NEPTUNE Canada: Project Details

Project Details

Ocean Crustal Hydrogeology

Principal investigator(s)

Kelin Wang & Earl Davis

Co-investigators

Earl Davis (Pacific Geoscience Centre, Geological Survey of Canada), Alison LaBonte (NEPTUNE, University of Victoria), Andrew Fisher (Department of Earth and Environmental Sciences, University of California at Santa Cruz), Keir Becker (Rosenstiel School of Marine and Atmospheric Science, University of Miami), Heiner Villinger (Department of Earth Science, University of Bremen), Geoffrey Wheat (National Oceanic and Atmospheric Administration, University of Alaska at Fairbanks), Kelin Wang (Pacific Geoscience Centre, Geological Survey of Canada), Martin Heesemann (Pacific Geoscience Centre, Geological Survey of Canada)

Discussion & Collaboration

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Abstract

Deep-ocean boreholes provide unique opportunities for determining the physical, chemical, and biological state and properties of water-rich sub-seafloor formations, and the tectonic setting and lithologic structure of the northern Juan de Fuca plate offers an ideal situation for borehole hydrologic observations. All types of active plate boundaries - divergent, convergent, and transform - are present at the margins of this plate. The scale of the Juan de Fuca Plate system is also optimal for studying interrelationships of processes within the plate and along its boundaries. For example, simultaneous observations of borehole pressure signals and seismicity over the Juan de Fuca Plate may reveal the relationship between dynamic processes such as episodic plate motion, internal plate strain, and earthquakes. Finally, the plate, including parts of the ridge axis, is blanketed by low-permeability turbidite sediments which form a barrier to the exchange of water between the highly permeable igneous crust beneath the sediments and the ocean overhead. This confinement facilitates quantitative experiments that are not possible in sediment-free settings, and the small scale of the plate permits meaningfully related measurements from the ridge to the subduction zone.

To take advantage of the setting, ten holes have been drilled across the Juan de Fuca Plate between 1991 and 2003 by the Ocean and Integrated Ocean Drilling Programs (ODP/IODP) expressly for the installation of "CORK" (Circulation Obviation Retrofit Kit) hydrologic observatories for long-term formation temperature and pressure monitoring, fluid sampling, active hydrologic testing, and microbiological sampling and incubation. Experiments carried out to date have provided a wealth of information about the driving forces for flow generated by thermal buoyancy and tectonic consolidation, the rates and direction of flow through sedimentary and igneous crustal sections, the hydrologic and elastic properties of these sections (e.g. permeability and compressibility), and the age and composition of formation fluids. Original goals of these experiments have been surpassed to the point that many new objectives have been added for recent and future installations. Cross-hole tests have been designed to discover the details of fluid flow pathways within the igneous crust. Borehole pressure and temperature measurements are now being used as quantitative indicators of oceanographically, tectonically, and seismically stimulated transient formation strain, fluid flow, chemical reaction, and microbiological nutrient supply.

To date, monitoring at all of the borehole sites has been carried out autonomously, with power supplied by batteries and stored data accessed periodically by submersibles and ROVs. We anticipate eventually connecting each to NEPTUNE, which will eliminate current constraints on sampling frequency imposed by power, data storage, and time accuracy. This will enable temporal changes in state to be determined up to seismic frequencies, and thus greatly enhance our ability to address new fundamental questions about the dynamics of the oceanic lithosphere and the subseafloor hydrosphere.

Study Sites/Locations

As a first step, a NEPTUNE node will be established near ODP Site 1027 situated roughly half way between the Juan de Fuca Ridge axis and the Cascadia subduction zone accretionary prism. In this area, autonomous monitoring is underway in four instrumented holes; this will be expanded to include two additional holes in the near future. The existing Holes 1026B, 1027C, 1301A, and 1301B penetrate the sediment section which ranges from 247 m to 614 m in thickness, and extend 50 to 330 m below the top of the upper igneous crust. Initial NEPTUNE monitoring will include a single borehole, Hole 1026B, instrumented for monitoring seafloor and formation pressures and temperatures, and an instrument cluster on the seafloor nearby were there will be a broad-band seismograph, hydrophone, current meter, and salinity sensor. Pressure and temperature sensors in other boreholes that are completed with "CORK" seals to allow monitoring and sampling in an unperturbed formation will be connected at a later time. In addition to its primary purpose for monitoring formation signals, the pressure instrumentation at Hole 1026B will provide a central monitoring point in a surrounding triangular array of bottom pressure, temperature, and salinity instruments for detecting tsunamis and studying long-wavelength oceanographic phenomena. At some borehole sites, experiments will involve more active use of holes penetrating the crust for fluid sampling, injection testing, and microbiological incubation. Initial and future borehole observations will link closely with monitoring planned by seismology, tsunami, geodetics, hydrates, physical oceanography, and electromagnetics groups.

Equipment Summary

The primary instrumentation for pressure and temperature monitoring includes high-fidelity sensors for monitoring crustal and/or sedimentary formation pressure and seafloor water temperature and pressure. Instruments built for already established and new boreholes are virtually identical to ones built for Tsunami monitoring. As part of the development of the pressure recording instruments for these experiments, a new digitizer has been developed that provides pressure and temperature resolution of better than 0.4 Pa (0.04 mm of equivalent water head) and 0.1 mK, respectively, at 1 Hz frequency (see attached instrument description). This unprecidented resolution will provide a new view into the hydrologic response to loading caused by ocean waves, tsunami, earthquake waves, and geodynamic strain.

How this project will foster collaboration

Close ties already exist among a broad range of investigators working with the borehole and seafloor data recorded to date with autonomous instruments (e.g., hydrogeologists, geochemists, oceanographers, and microbiologists). These ties will be strengthened, and new collaborations will be made possible as NEPTUNE moves the "bandwidth" of observations to much higher (e.g., seismic) frequencies and longer periods (multiple decades) than are currently possible. With the increased efficiency of operations, more sites can be supported than have been in the past with autonomous, submersible-serviced installations. Additional sites will make possible observations of coherent signals that have a characteristic scale of tens to hundreds of km, and it will awaken a wide variety of researchers to the importance of plate-scale phenomena over a broad frequency range.