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doi:10.2204/iodp.sp.328.2010

Observatory objectives and design

Motivation

As a result of this failure, the objectives of the CORK monitoring planned for Leg 146, namely to document the average pressure state of the prism created by tectonic strain and gravitational loading, the pressure gradient driving fluid expulsion and gas migration, and the thermal profile as a constraint on the vertical velocity of interstitial fluid flow, have never been met; hence, they continue to provide motivation for an observatory installation at Site 889. In addition, many advances have been made with monitoring experiments in other settings since the time of Leg 146 nearly 18 y ago, and these provide new elements of motivation. From a scientific perspective, long-term monitoring experiments at a number of sites in tectonically active settings (Juan de Fuca Ridge axis and flank, Mariana forearc prism, Costa Rica forearc prism, and Nankai accretionary prism) have revealed that formation pressure variations provide a quantitative proxy for volumetric strain. Transient events related to coseismic, postseismic, and aseismic deformation have been seen at all of these locations, and the observations are leading to a new understanding about the episodic nature of deformation, seismic energy efficiency, and regional interseismic strain accumulation. An example from the Nankai Trough (Fig. F6) shows the pressure response to coseismic, postseismic, and secular interseismic strain. With the relatively high frequency recording and precise timing possible with a NEPTUNE cable connection and the high resolution provided by current sensor technology, it is also possible to observe formation strain associated with seismic ground motion. This has been documented in numerous instances (Fig. F7).

We anticipate that signals similar to these examples will be present at Cascadia. An illustration of the likelihood of their presence is provided by the combination of Figures F8, F9, F10, and F11. Figure F8 shows Site 889 to be surrounded by high seismic activity. To the northwest of the site, strike-slip events are concentrated along the Nootka fault, the strike-slip boundary between the Juan de Fuca and Explorer oceanic plates. Along the continental margin, intraplate events occur in the overriding continental crust and in the oceanic crust of the subducting Juan de Fuca and Explorer plates (Fig. F8A). Further landward, seismic tremor occurs episodically along and above the top of the subducting plate, downdip of the thrust seismogenic zone (Fig. F8B). No events have yet been identified on the currently "locked" part of the subduction thrust interface, although in other subduction zone settings (Nankai and Costa Rica), slow slip crossing the "locked zone" has been documented (Davis and Villinger, 2006; Heesemann and Davis, submitted). Figure F9 demonstrates the utility of using pressure as a proxy for strain by way of the reaction at a CORKed site on the nearby Juan de Fuca Ridge flank to two seismogenic strain events 100 to 150 km away. The recurrence statistics of intraplate and Nootka fault earthquakes within 150 km of Hole 889C (Fig. F10) and the regular occurrence of slip events downdip of the locked portion of the subduction thrust (Fig. F11) show that strain-related signals and instances of formation pressure response to seismic ground motion should be plentiful in a relatively short period of time.

Beyond these scientific considerations, a number of technical factors fortify the justification for a geophysical observatory at this site. The high reliability of CORK instrumentation has been demonstrated through successful long-term operations at many sites. Instruments deployed during ODP Leg 196 (Nankai Trough) have been operating for >7 y, and those deployed during ODP Legs 168 and 169 have been in operation for >13 y. Improvements in power consumption, memory capacity, and resolution now permit detection of much subtler signals than were previously possible. And in this instance, connection to the NEPTUNE observatory cable infrastructure will open up great opportunities. Much higher sampling frequency will be achieved, allowing observations to reach into the seismic frequency band (Fig. F7) and to be placed in context of colocated seismic and hydrologic records that are being collected with a broadband seismometer and a variety of seafloor vent monitoring instruments roughly 3.5 km from the Site 889 Advanced CORK (ACORK) borehole observatory.

Observatory configuration

Most CORK installations to date have been configured to meet a broad suite of requirements, including passive geophysical monitoring, active hydrologic testing, and formation-fluid chemical and microbiological sampling (see reviews by Kastner et al., 2006; Becker and Davis, 2005; Fisher et al., 2005). Unfortunately, large perturbations can occur when fluids are allowed to be produced from the formation, particularly when monitoring screens are situated in low-permeability material. Direct effects arise from any pressure drop associated with production, and indirect effects arise from thermal perturbations caused by flow (see discussion in Davis and Becker, 2007). The latter can be caused by the anomalous buoyancy of the water in the umbilical screens that connect to the seafloor sensors and from transient thermal expansion of the fluid in the umbilical and screens that is confined by the low-permeability material surrounding the screens. To avoid these problems, this observatory will be devoted to passive geophysical monitoring exclusively; fluid sampling and active experiments, specifically those proposed in Proposal 553-Full2 (www.iodp.org/597/), will be carried out at a later date in separate holes. It is our hope that justification for paired monitoring and sampling holes will be provided by the early results of this expedition's passive monitoring effort; we anticipate that the Expedition 328 efforts will be neither redundant nor conflicting but fully complementary with those of a more extensive future program.

The primary components of the ACORK system to be deployed are shown in Fig. F12. Four screens will be centered at depths of 155, 205, 245, and 295 mbsf; two are above and two are below the gas/gas hydrate boundary at 225 mbsf, and all are within the accretionary prism lithologic unit (Fig. F4). Screens will be virtually identical to those used for Leg 196, with 7.6 m long sections of filter screens built on standard 11.24 m long 10¾ inch diameter solid casing joints. Carbolite (aluminum oxide ceramic) "sand" is packed in the ~2 cm annulus between the casing and a screen formed of wire wrapped around and welded to radial webs.

Formation pressure signals are transmitted to seafloor sensors via ¼ inch outer diameter 0.035 inch wall 316 stainless steel umbilical tubing. Packers between screens will not be used at Site 889; based on results from the ACORK at ODP Site 808 and previous experience at Site 889, we are confident that hole collapse will provide a good seal between the ACORK casing and the formation.

Pressure monitoring instruments will be installed in the wellhead frame on the ship and deployed with the ACORK casing string. Underwater-mateable hydraulic connectors will allow the instrument package to be removed and replaced in the event that repairs or service are ever required. Gas will be purged from the umbilical tubing through lockable check valves at the highest point of the wellhead plumbing. Three-way valves will connect the umbilical lines to the instrumentation: in the "formation" position, these will connect the formation to the sensors; in the "hydrostatic" position, the formation lines will be closed and the sensors will be connected to the local ocean. The logging instrumentation will include individual sensors (Paroscientific 8B 4000-2 quartz transducers) to monitor pressures at the seafloor and at each of the formation screens. Frequency output from these will be digitized with high-resolution (~10 ppb full scale = 0.4 Pa) low-power "Precision Period Counter" cards (Bennest Enterprises, Ltd.), and stored in MT-01 flash memory (Minerva Technologies, Ltd.). The records shown in Figure F7 were recorded by an identical unit. Bottom water temperature will be determined from a temperature-sensitive frequency channel of the Paroscientific sensors and with a highly stable platinum thermometer. Battery capacity will be sufficient to power the system for roughly 10 y at a sampling period of 10 s. An onboard voltage detection circuit will switch the system into a high-rate (1 Hz) sampling mode and idle the batteries when external power from a NEPTUNE connection is made (anticipated within the first year of operation).

At the top of the ACORK wellhead structure will be a ~30 inch diameter reentry cone, needed for installation of the bridge plug in the 10¾ inch casing. This will later facilitate submersible-controlled wireline installation of deep instrument packages, including a thermistor cable, tilt sensors, and a seismometer for low-frequency seismology.

Screen spacing

The rationale for the ACORK screen configuration deserves some discussion. At the simplest level, multiple monitoring points will allow determinations of the average vertical pressure gradient generated by prism thickening and driving vertical fluid flow and the contrast in gradient between the section above and below the level of gas hydrate stability associated with a contrast in permeability if one exists. The combination of the 7.6 m length of the screens and their ~50 m separation should make such gradient determinations relatively insensitive to localized heterogeneities associated with fractures, turbidite layering, and lenses of massive hydrate accumulation.

Data from below and above the gas/gas hydrate boundary will also constrain the contrast in mechanical properties of gas and gas hydrate–bearing sediments and provide independent information about the effective permeabilities of the sections above and below the boundary. The way this can be done is summarized in Figure F13, which begins with a schematic illustration of how variable loading either at the seafloor (e.g., tides and ocean waves) or within the formation (tectonic strain and seismic waves) is transmitted to formation pore water and how local contrasts in loading response causes transient pressure gradients to be established (Fig. F13A). The instantaneous (elastic) response to seafloor loading = γ (referred to as the loading efficiency) (Fig. F13B) depends on porosity, Poisson's ratio, the compressibility of the solid grain constituents, the compressibility of the sediment or rock framework, and the compressibility of the interstitial fluid or fluid + gas mixture. With the first three of these being well known, absolute values and contrasts in observed loading efficiency can be used to constrain the effects of gas on the elastic properties of the fluid (and hence gas content) and the effect of hydrates on the elastic properties of the matrix (and hence average hydrate content).

In simple cases where a sharp mechanical properties contrast is present (e.g., at the seafloor or at the gas/gas hydrate boundary), a transient pressure gradient will be established and interstitial water will flow (Fig. F13A). A damped diffusional wave will propagate away from the boundary, adding a component to the signal (Fig. F13B) that decays with distance (Fig. F13C). At large distances, the response is purely elastic (γ) and constrains such things as the matrix compressibility, the gas content (Fig. F13D), and the coefficient that defines how tectonic deformation loads the interstitial water (Fig. F13E). At intermediate distances, the characteristic diffusion scale length, l, (Fig. F13C) depends on the hydraulic diffusivity of the formation, η, and the period of the loading signal, P, as l = (π η P)½. The broad bandwidth of ocean wave and tidal loading, for which periods range from seconds to weeks, combined with the distribution of the screens around the gas/gas hydrate boundary, should allow much to be learned about the formation elastic and hydrologic properties.

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