171A Scientific Prospectus


LWD investigations of the Barbados prism will build on existing LWD measurements. Proposed LWD sites will focus on determining the characteristics of the negative polarity reflections at Barbados, measuring the physical properties of faults, and determining the physical properties of the incoming sedimentary sequence. The sum of all penetrations will provide an overview of prism consolidation and velocity-porosity relationships. In prior drilling through the North Barbados Ridge accretionary prism during Leg 156, 1152 m of logs in Hole 947A and 948A was obtained using LWD technology. Leg 171A is specifically designed to acquire more LWD data in additional holes in the Barbados accretionary prism. There will be no coring on this leg.

Based on previous experience in accretionary prism environments, we anticipate that operational problems will be encountered during LWD such that we will only complete the four primary sites. In the unlikely event that no problems are encountered (as was assumed when producing the time estimates in Table 1), additional LWD has the highest priority. However, current cost constraints may limit any LWD penetrations in addition to the four primary sites. Accordingly, a second-priority activity is acquisition of wireline sonic velocity data.

The proposed tools will be the same as those used during Leg 156, directly measuring in situ resistivity, porosity, density, and natural gamma ray. An LWD sonic tool is not available for this leg. Sonic velocity information is available from Site 948, logged during Leg 156. If possible, new sonic velocity data will be obtained using wireline logging. Leg 171 will start with a complete set of LWD tools and a backup. In the event of a tool loss during the first site, a backup tool will be resupplied to the ship from Trinidad. If a tool is lost at or after the second site, no time will be available for replacement. Estimated operational times are shown in Table 1, and assume no significant hole problems.

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