OPERATIONS STRATEGY

Following is a summary of Leg 205 operations at the two sites to be reoccupied during the Costa Rica hydrogeology project (Table T1). More complete information regarding these sites can be found in the site chapters in the Leg 205 Initial Reports volume (Morris, Villinger, Klaus, et al., 2003).

Site 1253 is located ~200 m seaward of the deformation front in the deepest part of the Middle America Trench (Figs. F3, F4) Operationally, the primary goal for this site was to recore the sediments immediately above the sill encountered during Leg 170, drill and core for the first time through the sediments below the sill, and core >100 m into the oceanic sections. The other major task was to install a CORK-II observatory in the deep igneous section; coring and logging information was used to identify depths to set the packer and osmotic fluid and gas samplers.

One hole was drilled at Site 1253, which was partially cored and into which we installed a long-term hydrologic borehole observatory. After setting a reentry cone and 16¹/₂ inch casing into the seafloor, the hole was reentered with the rotary core barrel (RCB) and drilled without coring to ~370 mbsf. RCB coring below 370 mbsf penetrated 30 m of calcareous and locally clay rich sediments with intermittent ash layers (average recovery = 75%) before encountering a gabbro sill between 400 and 431 mbsf (average recovery = 74%). Below the sill was ~30 m of partially lithified calcareous sediments with intermittent ash layers (average recovery = 20%). This interval was followed by coring ~140 m into a second igneous unit (average recovery = 75%) with local zones of 55%–50% recovery.

After coring, operations focused on preparing the hole for downhole logging and CORK-II installation. The hole was opened to 14³/₄ inches; 10³/₄ inch casing was installed to ~413 mbsf and cemented in place to inhibit communication between the borehole and the formation. After drilling out the cement shoe and drilling a rat hole with an RCB bit, the hole was logged.

After logging, the CORK-II components were assembled, including a 4¹/₂ inch casing screen, casing packer, and casing made up to the instrument hanger. The entire assembly was lowered into the hole and latched in to seal the borehole outside of the 4¹/₂ inch casing. The OsmoSampler with integral temperature sensors was lowered through the center of, and latched into the bottom of, the 4¹/₂ inch casing. The final operation was to inflate the packers and shift spool valves connecting the CORK-II pressure monitoring system to the formation, completely sealing the zone to be monitored. Problems with the go-devil used for this step made it difficult to determine whether the packer had inflated or the valves had turned for pressure monitoring. Alvin dives since then have confirmed that the installation is fully operational. Three absolute pressure gauges including a data logger are installed in the instrument hanger head. One sensor monitors pressure within the sealed-off fluid sampling zone at the bottom of the hole, one monitors pressure variations in the borehole above the sealed-off section, and the third sensor provides seafloor reference pressures. One additional sampling line extends from the CORK-II head down to the screened interval below the packer and is available for future pressure/fluid sampling purposes. The specifics of the CORK-II installation, relative to the structure and petrology of the igneous sections, are discussed in more detail below.

Details of the CORK-II installation in Hole 1253A are shown in Figure F5, and petrological and structural characteristics of key depths are shown in Figures F6 and F7. The center of the packer was set at ~473 mbsf, with the inflatable element between 471.5 and 475.5 mbsf. Cores show this to be a high-recovery interval of massive rock with relatively few fractures. The upper OsmoSampler, inside a 7.35 m long screen, is set between 497 and 504 mbsf. A fluid sampling line runs from a 2 m pressure screen within the casing screen to the CORK-II wellhead. The lower sampler dangles in the open hole between 512.1 and 519.5 mbsf. The placement of the osmotic samplers was determined using a combination of scientific and operational constraints. Originally, the intervals 513–521 (now OsmoSampler 2) and 560–568 mbsf were targeted. However, logging tools encountered a bridge at 530 mbsf, restricting OsmoSampler deployment to shallower levels. The upper pressure screen above the packer was set between two igneous subunits, where sediments collapsing around the screen should make an effective seal. The final installation configuration for this modified CORK-II geochemical and hydrologic borehole observatory is shown in Figure F5.

Site 1255 is located ~0.4 km arcward of the deformation front in a water depth of 4311.6 m and close to the Site 1043 holes drilled during Leg 170 (Kimura, Silver, Blum, et al., 1997). Hole 1255A is ~20 m east of Hole 1043A and ~30 m northwest of Hole 1043B (Figs. F3, F8, F9). In Hole 1043A the complete section was cored to 282 mbsf in the underthrust sequence (Unit U3), whereas Hole 1043B was logged using logging while drilling (LWD) to 482 mbsf, the top of igneous basement. Both holes penetrated the décollement, and their results were used to plan drilling strategy and installation of the CORK-II observatory.

After setting the reentry cone in Hole 1255A, the hole was deepened to 123 mbsf with a 14³/₄ inch bit, followed by installation of 10³/₄ inch casing to 117 mbsf and cemented it in. Coring started at 123 mbsf, after drilling out the cement shoe, and stopped at 157 mbsf, when a sudden increase in penetration rate during cutting of the fourth core indicated that the underthrust sediments had been reached. Installation of the CORK-II was successful and was completed with deployment of the remotely operated vehicle (ROV) platform. The observatory configuration is shown in Figure F10. The center of the packer is at 129 mbsf and the center of the screen at 140 mbsf, in the middle of the geochemical anomaly determined from data from Sites 1255 and 1043. A second pressure port inside a small screen was installed just above the upper packer. A postcruise Alvin dive showed the installation to be fully operational, and pressure data showed a return to hydrostatic conditions within the borehole.

Eight Alvin dives were planned for recovering and replacing the OsmoSamplers and temperature loggers at Sites 1253 and 1255. The intent was to place a winch on top of the wellhead, latch on to the instrument string with the running tool, and use the winch to break the seal and pull the OsmoSamplers to the wellhead, where they could be floated to the surface. Replacement samplers dropped by elevator would then be guided hand-over-hand into the 4.5 inch casing and allowed to free fall to seat. The Alvin and Atlantis crews performed superbly, but we encountered several problems. During dive 1 we installed the winch. During dive 2 we were unable to latch the running tool into the sampler despite repeated attempts; slack and additional play in the winch line suggested soft debris, possibly rust brushed from the 4.5 inch casing by passage of the tool and line atop the samplers, occluding the latch. The running tool was recovered and additional jars added in an attempt to penetrate the debris with a heavier tool. Eventually, after overcoming several other problems, the running tool latched into the OsmoSampler package, as determined from pull on the winching motor. A design incompatibility between the winch and optimal Alvin operations resulted in the OsmoSamplers being dropped back into the hole after being winched up 70–100 m. During penultimate dive 7, we made a brief attempt to retrieve the OsmoSamplers, but time limitations made them impossible to recover, given the need to secure the sites and recover materials on bottom.

Ultimately, pressure data were downloaded at both sites; Site 1255 was left in its original condition, and Site 1253 was left with the OsmoSamplers seated at depth, the tools and ~550 m of Spectra line attached, and a ring and float attached ~20 m above the wellhead. The impact of the engineering and borehole complications were, of course, exacerbated by Alvin's limited bottom time in deep water and power. Although these factors were recognized before scheduling ship time, Jason was fully booked through and beyond the 2 y window of the OsmoSampler and temperature logger configuration. Lessons learned from this Atlantis cruise benefited final engineering design and fabrication for IODP Expedition 301.

Sites 1253 and 1255 were left ready for OsmoSampler recovery and replacement by the JOIDES Resolution or ROV. The drillship provides heavier wireline tools and therefore more jarring action to penetrate the soft debris and latch in. Our experience suggests that it should be simple to latch in, but bailing soft debris from the hole is a possibility if necessary. Using the JOIDES Resolution to recover and replace the OsmoSamplers allows us to install lines to the seafloor that will make future ROV/submersible recoveries feasible without the submersible winch system, which has been problematic as presently configured. Continuous operations allow for time-efficient recovery and reinstallation.

The operations time estimate for OsmoSampler recovery and replacement is listed in Table T1. All operations are relatively straightforward and have been performed using the JOIDES Resolution in the past. The time estimate for operations at the Leg 205 sites is ~3 days (see Table T1). Costa Rica Sites 1253 and 1255 are conveniently located relative to the drillship transit following IODP Expedition 301, after the Astoria, Oregon, port call en route to the Panama Canal.

In addition to replacing the OsmoSamplers at Sites 1253 and 1255, we anticipate deploying a long-term current meter to record near-bottom currents close to the sites, subject to approval by Costa Rican authorities. The motivation for the experiment derives from a long-term record of bottom water temperature obtained from the first phase of borehole hydrologic monitoring, which reveals coherent variability between the two instrumented sites (ODP Sites 1253 and 1255) that is characterized by slow increases in temperature followed by abrupt decreases. Strong currents aligned with the trench axis are suspected. Bottom water temperature monitoring, along with seafloor fluid sampling, will continue throughout the current meter deployment period. Together, these data should provide new insights into deep-ocean water transport along continental margins. The current meter, which consists of a Nortek "Aquadopp" acoustic doppler sensor, two acoustic release units, a float, and a railway wheel weight, will be deployed near Site 1253. At the time of this prospectus, the approval request is pending.