Foundational experiences and recent advances in long-term deep-ocean borehole observatories for hydrologic, geodetic, and seismic monitoring

Abstract

For nearly three decades, various phases of the scientific Ocean Drilling Programs have deployed sealed-hole observatories in deep-ocean boreholes for longterm subseafloor monitoring to address a range of hydrologic and geodynamic objectives. We summarize the scientific motivation for these observatories and review some important early results from those installed in young oceanic crust and subduction zones. We also summarize the evolution of the borehole observatory designs and associated instrumentation, from simple single-interval installations with autonomous low-rate temperature and pressure monitoring to recent multiple-zone installations with sophisticated downhole instrument packages connected to seafloor cabled networks that provide power and high-rate, real-time data access. We emphasize recent advances, illustrated with example data drawn mainly from transects of borehole observatories offshore Japan and Cascadia. These examples illustrate the value of borehole observatory data in resolving a wide range of crustal geodynamic responses from long periods of gradual geodetic change and accumulation of stress to episodes of rapid deformation associated with both seafloor spreading and subduction processes.