DRILLING STRATEGYThe primary sites will be drilled first on the rise (Site PBD-12B), then on the shelf (Site PBS-2A), and finally on the slope (Site PBF-6A). Depending on ice conditions, this plan will be modified as necessary. Shallow-water guidelines do apply to some of the alternate sites. Because of these guidelines, we may be unable to meet the site objectives if the heave constraints are exceeded.
Average conditions in Antarctic waters can be predicted with reasonable confidence; however, interannual variability can be high and the situation at a specific location can vary dramatically. Therefore, we have several layers of contingencies. The primary drilling strategy consists of three sites (Table 1; Fig. 14) with alternates fairly close by that will allow drilling in case of small scale ice problems and drilling difficulties (Fig. 15). We propose an additional strategy to meet the objectives in a less than ideal way in case very heavy ice conditions are encountered on the Prydz Bay shelf. Proposed Site PBD-15A was selected to drill to a depth at which Eocene sediments are thought to occur. A second site was selected on the Mac Robertson Shelf (proposed Site PBS-5A) at which middle Eocene fossils were recovered by gravity coring. If the primary objectives are achieved, time is still available, and ice conditions permit, the Holocene sequence in Iceberg Alley will be cored.
The Prydz Bay shelf site (proposed Site PBS-2A) will be cored by rotary core barrel (RCB) from the outset because poor results were achieved during Leg 119 using the advanced hydraulic piston corer (APC) on the shelf. For the slope and rise sites (Sites PBF-6A and PBD-12B), coring will be with APC to refusal then extended core barrel (XCB) coring to refusal. A new hole will be drilled for RCB coring to total depth. If time permits, we will attempt to triple APC the Mac Robertson Shelf site (Site PBS-3A).
To 188 Logging and Underway Geophysics
To 188 Table of Contents