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doi:10.2204/iodp.proc.301.103.2005

Sensors and sampling

Expedition 301 CORKs were designed to include minimal instrumentation on the CORK head during deployment. We took this approach for several reasons. First, the CORK planning, design, construction, and testing process prior to Expedition 301 was considerably shorter than in the past and we were concerned that rushing development of sensors and instrumentation might compromise these systems. Second, leaving most of the pressure and chemical sampling instrumentation off of the CORK heads made deployment easier and allowed us to leave the lines open to purge air, as described above. We were also concerned that the CORK systems should remain sealed across most of the monitored intervals during the first 12 months after deployment so that we could determine with confidence the magnitude and sign of slight differences in formation fluid pressure relative to hydrostatic and between zones. We attached osmotic fluid samplers to the open CORK heads valves during initial deployment, but most of these instruments were recovered during subsequent ROV operations, and the associated valves were closed.

Pressure measurements

Earlier generations of CORK systems used pressure gauges and data loggers capable of measuring absolute pressure with resolution of a few kilopascals and storing several megabytes of data (Becker and Davis, this volume). Measurements were made at (user programmable) intervals of 10 s to 1 day. New pressure measurement and logging systems were developed for Expedition 301 CORKs and deployed by ROV after the drilling expedition. Specifications for Expedition 301 CORK pressure instrumentation are summarized in Tables T1 and T2.

Expedition 301 pressure instruments are built around Paroscientific Digiquartz absolute pressures sensors, like earlier generations of instruments. The new instruments can monitor multiple depth intervals on separate channels and have greater memory capacity, lower power consumption, and faster communication and data download rates. This permits them to sample much more frequently for longer periods than was possible in the past (15–60 s versus hourly sampling in Leg 168 instruments). The new instruments also have greater pressure resolution and are less sensitive to temperature variations.

The Expedition 301 instruments store data on flash memory cards; 256 megabyte capacity was used, but greater capacity is available. In addition, a new analog-to-digital converter (ADC) for the pressure sensors provides a frequency resolution of ~2 ppb with an 800 ms measurement. Applied to the full dynamic range of the pressure transducers, this is equivalent to pressure resolution of 20 ppb (2 Pa or 0.2 mm of water), an improvement over previous CORK instrumentation by a factor of 50. Whether the transducers themselves are quiet at this level will be determined during coming years. This level of sensitivity could permit new studies of oceanographic (infragravity waves, tsunami, and turbulence), seismic (surface waves) and crustal hydrologic phenomena.

Hydraulic connections for the new pressure measurement and logging systems are made using custom-designed submersible-mateable connectors. Because the new pressure instruments require less power and can use smaller batteries, the sensor and logger housings are smaller and more portable. This allows the instruments to be carried to and mated with the CORK head plumbing by submersible if they are not mounted at the time of drilling and replaced later if necessary. A T-handle is used to control a valve on the pressure measurement line in the CORK head. The handle is turned 90° counterclockwise from vertical to monitor the formation and 90° clockwise from vertical to expose the pressure sensors to seafloor pressure for a short time (during a submersible or ROV servicing visit), allowing instrument offset and drift to be checked.

Temperature measurements

Autonomous temperature sensors and data loggers were deployed in all Expedition 301 CORK systems. We elected to take this approach, rather than using a thermistor-instrumented cable attached to a central data logger, for several reasons. First, planning and cable construction would have been extremely rushed in the short time available before the start of Expedition 301, leaving little time for seasoning and testing. Also, we did not know the final hole depths or the depths that would be of greatest interest for temperature measurement, so it would have been impossible to position thermistor breakouts at the right locations. Field shortening and splicing conventional braided line is easier and more secure than modifying a multiconductor thermistor cable. It is also easier to attach braided line to plugs and sinker bars; the line can be attached with a shackle or thimble, and there is no need to penetrate the plugs for passage of conductors.

We also needed to monitor temperatures at the depths of the osmotic samplers because their sampling rates depend on the temperature-dependent viscosity of seawater. This required purchase of at least a small number of autonomous temperature loggers, but there were no suitable off-the-shelf instruments available, and tool manufacturers generally have a minimum order and significant time requirements for custom developments. We had already determined from Leg 168 measurements and subsequent CORK monitoring that temperatures in uppermost basement in this area are ~62°–65°C (Davis and Becker, 2002; Fisher et al., 1997; Shipboard Scientific Party, 1997), so there was no need to repeat this experiment.

We were not sure that a single vendor would be able to supply the necessary tools in time for the expedition, given the short lead time, so we contracted with two vendors to prepare temperature instruments. Antares, Datensysteme GmbH, had previously built autonomous temperature loggers that were deployed in CORKs during Leg 205. These tools are similar in design to outrigger probes built for thermal gradient measurements on core barrels (Pfender and Villinger, 2002), but we asked Antares to design and build modified tools with a titanium pressure case, two O-ring seals, a 5 y lithium battery, and a thermistor circuit having a dynamic range of ~40°–100°C. We anticipated formation temperatures at depth no greater than ~80°–90°C. Choosing a limited dynamic range allowed us to maintain nominal tool resolution of 1 mK. The new tools had other specifications very similar to the older tools (Pfender and Villinger, 2002): a 16 bit ADC, nonvolatile memory for up to 64,000 readings, and a user-programmable sampling interval of 1 s to 255 min. The new tools are about the size of a wide-tipped marking pen, slightly wider and longer than the older tools. The tools are programmed and operated using a through-case serial connection, with custom software run on a personal computer (PC), so the pressure cases need never be opened to operate the tools, check battery power, or access data.

The other temperature tools prepared for Expedition 301 were constructed by Onset Computer Corporation. These tools are off-the-shelf temperature loggers (HOBO model U12) modified with a titanium pressure case and longer life battery for use in the deep sea. The Onset tools have a wider temperature range than the Antares tools (–40°–100°C), temperature resolution of 0.02°–0.1°C, and nominal accuracy of 0.2°–0.6°C across the range of anticipated CORK observatory temperatures. Like the Antares tools, the Onset tools are programmed and data are accessed using software on a PC, but the Onset tools must be opened to allow communication with a computer. The Onset tools are about the same diameter as the Antares tools, but are half as long.

Eighteen of the new Antares tools and ten Onset tools were purchased, and combinations of these tools were deployed with each CORK instrument string, as described later. Because the temperature tools arrived just before the pre-Expedition 301 port call, we did not have time to check tool calibration prior to the start of the expedition. We brought to sea a stirred-fluid temperature calibration bath (Hart Scientific model 7011) capable of maintaining temperature stability of ±0.001°–0.01°C over a temperature range of 0°–100°C and used this system to test the temperature loggers prior to deployment. Once the bath was stable at a target temperature, absolute bath temperature was measured and logged using a National Institutes of Standards and Technology-traceable resistance temperature device probe (Hart Scientific model 1521) with an accuracy of ±0.001°C. The Onset tools were calibrated at eight temperatures in the range of 20°–80°C, whereas the Antares tools were calibrated at six temperatures in the range of 42°–80°C. Data from one typical example of each kind of tool are shown in Figure F5.

Comparison of bath and apparent tool temperatures (based on factory calibration) showed that the tools required additional calibration to achieve accuracy goals for the CORK experiments (ideally ≤0.01°–0.02°C). The Onset tools were generally within 0.01°–0.3°C of the bath temperature across the calibration range, but some tools were off by as much as 1.15°C. The Antares tools were within 0.15°C of the bath temperature across the calibration range. Individual tools of both kinds exhibited systematic bias to their miscalibration, with offsets generally increasing in magnitude at higher temperatures. We crossplotted apparent tool and bath temperatures and fit second- and third-order polynomials to the data, to allow calibration corrections to be applied after data were recovered. This reduced the typical residual temperature error to 0.01°–0.03°C for the Onset tools and 0.001°–0.003°C for the Antares tools.

Autonomous temperature tools were deployed in two configurations with the CORK instrument strings (Fig. F6): inside polyvinyl chloride (PVC) housings containing OsmoSamplers and microbiological growth systems and on Spectra line above these systems.

Fluid sampling and in situ tracer injection

The CORK fluid sampling program comprised two approaches: (1) pumps placed at depth below the CORK seals and (2) pumps placed on the CORK head. The first kind of system allows fluid to be collected within a borehole at in situ temperature, pressure, and chemical conditions but requires opening of the CORK seal to recover the samples. The second kind of system uses valves and small-bore tubing to draw fluids from depth, making it possible for samplers on the CORK head to be recovered and redeployed using a submersible or ROV. The heart of each of these sampling systems is one or more OsmoSamplers.

OsmoSamplers are designed to sample fluid continuously for a specified time, using osmotic pressure created across a semipermeable membrane (by solutions of differing salinity) to continuously draw sample through a small-bore tubing (Jannasch et al., 2004; Theeuwes and Yum, 1976). The rate of pumping depends mainly on the osmotic gradient (held constant over the life of the instrument) and temperature (which varies slightly in crustal boreholes and is monitored continuously). Sampling systems are designed to maintain a saturated salt solution (noniodized table salt; NaCl) with excess salt on one side of the membrane and distilled water on the other side. The distilled water reservoir is connected to small-bore tubing that is initially filled with distilled deionized water, thus maintaining its salt free condition. OsmoSamplers have been deployed and recovered successfully in a variety of settings, including estuaries, seamounts, seafloor spreading centers, and deep ocean boreholes (e.g., Jannasch et al., 2003; Spinelli et al., 2002; Wheat et al., 2003, 2000). OsmoSamplers work well for long-term deployments in these settings because they require no external power and have no moving parts.

Borehole OsmoSamplers

Borehole OsmoSampler systems (Fig. F7) were designed to pass through drill pipe and the inner CORK casing, including the smallest restriction (3 inches) at the bottom plug seat. OsmoSamplers also had to be recoverable using a relatively light weight deep-sea vehicle and leave the hole accessible for future deployments. We designed the units to be light by minimizing the amount of steel in the structure, allowing the units to be lifted by one or two people on deck, and with buoyant floats during seafloor recovery. The latter is important for avoiding the time-intensive handling of a rope-pulling system from a surface ship.

An additional design constraint was that fluids were to be sampled continuously for 5 y. Samples are collected either in 1.2 mm internal diameter (ID) Teflon or copper tubing. Teflon tubing is used to collect fluids for dissolved ion and isotopic analyses; copper tubing is used to collect fluids for dissolved gas analyses. The number of osmotic pumps used in each downhole system was selected on the basis of earlier results, a desire to collect ~3.5 mL of fluid per week, and practical limitations on total tubing and system length. Individual sections of tubing are coiled and joined, with as much as 912 m of tubing capable of collecting 1.0 L of fluid.

Downhole OsmoSampler systems were designed to survive for 5 y at 65°–75°C within hydrothermally altered formation fluid. This estimated temperature range is based on earlier studies of upper basement conditions in Hole 1026B (e.g., Davis and Becker, 2002; Fisher et al., 1997); higher temperatures are possible deeper within the crust if vigorous convection is limited to uppermost basement. The materials chosen for fabrication of downhole OsmoSamplers are compatible with deployment in formation fluids that are reduced, are depleted in dissolved oxygen and nitrate, have no measurable hydrogen sulfide and a near-neutral pH, and contain ~20% of the dissolved CO₂ found in bottom seawater in this area (Wheat et al., 2004).

On the basis of these constraints and expected conditions, we used a 3 inch outer diameter (OD) high-density polyethylene (HDPE) sleeve to protect OsmoSampler pumps and sample coils, acrylic to house the pumps, ¾ inch OD PVC schedule 80 to protect the inner diameter of the pumps and coils, a ½ inch grooved steel rod for a strength member, HDPE and steel endcaps, and ½ inch galvanized steel shackles with plastic isolators to join the units with the stainless steel bottom seal plug and sinker bar (OD = 3 inches; length = 12 ft; weight = 280 lb in air and 250 lb in water). An HDPE sleeve was used rather than polycarbonate because a polycarbonate sleeve used to house the Hole 1027C OsmoSampler deployed during Leg 168 shattered during recovery, exposing the sample coils and pumps. Acrylic was used to house the membranes because its thermal expansion coefficient is similar to that of water. A steel strength member was used to provide structural support and should survive for 5 y in a reduced environment. In fact, the CORK body that was recovered from Hole 1026B after 8 y was in excellent shape, as was the stainless steel sinker bar recovered after 3 y from Hole 1027C (Wheat et al., 2003). Galvanized shackles were used because of their strength and corrosion resistance.

Four different kinds of OsmoSampler units were constructed for Expedition 301: gas sampling, microbiological, tracer injection, and acid addition. The number and type of each sampler deployed in each hole during Expedition 301 are discussed later. The gas sampling system includes copper tubing arranged in three coils (each 304 m long) and a single pump.

The microbiology units consist of four pumps, each attached to a PVC flow cell that contains four mineral samples intended to serve as growth or attachment substrate. Given the desired flow rate, the number of materials and controls to be tested, and limitations on system length, it was not possible to have the pumps filled by a distilled water reservoir of sample coils. Instead, the distilled water reservoir was enlarged so that a strong gradient could be maintained throughout the experiment between the saturated salt solution and the formation fluids drawn into the reservoir. Design of these microbiological systems, including selection of substrate, is discussed in greater detail in the next section.

Tracer injection systems were designed to help assess rates of fluid exchange within isolated intervals of the boreholes. Anticipated exchange rates in boreholes below Expedition 301 CORKs were estimated on the basis of rates determined below CORKs in Holes 1024C and 1027C (Wheat et al., 2003). We considered these earlier estimates to be lower bounds on borehole exchange rates in Holes U1301A and U1301B, perhaps by two orders of magnitude, because of the greater crustal intervals of high-permeability rock exposed in the latter holes. Each Expedition 301 tracer injection system consists of three Teflon sample coils connected to a pump with the outflow connected to three additional Teflon coils. The latter three coils were each filled with a tracer (Table T3). We used Cs, Rb, and rare earth elements (REE) as tracers because of their low concentrations in seawater and basement formation fluids in this area, their ease of measurement using conventional analytical techniques (e.g., inductively coupled plasma–mass spectrometry [ICP-MS]), and their long-term stability within Teflon sample coils. Tracer concentrations were chosen to provide a measurable chemical anomaly for at least one of three elements given a reasonable range of volumetric exchange rates within the monitored intervals (Fig. F8). Different tracers were used in the different holes so that we could determine whether crossinterval or crosshole exchange occurred during the monitoring period.

The acid addition systems acidify samples during collection in order to minimize precipitation and microbial growth, either of which might clog the sample tubing. These systems comprise two pumps and four Teflon coils. An acid addition pump uses membranes having a smaller surface area, made of a different material, resulting in a flow rate ~10% of the rate generated by the sampling pump. The outflow from the acid addition pump is attached to a Teflon coil that is filled with 40 mL/L of subboiled 6N HCl. This coil is joined by a tee with the three Teflon sample coils and the sample pump. The third arm of the tee is open to the formation. The sample is diluted by 10% as a result of the acid addition.

CORK head OsmoSamplers

OsmoSamplers placed on the CORK heads were designed to fit in one of three bays (Figs. F2, F9). Major issues in the design of these systems included the type and size of tubing that connects the hydraulic horizon to the seafloor, ability to maintain pressure and redox conditions of the sampled horizon, length of screened horizons, size of the bays within the CORK body, dead space between connections, longevity of hardware fixed to the CORK, metal compatibility, and fluid sampling rate.

A particular hydraulic horizon is connected to the seafloor via stainless steel tubing. Each of these stainless steel tubes is connected to a stainless steel miniscreen within the hydraulic horizon and a stainless steel union/reducer on the CORK body at the base of the chemistry bay. Several diameters of tubing were used, depending on the umbilical available. Larger (≥¼ inch) tubing is desirable for holes that are overpressured and producing because this will allow fluid production with minimal head loss or clogging. Small bore tubing (≤¹⁄₁₆ inch) is necessary for holes that are underpressured to minimize the amount of water that must be pumped at the seafloor before sampling in situ fluids.

The fluid sampling bay of each Expedition 301 CORK can accommodate up to three CORK head OsmoSampler units, one for each of the three separate hydrologic intervals (Fig. F9). Even though some Expedition 301 CORKs were intended to sample fewer than three isolated intervals, three OsmoSampler locations were included on each CORK head for redundancy and consistency of design. A CORK head OsmoSampler unit starts at the connection between the plumbing from the hydraulic horizon to unions/reducers at the base of the chemistry bay. Small bore (¹⁄₁₆ inch) stainless steel tubing was run from these unions/reducers to ¹⁄₁₆ inch two-way stainless steel (T-handle) valves that are closed when horizontal and open when vertical (clockwise 90° to close). These valves are connected by small-bore polyetheretherketone (PEEK) or stainless steel tubing (¹⁄₁₆ inch) to titanium nipples. We used different tubing materials on different CORK intervals to test the long-term stability of PEEK versus stainless steel in this environment. The nipples have a ¹⁄₁₆ inch hole drilled into the side. The nipples and the plastic sheet they are fastened to are designed to connect to a corresponding plate supporting an OsmoSampler. To allow OsmoSampler replacement by submersible or ROV, we designed plastic stabilizers to guide the plate into position. The plate is locked in place with a T-handle. The locked position is horizontal, rotating 90° in either direction (to vertical) to unlock.

The OsmoSampler plate holds an OsmoSampler and connects it to the formation via plastic covers that fit over the small-bore hole in the titanium nipples. The holes are surrounded by two O-rings, providing a seal that holds at pressures >500 kPa, considerably greater than differential pressures anticipated in the isolated intervals of Expedition 301 CORKs. Each OsmoSampler plate covers two titanium nipples, each of which samples the same depth horizon but uses different tubing. This allows the sampler to draw fluid from one nipple and expel fluid (outflow or waste brine) to the other nipple while maintaining in situ pressure, a capability that will be useful for CORK observatories isolating underpressured systems. We did not use this capability during Expedition 301 because we did not deploy a CORK in Hole 1027C, which is known to be underpressured.

CORK head OsmoSamplers fit within the bay and do not extend beyond the protective gussets. This limits the diameter of the sampler and the number of membranes that can be used. We were able to fit 12 membranes and ~1000 ft of 1.2 mm ID Teflon sample tubing within design specifications. These samplers will pump ~4.0 mL/week at in situ temperatures. The size restriction is only necessary for deployment; samplers deployed by submersible or ROV after CORK deployment need not fit completely within the CORK head bay.

Microbiological sampling and growth experiments

The establishment of subseafloor observatories within upper oceanic crust offers an unprecedented opportunity to investigate microbial processes in basement. The CORK installation at Hole 1026B yielded some of the first evidence of active microbial communities in igneous basement of the upper oceanic crust (Cowen et al., 2003), but those samples were collected at the seafloor, after fluid had passed through long sections of casing and tubing and interacted extensively with steel and other materials (Wheat et al., 2004). CORK systems deployed during Expedition 301 include in situ growth experiments to investigate microbial colonization and alteration of minerals. Many researchers have used textural observations to investigate the role of microbiology in alteration of oceanic crust (e.g., Fisk et al., 1998, 2000; Furnes and Staudigel, 1999; Staudigel et al., 1998; Thorseth et al., 1995). The in situ microbiological growth experiments designed for Expedition 301 are intended to enable better characterization of the rate of microbial alteration of minerals and to investigate the role of mineralogy in controlling microbial alteration. These experiments comprised two kinds of systems: (1) passive experiments in which fluids are allowed to pass over polished sections of various rock or mineral samples located inside a perforated HDPE sleeve between OsmoSamplers and (2) flow cells in which fluids are pumped across rock and mineral samples using OsmoSamplers (Fig. F10).

For the passive experiments, polished samples of basalt, dunite, biotite, and pyrite, each ~0.5 cm² in exposed surface area, were mounted on plates (Fig. F10A). For each experimental apparatus, four plates were attached to a holder that fit over the inner strength member holding a set of downhole OsmoSamplers. All components of these assemblies, including screws, were composed of polycarbonate and were autoclaved prior to shipping to port.

The flow cell experiments use downhole OsmoSamplers to pump crustal fluid though chambers packed with various minerals. Each flow cell contains four chambers filled with crushed and sieved mineral samples (Table T4). Fluid enters the flow cell through a 0.12 mm ID Teflon inlet tube and passes through each of the mineral packages in series (in the order shown on Table T4). The flow cells were designed and assembled in pairs such that each biotic experiment is accompanied by an identical unit with the addition of a 0.2 µm filter on the inlet. The filter makes the paired flow cell an abiotic control that will allow us to better distinguish between biotic and abiotic alteration textures of the minerals. Where space allowed, there is a dedicated OsmoSampler for each flow cell. In a few cases, two flow cells are attached to one OsmoSampler in series. In series installations, the crustal fluid passes through all four chambers of the biotic flow cell, through the filter, and into the abiotic flow cell (Fig. F10B).

The flow cell housing is constructed of PVC, and the cells are plumbed in series with 0.12 mm ID Teflon tubing. Minerals are packaged in plastic tubes with foam plugs at each end to act as coarse particle filters and flow diffusers. Tubing and mineral packages were autoclaved prior to deployment. Flow cell housings were rinsed in deionized water and wiped with ethanol. The flow cells were preloaded with filtered seawater (0.2 µm) to ensure that they did not contain air bubbles when deployed. Natural seawater was used rather than deionized water because we wanted to achieve relatively even salinity throughout each cell for the duration of the experiment.

In addition to the downhole growth and alteration systems described above, uphole microbiological sampling capabilities were designed into the Expedition 301 CORK systems. One of the three sampling and monitoring bays on each CORK head was dedicated to microbiological and other sampling. No CORK head microbiological sampling systems were prepared for use during Expedition 301 CORKs; microbiological sampling at the seafloor will occur after formation pressures and chemistry recover from the disturbance resulting from drilling. When pressures have recovered and these CORK systems are vented for extensive periods in the future as part of long-term hydrogeologic flow experiments, there will be excellent opportunities to sample formation fluids for microbiology.

As described earlier, dedicated microbiological sampling tubing was purchased for use with CORKs to be placed in Holes U1301B and 1027C, the holes thought to provide the best opportunity to collect relatively pristine microbiological samples. This line is composed of ½ inch Tefzel inside braided wire and an outer layer of plastic (Fig. F4). One set of pass-throughs that penetrate the upper CORK seal and each of the CORK casing packers was constructed from titanium fittings and tubing so as to minimize opportunities for microbial reactions during passage of fluids from depth to the seafloor. In addition, in Hole U1301B, the section of 4½ inch CORK casing located next to the small-diameter screen through which microbiological samples are to be collected was sheathed in plastic shrink-wrap tubing prior to deployment. The microbiological sampling valve and fitting in the CORK heads for Holes 1026B and U1301A are connected to stainless steel sample tubing. These holes are contaminated by a relatively high area of exposure to metal, grease, and other drilling materials (particularly Hole 1026B, which includes steel casing and liner through the entire basement section), so use of the expensive, dedicated microbiological line was not justified.

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