LOGGING/DOWNHOLE MEASUREMENTS PLAN
The current operations plan for Expedition 303 includes downhole logging at the first proposed site, currently ORPH3A (alternatively, if a different site is drilled first then that site will be logged in place of ORPH3A). The two standard IODP tool string configurations will be deployed. The triple combination (triple combo) tool string logs formation resistivity, density, porosity, natural gamma radiation, and borehole diameter and will be run first, followed by the Formation MicroScanner (FMS)-sonic tool string, which provides an oriented 360° resistivity image of the borehole wall, logs of formation acoustic velocity, natural gamma ray, and borehole diameter. The Lamont-Doherty Earth Observatory (LDEO) high-resolution Multisensor Gamma Tool (MGT) will be deployed on the top of the triple combo tool string and run on a separate pass. Logging operations at additional sites and during Expedition 306 will take place based upon an assessment of the results from proposed Site ORPH3A. Preferred sites for logging during Expedition 306 (assuming the ideal operational plan) are IRD3A and one of the GAR sites.
Assessment of logging at additional sites will require consideration of multiple issues. The objectives of these cruises are intimately linked to obtaining complete recovery of the sedimentary section, requiring at least three, and possibly four holes at each site. Thus, there is an obvious impact when opting between these two operational activities for these cruises; when logging operations are in progress, core is not being acquired. In addition, because the base of the drill pipe must be securely in the hole during logging operations (typically 100 mbsf, but in some conditions somewhat shallower), the upper part of the sedimentary section is not logged.
On the other hand, logging data will assist in the construction of a robust stratigraphic framework for the site through detailed core-log integration. This will allow core data to be plotted on a true depth scale, thereby allowing an assessment of core expansion and identification of core gaps. Ensuring quality control of spliced core records is of fundamental importance to the success of this expedition and can only be fully achieved through comparison with logging data. Additionally, the high sedimentation rates expected at some of the proposed sites will allow the identification of Milankovitch cycles in much of the standard logging data (including natural gamma radiation, density, and porosity). Environmental changes occurring over millennial time periods will be identifiable in the FMS data. Obviously, the importance of logging data only increases if core recovery is less than ideal.
In summary, additional logging after the first site will entail evaluation by the science party of core and logging data quality and the impact of weather conditions on coring and logging operations, previous logging in the area, depth of coring penetration, and operational time constraints, all in light of the issues discussed above. This will likely be required on a site-by-site basis. Further details on logging tools and their applications can be found on the LDEO Borehole Research Group (BRG) Web site (www.ldeo.columbia.edu/BRG/).
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