173 Scientific Prospectus
DRILLING STRATEGY
The principal overall objective of this leg is to complete the transect of sites begun during Leg 149, that sampled basement from continental to oceanic crust. At all sites (except Sites 901 and IBERIA08B) the sediments will be drilled to within about 100 m of acoustic basement before coring commences, because the sediments were already extensively cored during Leg 149. The rotary core barrel (RCB) will be used at all sites. Drilling is anticipated to penetrate several tens of meters into basement. Site IBERIA09A or Site IBERIA09B will require a basement penetration of about 400 m or 600 m respectively. Here, we plan to use a reentry cone and to case most, or all, of the sedimentary section. A pilot hole will be drilled into the top of basement and then logged. Further basement coring will proceed in a separate hole after casing has been set. The basement section of this hole will also be logged on completion.
No downhole measurements are planned; heatflow measurements were already made during Leg 149.
The ship is expected to occupy each site using Global Positioning System coordinates alone without the need for a pre-site survey since good multichannel seismics already exists over and around the sites.
There will probably be time on Leg 173 to drill only three or four sites (Table 1). The highest priority sites are Site IBERIA09A and Site IBERIA09B but only one of these sites will be drilled. The final choice will depend on additional processing (pre-stack depth migration) of new seismic lines in the area. However, the chosen site will be drilled second because the 18 hr transit from Lisbon does not allow the drill crew enough time to prepare the reentry cone and associated hardware. Site IBERIA08B will be drilled first. In addition we plan to drill Sites IBERIA07A and IBERIA10A if time allows.
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