The sites are located within a grid of multichannel seismic (MCS) lines and multibeam bathymetry (Fig. 3). The northern site (ACE-9a) is designed to penetrate the Pliocene-Quaternary hemipelagic cover sequence into the prerift section of Miocene forearc clastics. Penetration into greenschist facies metamorphic basement beneath the 300-m-thick Pliocene-Quaternary section at Site ACE 3c is planned to be 100+ m or until bit destruction. Site ACE-8a includes a triple-casing reentry hole (Fig. 6) that will also sample basement.
The plan is to start with a jet-in test at Site 8a to determine how much initial casing can be washed in with the reentry cone at the Site 8a reentry hole. We will then core and log, to the maximum depth possible, at the Site ACE-8a pilot holes. This will be followed by coring and logging at Site ACE-9a before returning to Site ACE-8a for operations at the cased Site ACE-8a reentry hole. Site ACE-3c will be the final hole of the leg.
Casing at the Site ACE-8a reentry hole is needed in anticipation of possibly unstable sediments (resulting from rapid deposition and faulting), an active fault zone (probably overpressured), and anisotropic basement (probably sheared and altered). We expect that this will be the most challenging drilling operation during Leg 180. Uncertainties regarding the degree of consolidation of the seafloor sediments may have considerable impact on our ability to install the reentry cone and attached conductor pipe (20-in. casing). The jet-in test at the begining of the leg will provide information to determine the appropriate type of seafloor structure and deployment technique. Our plan is to wash the reentry cone and casing into the seafloor. If the jet-in test indicates this is not possible, the installation process could become more complex and time consuming, potentially impacting other cruise activities.
The vertical seismic profile (VSP) and packer experiments are dependent upon successfully cementing around the long casing strings. The cement characteristics are dependent on the local temperature gradient, which is unknown. Therefore, we plan to have specially blended cement and additives on board to allow shipboard adjustment of the cement characteristics.
The deepest casing string was designed to use liner (10 3/4 in.) hanger technology instead of conventional casing hanger hardware. Installing a very long casing string through a potentially unstable detachment fault is problematic. Use of the liner hanger allows us to install the casing at the deepest possible point that the formation will allow without requiring that a predetermined length of hole remain open.
Alternative Sites ACE-1c and ACE-7b exist to increase the stratigraphic resolution of the onlap sequences on the northern margin, if they are drilled.
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