DRILLING STRATEGY

IODP Expeditions 304 and 305 were planned based on a single proposal to core in two fundamentally different hard rock environments. One site is in the hanging wall to a low-angle detachment fault that has exposed deep crustal rocks on the Mid-Atlantic Ridge flank. The second target is the footwall to this fault, where the deep ocean crust and upper mantle have been exhumed. Lithologies (beneath a 1–2 m thick carapace of carbonates) are expected to be fractured pillow basalt in the hanging wall and altered gabbro and variably serpentinized peridotite in the footwall. High recovery is a priority, so we have prepared all operations estimates with the intent to recover cores in nominally 4.5 m intervals (half cores). Both sites will have penetrations in excess of 200 m (>400 and >700 m, respectively), thus requiring the ability make multiple reentries into each borehole.

Installing reentry templates in hard rock has historically met with limited success, but toward the end of ODP some progress was made in the deployment of hammer-in casing and reentry funnel assemblies. Although these tools are still developmental and have not been deployed in basalt, gabbro, or peridotite, the operations team for Expeditions 304 and 305 considers a hard rock reentry system (HRRS) as the most viable approach to assuring installation of a reentry template. An alternative approach, if the HRRS is unsuccessful, would be to drill a large-diameter borehole, attempt a single bare hole reentry, and deploy a free-fall funnel with a short casing. Another constraint on our operations planning is the assumption that we are not likely to be able to reach our depth target in the footwall site (>700 m) in 40–41 operational days (scheduled time on site). It is possible to reach this target, however, if some time during Expedition 304 is allocated to hole preparation and uppermost section recovery for Expedition 305.

Our first occupation during Expedition 304 will be the footwall site (proposed Site AMFW-01A) (Table T1). We will attempt a pilot hole, coring to ~130 meters below seafloor (mbsf) to ensure we have selected a drilling location consistent with our objectives (the sediment cover precludes direct observation and we would prefer to avoid initiating a hole with casing in basaltic rubble). Conventional rotary coring (with the exception of recovering 4.5 m cores) to this depth will provide information on formation integrity for our casing deployment and will be sufficient for a wireline logging program in the upper part of the corrugated dome. The goal of ~130 mbsf is also the maximum depth we are likely to reach on a single bit run. Once we have established a deep-penetration drilling target, we will offset a few meters and deploy the HRRS with large-diameter casing to nominally 20 mbsf. A large-diameter hole will then be drilled to ~130 mbsf, and casing will be installed to within a few meters of the bottom of the hole (~120 mbsf) to ensure the upper part of the borehole will not collapse. This will complete our optimum operations strategy at the footwall site during Expedition 304.

Once we have established the reentry template in the footwall site, we will make the short transit to the hanging wall site (proposed Site AMHW-01A) and complete operations there. At proposed Site AMHW-01A, the HRRS will be set to ~20 mbsf (since we have no reason to believe anything but basalt is present in the hanging wall, no pilot hole is required). Conventional rotary coring (with the exception of recovering cores in 4.5 m increments) will follow to a depth of deepen the hole to fractured basalt, we expect to open the borehole and install a casing string to ~120 mbsf. With the remaining time during Expedition 304, the borehole at proposed Site AMHW-01A will be deepened, targeting penetration through the detachment fault. Operations during Expedition 304 will conclude with multiple logging runs at this site.

The operational strategy for Expedition 305 is straightforward (Table T2). After reoccupying the borehole in the footwall (proposed Site AMFW-01A), we will core as deep as possible, leaving sufficient time at the end of coring to allow for multiple logging runs, including a borehole vertical seismic profile experiment. It is possible that an array of ocean bottom seismometers (OBS) may be deployed near our drill site as part of a seismic-while-drilling experiment. A few hours of operations time may be sequestered at the conclusion of coring and downhole experiments to recover these OBS.

Owing to the inherent risks of hard rock coring, we have developed plans for alternative operations. Achieving the target at proposed Site AMFW-01A is a high priority for the project. Since it is relatively deep and reaching that target relies on at least some preparation during Expedition 304, we have elected to begin operations during Expedition 304 at this site. Our contingency strategy allows for multiple time-dependent options based on the level of operational success achieved.

If the pilot hole at the footwall site fails to reach a depth sufficient to reach the objectives outlined in our primary operations plan, it should be early enough in the expedition to attempt another pilot hole within the same operational area (proposed Site AMFW-01A). Our strategy will be to continue attempts to establish a pilot hole until successful or until 12 days on site have expired (leaving the required 28 days to complete operations at the proposed hanging wall Site AMHW-01A). Once the pilot hole is successful (including multiple logging tool runs if possible), we will complete as much of the casing operation at an adjacent HRRS site as possible until time constraints require us to move to the hanging wall site.

Since establishing a reentry template during Expedition 304 is important to the objectives of Expedition 305, in the event of failure of the first HRRS site (according to our primary operations plan this could occur during coring or casing attempts), we expect to attempt to set a second HRRS nearby, if the cause of failure of the initial deep penetration borehole does not preclude a second attempt. We will conclude as much of this operation as possible before moving to the hanging wall site in time to achieve complete operations there.

If attempts to reach the detachment fault at the hanging wall are unsuccessful, we will not attempt to deploy a second reentry template; doing so would not leave enough time for the target depth to be reached at that site. The objective of penetrating the unexposed detachment fault during Expedition 304 is likely to be abandoned if the original drilling strategy at proposed Site AMHW-01A fails.

Two options that we consider important to completing the other objectives of these expeditions are to return to the footwall and either continue deepening the existing borehole (or completing casing operations if required) or to attempt to recover the uppermost carapace of the footwall (see "Scientific Rationale for Drilling"). This can be accomplished via a few short offset holes or a transect of spreading direction–parallel shallow-penetration holes within a few kilometers of the primary footwall site, possibly recovered with XCB or motor-driven core barrel technology. A decision on which of these options to pursue will depend on the time remaining in the expedition.

Contingency planning for Expedition 305 also presents a series of options if the deep-penetration hole fails; the choice would be based on the amount of time remaining. Our first-priority alternative is to attempt another deep-penetration site. However, a minimum of 35 days is required to reach 800 mbsf and 27 days is required to reach 600 mbsf. If the failure occurs more than a week into Expedition 305, we will not likely be able to attain 800 mbsf. If there is insufficient time to reach our depth target, several of the primary objectives of the expeditions can be addressed by a series of single-bit or shallow-penetration holes. If time is available, we will consider a transect of penetration-to-bit-destruction holes on the central dome of the footwall, including logging operations if possible. Our highest-priority transect would be a spreading direction–parallel series of holes (see "Scientific Rationale for Drilling"). If time is still available, we would also consider a transect of strike-parallel holes. If only a short time is available, we would consider a series of short offset holes to recover the upper carapace of the central dome or, potentially, the southeast shoulder of the massif.

The number of contingency sites and the order in which they might be occupied will depend on objectives achieved or addressed at the primary sites. We intend to occupy the alternate sites only if there is insufficient time or tools to complete our deep-penetration targets. As stated above, our highest priority alternate target is a spreading direction–parallel transect across the dome. One, two, or three locations (proposed Sites Alt AMFW-02A, 03A, and 04A) might be drilled along seismic Line Meg-10 (Figs. F4, F7). As each occupation will take 3–5 days (cored to bit destruction; see Table T2), the number of sites (and thus the specific locations) cannot be defined with certainty until the amount of time available is determined. With only a few days (<5) available, the most likely scenario, given our current information, would be to select a target near the western end of our transect. Ultimately, the targets selected for drilling will be determined by discussion among the science coordination team at sea (Co-chief Scientists, Staff Scientist, and Operations Superintendent), the operations team on shore for the companion expedition, and IODP-TAMU management with communication through IODP-TAMU Headquarters to IODP Management International.

In the event there is sufficient time to complete the objectives of the spreading parallel–direction transect, the next priority contingency operation is targets (one or two depending on time available) along the massif parallel to the ridge axis. Structure at these alternate sites is illustrated in Figure F8 (proposed Sites Alt AMFW-05A and 06A) along seismic Line Meg-4. The specific location of these targets will depend on the results of previous operations and a subsea camera survey.

The final contingency site we include (proposed Site Alt AMFW-07A) is the potential of shallow penetration (a few meters) on the southeast shoulder of the Atlantis Massif, located along seismic Line Meg-9 (Figs. F7, F8), for comparison to the surface of the detachment fault at other sites on the northern limb of the massif.

In the event of catastrophic failure of either deep-penetration site, we can still achieve some of the science objectives of these expeditions if the resistivity-at-the-bit (RAB) logging-while-drilling tool is available. If this tool can only be available for a single expedition, we prefer to have it on Expedition 305 as an ultimate option if all attempts to core and log through the detachment or into the central dome have been unsuccessful (see "Logging Strategy").