-in casing screen downhole.
Two OsmoSampler packages were deployed in Hole 1253A with the center of the upper one inside the screen at 500 mbsf and the lower one in the open hole at 516 mbsf (Fig. F21). A packer was set in the lower igneous unit at 478 mbsf. Two weak links were used in each OsmoSampler package in order to maximize chances of recovering at least the upper OsmoSampler (located within the screened interval) in case the hole collapsed around the lower OsmoSampler or sinker bar. First, the upper sampler was attached with a braided 3/8-in nylon rope to the latch/running tool assembly. Next, a sinker bar (3.7 m long; 131 kg weight) was attached to the lower OsmoSampler unit with a braided 3/8-in nylon rope and a weak link of 408-kg (900 lb) breaking strength. Another weak link with a breaking strength of 680 kg (1500 lb) was attached to the bottom of the upper OsmoSampler followed by 13.4 m of 3/8-in braided nylon rope. The lower OsmoSampler was then attached to the rope. The entire assembly (27.31 m overall length) was slowly lowered down the drill string in steps to prevent differential pressure from damaging the osmotic pumps and lowered further until the latch/running tool subassembly landed in the XN latch nipple on top of the 4-in casing screen downhole.
Installation of the CORK-II observatory, consisting of an OsmoSampler package with OsmoFlowmeter, started immediately after coring to 157 mbsf by assembling the 4-in casing containing the screen and the packer element. With the casing shoe at 117.4 mbsf, the plan (Fig. F22) was to set the center of the packer at 129 mbsf and the center of the screen at 140 mbsf, in the middle of the geochemical anomaly as determined from Site 1255 data and Site 1043 results. The second pressure port inside a small screen was installed just above the upper packer. The final step was to attach the CORK-II head to the assembled casing string and lower the installation into the cased and open hole. No problems were encountered this time. Before latching the CORK-II head into the reentry cone, the OsmoSampler seat was deployed and latched, and then the OsmoSampler including the flowmeter was lowered slowly to allow proper equilibration of the samplers to ambient temperatures and pressures. Once the OsmoSampler was in place, the CORK-II head was latched, the packer was inflated, and the spool valves were shifted to start in situ pressure monitoring. The last step was the deployment of the ROV platform before the drill string was decoupled from the CORK-II.
Because of the likelihood that the spool valves had not shifted and the packer had not inflated at the time of the CORK-II installation at Site 1253, a visit to the site was planned with the manned submersible Alvin in November 2002, ~1 month after the installation of the CORK-IIs. Data recovery operations at wireline-installed CORKs in Holes 504B and 896A, located on the southern flank of the Costa Rica Rift, had been fortuitously scheduled for this time. The proximity of the Atlantis/Alvin port call to the margin sites and efficiency of operations at the rift flank sites made the visit to the margin sites possible in the time allotted to that cruise. Participants in that program included K. Becker (chief scientist, Rosenstiel School of Marine and Atmospheric Science, University of Miami, United States), T. Pettigrew (ODP, College Station, United States), and E. Davis and R. Meldrum (both of the Geological Survey of Canada). Edited videos of both dives can be found in Figures F31 and F32 in the "Leg 205 Summary" chapter.
We prepared for the spool-valve problem at Site 1253 with two parallel solutions. The first would make use of a pump mated to the hydraulic connection at the top of the CORK-II assembly that accesses the packer-filling line (the very line that was believed not to have been pressurized when the packer-inflation go-devil failed to reach its landing point during deployment). This pump was designed and built by B. Carson and L. Holloway for formation testing in the CORKed Hole 949C in the Barbados accretionary prism. If this operation were to fail, a second approach would employ a stand-alone pressure sensor and data logger constructed by E. Davis and R. Macdonald that would be coupled to the redundant lower CORK-II formation-screen fluid sampling line that was plumbed to the seafloor through a valved port. This sensor/logger unit was equipped with an underwater-mateable electrical connector identical to that on the CORK-II unit itself to facilitate data downloading at the time of future site visits.
As it turned out, visual inspection of the CORK-II head at Site 1253 showed that both spool valves had indeed shifted (although the left-hand piston was ~
in above the level of the right) despite the go-devil never properly landing. The pump was coupled and run for 16.5 min as an ultimate precaution (this served to shift the left-hand spool valve fully down to the same level as the right), but following an inspection of the downloaded data, it became clear that the installation was successful from the beginning. The success was evident in the way of attenuated tidal pressure signals in basement and small, but well-resolved, average pressure differentials (several kilopascals) between the two basement levels separated by the packer.
The time remaining after operations at Site 1253 was just enough to allow for a visit to Site 1255 before the end of the dive. Because it is located at the bottom of a local topographic slope or in a local depression, the site was somewhat difficult to find using the sonar scanner on Alvin. Here two more mysteries awaited. First, despite the T-handles having been secured by rubber bands at the time of deployment, the top-left sampling valve was found to be rotated roughly 10° from horizontal. This brought back bad memories of Leg 196; the cause of this behavior remains unknown. Fortunately, in this instance, the rotation was insufficient to cause the valve to have leaked. The second mystery was that the formation pressure registered by the sensor connected to the deeper (OsmoSampler) level indicated leakage somewhere in the CORK-II plumbing until ~36 hr before the submersible arrived. Pressures at the sensor remained close to hydrostatic, and the tidal signal remained unattenuated relative to seafloor pressure until a time near the end of this first recording period, when the sensor pressure rose to a level somewhat higher than that observed in the upper interval (roughly 180 kPa) and displayed an attenuated tidal variation. It is with considerable confidence that we now look forward to the first real phase of data recording and first OsmoSampler recovery at both of these sites.
