LOGGING/DOWNHOLE MEASUREMENTS PLAN
Continuous downhole logging measurements will be critically important to the scientific objectives of Expedition 307, particularly because of the potential of low core recovery at proposed Site PORC-03A. The logging program will attempt to establish lithological boundaries, seismic integration, and stratigraphic and sedimentological data.
The operations plan for Expedition 307 includes downhole logging at all three proposed sites. The two standard IODP tool string configurations will be deployed at each site. The first run will be the triple combo tool string, which logs formation resistivity, density, porosity, natural gamma radiation, and borehole diameter, followed by the FMS-sonic tool string, which provides an oriented 360° resistivity image of the borehole wall and logs of formation acoustic velocity, natural gamma ray, and borehole diameter (pending approval). The well seismic tool (WST) will be run at proposed Site PORC-03A as a third tool string to conduct a zero-offset VSP (pending approval), which will also require the use of an air gun.
The standard sampling interval of the logging tools is 15 cm, with a typical vertical resolution of 40 cm. For higher vertical resolution, the FMS resistivity images allow subcentimeter resolution, and the Lamont Doherty Earth Observatory (LDEO) Multisensor Gamma Tool (MGT) (~15 cm vertical resolution) can be included in the triple combo tool string.
Combining sonic velocity and density logs will allow modeling of a synthetic seismic profile at each site. Moreover, the sonic logs will improve the depth to traveltime conversion and thus correlation of the holes with the seismic profiles. At proposed Site PORC-03A, the velocity structure inside the mound will be further constrained by the zero-offset VSP.
For core-logging correlation, density, resistivity, and natural gamma radiation logs can be matched to the equivalent core measurements made on the multisensor track (MST). The logs will provide complete coverage in parts of the section that may not have full core recovery, which particularly may be the case at proposed Site PORC-03A, where limited core recovery is a possibility due to high coral density. This set of logs might thus reveal a more realistic stratigraphy of the internal buildup of the mound and the mound substrate (open-hole logs will be obtained below the position of the drill pipe, set at 5080 meters below seafloor [mbsf]).
FMS resistivity images will be useful to identify the presence or absence of coral macrofossils, which will be visible in the images as resistive features in a conductive matrix. The strike and dip of inclined features (beds, fractures, etc.) are also provided by FMS data. Hardgrounds will probably be identified as hard, resistive layers with a high uranium natural gamma radiation signature.
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