Mission results from the first GEOSTAR observatory (Adriatic Sea, 1998)

We assess the first mission of the GEOSTAR (GEophysical and Oceanographic STation for Abyssal Research) deep-sea multidisciplinary observatory for its technical capacity, performance and quality of recorded data. The functioning of the system was verified by analyzing oceanographic, seismological and geomagnetic measurements. Despite the mission's short duration (21 days), its data demonstrated the observatory's technological reliability and scientific value. After analyzing the oceanographic data, we found two different regimes of seawater circulation and a sharp and deepening pycnocline, linked to a down-welling phenomenon. The reliability of the magnetic and seismological measurements was evaluated by comparison with those made using on-land sensors. Such comparison of magnetic signals recorded by permanent land geomagnetic stations and GEOSTAR during a "quiet" day and one with a magnetic storm confirmed the correct functioning of the sensor and allowed us to estimate the seafloor observatory's orientation. The magnitudes of regional seismic events recorded by our GEOSTAR seismometer agreed with those computed from land stations. GEOSTAR has thus proven itself reliable for integrating other deep-sea observation systems, such as modular observatories, arrays, and instrumented submarine cables