Collaborative Research: Continental Shelf Geodesy: Continued Development of a Low Cost Sea Floor Geodetic System Based on GPS

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Existing technology for sea floor geodesy (measuring small motions of the sea floor) has limitations in terms precision and cost. This is especially true in the shallow continental shelf environment, where highly variable salinity, temperature and density of the ocean cause problems for acoustic ranging and sea floor pressure techniques. Potential applications of the proposed system include monitoring of underwater volcanoes, offshore oil and gas fields, and the shallow offshore portion of subduction zones. Measuring strain accumulation and release processes in the shallow offshore region of subduction zones is especially important because it has the potential to improve our understanding of giant subduction zone earthquakes and tsunamis, as well as improve our ability to forecast these catastrophic events.The proposed design is based on a successful Italian concept that uses high precision GPS mounted on a semi-rigid structure and moored to the sea floor. That system has been successfully tested up to 140 meters water depth on the flanks of an active volcano, but only acquired vertical component data. The Italian design has been modified to reduce costs and enable measurement of the full three-dimensional displacement vector. Initial tests have demonstrated that a simple rigid spar design is suitable for water depths up to 40 meters. A steel spar is attached by shackle to a heavy seafloor anchor and is kept near vertical by a near-surface float. GPS on top of the buoy measures instantaneous position, while tilt and heading sensors allow correction for buoy motion, enabling daily estimates of the position of the sea floor anchor precise to 1-2 cm. The new research proposed here aims to develop and demonstrate a deeper water version, suitable for water depths up to 150 meters, by adding a cable with orientation sensors to the buoy.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.