Shallow Water Seafloor Geodesy: GPS on An Anchored Spar Buoy

Measuring seafloor motion in shallow coastal water is challenging due to strong and highly variable oceanographic effects. Such measurements are potentially useful for monitoring near-shore coastal subsidence, subsidence due to petroleum withdrawal, strain accumulation/release processes in marine shelves and submerged volcanoes, and certain fresh water applications, such as volcano deformation in caldera-hosted lakes. I participated in a project to develop a seafloor geodetic system for this environment based on an anchored spar buoy topped by high precision GPS. Orientation of the buoy is measured using a digital compass that provides heading, pitch, and roll information. The combined orientation and GPS tracking data are used to recover the three-dimensional position of the seafloor marker (anchor). A test system has been deployed in Tampa Bay, Florida, for over one year, and has weathered several major storms without incident. Even in the presence of strong tidal currents which can deflect the buoy several meters from vertical, daily repeatability in the corrected three- component position estimates is 1–2 cm or better. Except for the rapid motion during the first month after deployment due to settling, other large anchor displacements correspond to extreme weather events, and are likely associated with current-induced scour activity