Further information about the tools and their applications is available at iodp.ldeo.columbia.edu/TOOLS_LABS/index.html. Operational time estimates can be found in Table T1. Site BON-1A has a ~260 m sediment overburden that will be cored. In the event of poor core recovery, this interval may be logged with the triple combo tool string prior to installation of the reentry system. Should recovery be good, a basic total NGR log of the interval may be acquired through the casing as part of logging the deep hole. Although the NGR signal obtained through the casing will be attenuated, the data should be adequate for observing broad trends and major changes through the sediment interval. This logging strategy will also be adopted for Site BON-2A, where the sediment overburden is ~120 m. The basement intervals for Sites BON-1A and BON-2A will be logged with a minimum of two tool strings (triple combo and FMS-sonic) to obtain a combination of standard measurements and borehole imagery. The UBI tool string may be deployed, depending on the condition of the borehole and the availability of time, as a third tool string or in combination with the triple combo. Formation temperature and core orientation measurementsFormation temperature measurements are planned for the APC sediment intervals at both Sites BON-1A and BON-2A. Three to five measurements per site will be acquired using the advanced piston corer temperature tool, which will allow reconstruction of the thermal gradient at each site. APC cores will be taken with nonmagnetic core barrels and oriented with the FlexIT tool for paleomagnetic studies. Nonmagnetic core barrels will be used until overpull limits are exceeded. |
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