Hybrid Optical–GNSS/Acoustic Method for Centimeter-Precision Seafloor Geodesy in Shallow Water

About 80% of shallow (depth < 60 km) earthquakes with Mw > 6.5 worldwide occur on offshore faults, highlighting the need for seafloor geodetic measurements to monitor strain accumulation before, during, and after major events. The current state-of-the-art technique for measuring horizontal seafloor deformation is GNSS/Acoustic (GNSS/A), which provides episodic measurements of absolute seafloor motion with centimeter-level precision at any depth. However, widespread application of GNSS/A remains limited by three main constraints: (1) high operational cost; (2) the inability to leave acoustic transponders in shallow waters (< 500 m) because of trawling activity; and (3) the need for long acquisition sessions to average out poorly modeled sound-speed variability in the water column. Here we present a new approach suitable for shallow water (< 300 m) that potentially enables centimeter-level seafloor geodesy at reduced cost and with shorter acquisition times. The method combines high-resolution optical imaging of the seafloor acquired by low-cost autonomous underwater vehicles (AUVs) with GNSS/A surveys. Acoustic beacons are used as ground control points, analogous to aerial photogrammetry, allowing georeferencing of the optical mosaics in a global reference frame. Natural markers such as rocks, reefs, and outcrops can then be re-imaged over time to measure displacement. Compared to classical GNSS/A, this approach uses acoustic beacons only during the survey, enabling multiple seafloor points to be monitored within a single experiment using a limited number of transponders. We will present results from a proof-of-concept experiment conducted in autumn 2025 near Toulon, southern France.