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doi:10.2204/iodp.proc.301.103.2005

Post-Expedition 301 CORK operations

Three weeks after completion of Expedition 301, Holes 1026B, 1027C, U1301A, and U1301B were visited by ROV during a Thomas G. Thompson cruise (1–16 September 2004) to carry out the following operations:

• Inspect reentry cones for signs of leakage

• Close CORK valves left open for purging of air and surface seawater

• Install CORK pressure monitoring instrumentation

• Recover OsmoSamplers installed at the time of CORK deployments

Each instrument package (whose ~60 kg weight in water was partially supported by releasable floatation) included a data logger, an underwater-mateable electrical connector, and pressure sensors connected to hydraulic couplers via a sheathed umbilical containing 1.5 mm (¹⁄₁₆ inch) OD stainless steel hydraulic lines (Fig. F20). These were dropped to the seafloor on an elevator (Fig. F20) from which individual units were removed with their flotation by the ROV ROPOS and carried to each of the new CORK sites. The elevator was dropped 100 m west of Hole U1301A; after removal of the third instrument package, the vertical mast holding the floats was removed to eliminate it as a potential hazard to submersibles or ROV tethers. At the end of the dive (spanning a total of 42 h on 7 and 8 September), a visit was made to Hole 1027C to download data recorded there since the last visit in August 2003.

Operational summary

Operations were planned for a single dive of ROPOS, although problems with hydraulic and electrical systems required two return trips of the vehicle to the deck. Outside of set-up and ROV maintenance time, the operations required 27 h, including one round trip from the ship to the seafloor.

The pressure logging instrument deployed on the reentry cone platform in Hole 1026B was set up to monitor a single basement interval (Figs. F11, F12, F13) and is programmed to log formation and seafloor pressure at a 15 s interval (Tables T1, T2). The single OsmoSampler connected to sampling line 6 was removed and recovered. All valves were closed with the exception of the three-way pressure monitoring line valve, which was left in the "monitoring" position. The upper few meters of Spectra line leading to the thermistors and OsmoSamplers was found not to be in the hole in the CORK head, and the top plug was hanging just over rim of the reentry cone. Although initially taut, the line went slack after picking up the top plug. The line could not be fed into the hole, so the plug was left on the reentry cone platform beside the CORK microbiology sampling bay. No sign of leakage from the top of the hole was observed and the differential pressure signal recorded during deployment and downloaded at the end of operations at this site looks clean (Fig. F21), so it is likely that the lower CORK plug is properly seated and sealed.

Connections to two monitoring intervals were made in Hole U1301A (Figs. F14, F15, F16); again, the instrument was placed on the reentry cone platform. Data from the formation sensors and a seafloor pressure sensor were set to record at 1 min intervals (Tables T1, T2). Unfortunately, no response via the RS-422 communications link was received from the logger after it was installed; a download of initial data was not possible, and the overall integrity of the seals in this CORK system could not be assessed. As in Hole 1026B, the upper CORK plug was hanging outside the CORK head, although this time the ROV was able to place the upper plug in position. There was no visible sign of leakage (e.g., shimmering water), and it seems likely that the installation is sealed. It is thought that the data logger is functioning properly because it passed deck testing prior to deployment, but we are preparing a replacement logger to be deployed by submersible in September 2005. The single OsmoSampler deployed during the drilling expedition (connected to sampling line 6 on the CORK head) was recovered, and all valves were closed except for the two pressure monitoring valves.

The reentry cone in Hole U1301B is located >1 m below the seafloor. The CORK head was modified during Expedition 301 to raise it by 2 m in order to allow for the positioning of the cone below the seafloor. Nevertheless, the depth of the cone and the added height of the cuttings pile made the pressure logging package difficult to reach when it was placed on the reentry cone platform, so it was positioned just outside the crest of the cuttings. Connections to three monitoring intervals were made at this site (Figs. F17, F18, F19). As in Hole U1301A, data from these and a seafloor pressure sensor are being recorded at 1 min intervals (Tables T1, T2). Also as in Hole U1301A, no response via the communications link was received from the logger and the overall integrity of the seals of this CORK system could not be assessed directly. No signs of fluid leakage were visible and it seems likely that the 10¾ inch casing is sealed to the formation at depth, in this instance by a combination of sediment collapse around the 16 inch casing string and cement pumped into the reentry cone near the end of Expedition 301. The cause for the lack of communications is similarly unknown, and this logging system will also be replaced in September 2005. One of the three CORK head OsmoSamplers deployed during the drilling expedition, monitoring the deepest crustal interval, was recovered, and the other two were left in place. All valves were closed except for the two pressure monitoring valves and those servicing the remaining two OsmoSamplers.

The visit to Hole 1027C, first instrumented in 1996 and operating continuously since, went according to plan. The cone sank in this hole much in the way that the one in Hole U1301B did, but here the main casing string was cemented in with the assembly held above the seafloor. Thus, in contrast to the nearly buried reentry cone in U1301B, the cone and mud skirt in Hole 1027C looms well above the local sediment surface. An underwater connection was made to the Hole 1027C data logger, and the most recently acquired data were downloaded.

Preliminary results

The combination of laboratory tests and initial data from the new pressure ADC and logging system installed in Hole 1026B show that the units have met or exceeded all design goals (Fig. F21). Power dissipation is very low; this will allow many years of operation at much higher sampling frequency than was possible with the previous generation of CORK pressure logging systems (Tables T1, T2). Pressure resolution is also much higher than with earlier systems. Figure F21A shows the latter part of the pressure record during deployment, from the time the Hole 1026B instrument was dropped to the seafloor to the time the hydraulic coupler was mated to the CORK. Values recorded for the two sensors track one another to within 0.5 kPa over the full range of pressure and temperature from on deck to the seafloor (0.1–27 MPa; 18°–1.8°C). A sense of instrument resolution can be gained with the detailed view in Figure F21B. Pressure resolution and noise are indicated by the comparison of differences between sequential values between the two sensors. More detailed analysis of recovered data shows coherent variations between sensors at a ~60 s period below a level of 0.01 kPa.

Data recorded at the time the instrument was coupled to the CORK head showed an initial 42 kPa of excess fluid pressure and then a relative decline of 1 kPa during the 20 min between coupling and data download (Fig. F21C). This excess pressure comprises a combination of the natural state of basement fluid and the anomalously warm and buoyant state of the hole that had been discharging basement fluid through the various ports open since installation. The pressure decline results from a combination of the initial decline in anomalous thermal buoyancy following closure of the valves and the changing tidal pressure differential. Judging from the magnitude of the excess pressure and the quiet nature of the formation signal, the quality of the seal in Hole 1026B appears to be excellent.

The data recovered from Hole 1027C bring the total history of continuous monitoring at this site to >8 y. Pressures recorded at the seafloor show signals from tides, weather, oceanographic mesoscale eddies, and seasonal wind-driven oscillations. This information (which constrains water mass) will be used in conjunction with satellite altimetry data (which constrains sea-surface elevation) to investigate ocean circulation processes. The formation pressure record includes response to seafloor loading over the broad oceanographic frequency band, as well as co- and postseismic strain signals associated with events on the Nootka fault in 1996 and the Juan de Fuca Ridge axis in 1999. The data collected this trip include what may be a formation pressure response to an Mw 5.8 earthquake ~200 km to the north on the Nootka fault on 15 July 2004. In addition, there appear to be significant pressure transients associated with Expedition 301 drilling, casing, and other activities (Fig. F21D). There is an overall increase in pressure in Hole 1027C during operations in Holes U1301A and U1301B and several abrupt changes in pressure after periods of rapid pumping, illustrating the extent of hydrogeologic "connection" between holes separated by ~2.4 km. This result is promising for planned crosshole experiments during future drilling and nondrilling expeditions. As with other such "uncontrolled" crosshole pressure signals, this one cannot be interpreted quantitatively because Holes U1301A and U1301B were not sealed during pumping. In fact, much of the fluid pumped probably came out of the holes at the seafloor and never entered the formation. We will monitor fluid flow volumes and rates during crosshole experiments completed during and after the next drilling expedition, allowing quantitative interpretation of pressure response at a large scale, in addition to monitoring fluid temperature, chemistry, and microbiology in multiple locations.

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