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We plan to acquire wireline logs from four of the seven sites proposed in the primary operations plan. The logging plan for the remaining site (with the shallowest penetration depths) is dependent on assessment, subsequent to coring operations, of the scientific value of acquiring logging data versus the available time. The two standard IODP tool string configurations will be deployed in all holes where logging operations are planned. These are the triple combo and Formation MicroScanner (FMS)-sonic tool strings. In the deeper holes two high-resolution Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG) tools used to acquire spectral gamma rays and magnetic susceptibility (currently under development) are planned to be added to the standard triple combo tool string. Deployment of these high-resolution tools in shallower holes will be determined on a site-to-site basis, taking into consideration the scientific value of these data, final coring depths, hole conditions, and available time.
In addition to the standard logging tools, the Versatile Seismic Imager (VSI) tool will be available for check shot surveys at the deeper drill sites (>200 m). Check shot surveys may help to integrate seismic reflection and velocity profiles with drilling and core data for crosshole and site correlation. Additional operational and planning time may be necessary to respond to air gun deployment and marine mammal policy issues, respectively.
The tool string configurations that are planned to be deployed are as follows:
The triple combo tool string acquires basic petrophysical parameters of the borehole and provides in situ measurements that can be used to recognize lithologic variations and can be correlated with analogous measurements on recovered core. Density measurements in combination with sonic velocities will be used to compute synthetic seismograms, linking borehole data with seismic lines. FMS images provide high-resolution (~1 cm) azimuthally oriented 360° resistivity images of the borehole walls that are valuable for interpreting sedimentary and structural features and comparing these features to the recovered core. The LDEO-BRG Multi-Sensor Spectral Gamma Ray Tool (MGT) will provide high-resolution continuous records of spectral gamma radiation. This should enable spectral analysis of climatic cycles and recognition of lithologic variations, as well as improved core-log integration. The integration of log and core data will provide a robust stratigraphic framework for each site.
Hole depth can affect the effectiveness of the deployed logging tool string and may determine the importance of allocating time to acquire logging data versus core acquisition. For example, logging shallow holes may be less effective because the drill pipe must remain securely in the hole during logging operations, resulting in loss of logging data in the uppermost section of the hole. Also, tools nearer the top of a tool string are not able to acquire data at the base of the hole. Therefore, in shallower holes, data unattainable from both top and bottom of the hole represent a more significant proportion of the total than in deeper holes where the length of the tool string presents less concern. Because near 100% recovery of the sedimentary sequence is anticipated for the shallow holes, logging will be of a lower priority for this expedition at the shallowest penetration site (e.g., BOW-15A). In contrast, the importance of logging increases in deeper holes and at sites where recovery levels are lower.
In the primary operations plan (Table T2), logging operations are proposed for four of the seven sites. Two sites are planned to be cored to a depth of ~700 mbsf (Sites BOW-12B and GAT-4C). For these sites the LDEO-BRG high-resolution spectral gamma tool (MST) is proposed to be run on the standard triple combo tool string. A checkshot survey is also proposed to be done at these two sites. For Sites UMK-4D and SHR-3B, where coring and drilling operations target ~200 mbsf, deployment of the two standard tool strings is proposed. Sites where the coring targets are shallower than 200 mbsf are omitted from the logging program. Final logging operations in these holes will be determined during the expedition, taking into consideration completeness of core recovery, hole condition, and final penetration depth. If time allows, one or both of the higher resolution LDEO-BRG tools may be deployed. A potential tool string that may alternatively be deployed in shallow holes (e.g., if final coring depth is less than ~200 mbsf) comprises a modified tool string with the high-resolution LDEO-BRG tools run in combination with the standard high-resolution lithodensity tool. This represents a shorter length of tool string suited for acquiring high-resolution data in a shallow hole. Logging operations for the back-up operations plan (Plan B, Table T3) are similar to the primary plan with the exception of downhole logging at Site GAT-3C (~400 mbsf) (see Table T3). Total logging time is comparable for both primary and back-up operations plans.
Further details on logging tools and their applications can be found on the LDEO-BRG Web site (www.ldeo.columbia.edu/BRG).