Seafloor geodesy in shallow water with 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 subduction zones and submerged volcanoes, and certain freshwater applications, such as volcano deformation in caldera-hosted lakes. We have developed a seafloor geodesy 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 1 year and has weathered several major storms without incident. Even in the presence of strong tidal currents which can deflect the top of the buoy several meters from vertical, daily repeatability in the corrected three-component position estimates for the anchor is 1-2 cm or better. Plain Language Summary To measure seafloor motion in shallow water, we built a spar buoy and put a GPS antenna and a digital compass (three-dimensional orientation sensor) on top of it. The buoy rests on the sea bottom using a heavy concrete ballast. Rotation and other movements of the buoy are measured by the digital compass and GPS. Position of the ballast can be calculated based on these measurements. We tested the system in Tampa Bay, Florida, and found that it is able to measure motion of the anchor with an uncertainty of 1-2 cm or smaller.