The operational strategy for Expeditions 309 and 312 is to core as deep as possible in Hole 1256D, including complete borehole coverage with conventional wireline logging tools (Table T1). Operational time for these expeditions includes 38 days on site for Expedition 309 and 35 days on site for Expedition 312. Using pipe-trip times appropriate for the water and hole depths and assuming 50 h of rotation on each of 1215 CC-9 rotary core barrel (RCB) coring bits (determined to be the best available for coring at Site 1256 during Leg 206), we expect to achieve a minimum depth of at least 1450 m into basement (1700 meters below seafloor [mbsf]). A penetration rate of 1.5 m/h has been assumed in this estimate, which was the average rate of penetration toward the end of drilling during Leg 206 and is similar to rates achieved during operations in sheeted dikes in Hole 504B during Legs 140 and 148 (Dick, Erzinger, Stokking, et al., 1992; Alt, Kinoshita, Stokking, et al., 1993).
For a spreading rate of >200 mm/y, the low-velocity zone interpreted as a melt lens is predicted to occur at a depth between 725 and 1000 m at the ridge axis (Fig. F4). The steep magnetic inclinations of the uppermost lavas sampled in Holes 1256C and 1256D (Wilson, Teagle, Acton, et al., 2003) as well as the ~100 m thickness of the ponded lava flow (Units 1256C-18 and 1256D-1) suggest that the magma flowed or was erupted a considerable distance off axis (~5 km). Assuming that there is at least 100200 m of additional lavas that flowed relatively short distances from the axis (12 km), we predict that gabbros representing the frozen axial low-velocity zone should occur at ~9251300 msb (11751550 mbsf), providing 150525 m of gabbro penetration if the proposed drilling scenario is achieved (Fig. F5).
Using the lowermost heat flow measurement taken in Hole 1256C (109 mW/m2), the temperature at the sediment/basement boundary (35°C), and appropriate thermal conductivities for lavas, dikes, and gabbroic rocks, we predict that the ambient temperature at 1700 mbsf in Hole 1256D should be ~100°140°C, significantly cooler than that encountered in Hole 504B (~170°C) at these depths.
Our operations schedule includes time for preliminary logging of Hole 1256D, which will require a dedicated round trip of the drill string, so that an equilibrium temperature profile and water sample can be recovered before the thermal structure of the crust is perturbed by drilling operations. We also expect to evaluate the diameter and eccentricity of the borehole with the triple combination (triple combo) tool string. In the unexpected case of significant fill and/or large changes in hole diameter, additional passes with Formation MicroScanner (FMS) or Ultrasonic Borehole Imager (UBI) may be made to evaluate the need for casing. A full suite of wireline logging tools will be deployed after the completion of drilling operations, following an order of operations similar to that used during Leg 206 (see "Triple Combination Tool String," "Formation MicroScannerDipole Sonic Imager Tool String," "Ultrasonic Borehole Imager," and "Three-Component Well Seismic Tool" in "Logging Strategy"). We also expect to run a three-component borehole magnetometer. We have requested funding to deploy the DMT Digital Color 360° CoreScan system or similar to digitally record the outer surface of all cores. Core-logging integration will be enhanced through the use of this system, as used during Leg 206.Contingency Plans Should Drilling Difficulties Arise in Hole 1256D
Although hole developments and drilling in Hole 1256D during Leg 206 were exhaustive and prepared the site for the best possible chance of successful deep drilling down to the gabbros, drilling upper oceanic basement will always be technically challenging and risk of hole collapse or drill string failure will always exist. As such, it is prudent that a range of contingency options be considered and approved precruise so that the best decisions can be made should the need arise.
The underlying philosophy for this experiment to drill a complete section of in situ upper oceanic crust was to construct a borehole engineered specifically for deep drilling. In Hole 1256D, the borehole infrastructure includes a large reentry cone supported by 95 m of 20 inch casing and 270 m of 16 inch casing that penetrates completely through the sedimentary overburden and is cemented ~20 m into the basement. This arrangement allows two further casing strings (133/8 inch and then 103/4 inch) to be deployed in the hole should the need arise to isolate and armor a section of basement. Our drilling strategy and contingency plans must protect the integrity of Hole 1256D, in which there has already been significant investment, as well as utilize the engineering opportunities allowed by the ocean bottom structures already in place.
Potential failings that may occur during Expeditions 309 and 312 fall into two broad categories: (1) failure of the drill string, requiring fishing operations, and (2) collapse and instability of the borehole itself. Drill string failures may be directly linked to borehole wall collapses. To mitigate against drill string failures we will adopt a conservative drilling strategy, with consistent core recovery taking priority over rapid penetration. Where necessary, half-cores (4.5 m) will be taken if core recovery becomes very low, so at least some rocks will be recovered from all intervals. The drilling operations team will ensure that Expeditions 309 and 312 will sail with the best possible arsenal of fishing tools (fishing jars, intensifiers), so should drill string or other equipment be lost in Hole 1256D we will have available all potentially useful tools to recover the lost equipment or mill it out.
Borehole failures are potentially more difficult to tackle. Casing a section of borehole a significant distance into Hole 1256D will require a very large expenditure of time and equipment. Deployment of the next-sized casing string (133/8 inch) will require the hole to be reamed to an internal diameter of 18.5 inches before casing can be inserted. Table T2 shows an estimation of the time required to open Hole 1256D to case a blockage at ~500 msb. Such an operation would take ~34 days, similar to the number of operational days on site for Expedition 309. Although installation of a smaller-diameter casing string (103/4 inch) would require less milling and would be less technically difficult, such an approach would not allow further casing strings to be inserted into Hole 1256D and would make redundant the difficult engineering operations required to cement 16 inch casing into basement during Leg 206.
It should be noted that a deep basement casing operation has never been attempted in scientific ocean drilling, and the best reaming strategies and tools remain unknown. As such, we will adopt one of the following strategies should a borehole breach occur that requires a significant casing string to be installed in Hole 1256D.1. To prepare for opening and casing Hole 1256D we will test hole opening strategies and tools including the deployment of 14.5 x 18.5 inch bicenter reamers coupled to 97/8 inch wobble bits and other potential tools to test their effectiveness and determine progress rates so that the best reaming and casing strategy can be developed ashore for future operations at the site.
Drilling a number of single-bit or free-fall funnel multi-bit holes in the environs of Hole 1256D to establish regional variation in upper crustal processes, lithologic structure, and seawater interactions would provide important information that is of high priority in the IODP Initial Science Plan. Determining the extent of the massive lava flow encountered in Holes 1256C and 1256D would constrain the size of off-axis eruptions. Drilling further afield in the GUATB-03 survey region would allow investigation of the significant variations in upper crustal velocities that were recognized across the survey region. The causes of these seismic velocity variations could be verified with a number of shallow holes combined with logging, providing important new knowledge on the range of crustal architectures present in upper oceanic basement formed at fast spreading rates. Specific sites shown in Figure F6 were selected from the pre-Leg 206 site survey cruise (Wilson et al., 2003) where crossing geophysical lines are available. Both of these sites were approved by the Site Survey Panel (SSP) and the Pollution Prevention Safety Panel (PPSP) as alternate sites for Leg 206. Slow seismic velocities in the southwest of the grid coincide with better-defined abyssal hill fabric and possibly more porous or rubbly material compared to Site 1256. As we anticipate drilling only shallow holes (200300 msb), deep thrust structures imaged on some lines will not be encountered. As the sediment overburden in this region has been thoroughly characterized and shown to be regionally uniform by drilling at Sites 1256, 844, and 845, we request that we be allowed to wash through most of the sediment pile and initiate coring just above basement. All holes >100 m will be logged with a full suite of wireline tools to return the maximum information on the crustal architecture.
Logging Hole 504B (1°13.6'N, 83°43.8'W). Hole 504B is presently the only crustal borehole to penetrate an entire section of lavas and sampled ~1 km of sheeted dikes. Because of tool failure during Leg 148, an FMS log of the entire hole was not achieved and the UBI was never deployed at that site. Because of its unique penetration, Hole 504B has been regarded as the oceanic crust reference site for more than a decade, but the absence of continuous imaging of the borehole walls and poor core recovery makes understanding the architecture of the oceanic basement at that site difficult and brings geochemical exchange budgets calculated from the distribution of the recovered cores into question. Transit to Hole 504B is ~2.5 days, and a full suite of logging operations would take ~5 days. Logging this site would provide important information critical to many key objectives regarding the architecture of the oceanic crust and chemical exchange between seawater and oceanic basement, highlighted in the initial science plan.
Deepening Hole 896A (1°13.0'N, 83°43.4'W). Hole 896A was drilled during Leg 148 (Shipboard Scientific Party, 1993) following termination of operations in Hole 504B. The hole is sited on a basement high, ~1 km southeast of Hole 504B, where heat flow and pore water studies indicate ongoing upwelling of basement fluids. The hole was drilled to investigate local-scale heterogeneity of the ocean crust around Site 504 and the effects of basement topography on the intensity of hydrothermal alteration. Three specific objectives were addressed in Hole 896A during Leg 148: (1) to examine the local variability in volcanic stratigraphy, areal extent of lava flows, and horizontal and vertical variations in igneous geochemistry; (2) to examine the effects of off-axis hydrothermal activity on the basement relating the composition of upwelling fluids in a high heat flow area to the alteration of the basement; and (3) to drill the second of a pair of deep basement holes from which future geophysical experiments between the paired holes can be conducted. Hole 896A is fitted with a reentry cone and 191 m of 113/4 inch casing. Basement was first encountered at 179 mbsf, and the hole was cored from 195.1 to 469 mbsf, 290 m into basement (Alt, Kinoshita, Stokking, et al., 1993). Deepening Hole 896A will provide important information on the variation of crustal structure in the vicinity of Hole 504B. The cores recovered so far have all incurred oxidative seawater alteration, but from Hole 504B one would predict that the transition to more reducing conditions occurs only a few tens of meters below the current depth of the hole. However, the depth of exchange with oxidative seawater may be dependent upon other hitherto unconstrained criteria, such as basement topography, volcanic stratigraphy, or sediment thickness. Knowledge of the variation of the penetration of oxidative seawater would be gained by deepening this hole, with important implications for refining geochemical budgets.
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