Acoustic Modems Installed at MoMAR Observatory

In October 2010, a collaboration between the French Research Institute for Exploitation of the Sea (IFREMER) and the Institut de physique du globe de Paris (IPGP - The Institute of Earth Physics of Paris), the MoMARSAT expedition, installed an array of oceanographic instruments at the Lucky Strike hydrothermal field offshore the Azores. The newly established MoMAR observatory will enable continuous monitoring of active volcanic and seismic processes, helping the scientists to better understand the dynamics of mid-ocean ridge hydrothermal systems and study the peculiar local fauna.

MoMAR is a twelve-month demonstration project, aimed at exploring the opportunities of fundamental research that involves multiple disciplines, ranging from geophysics to microbiology.

IFREMER initially developed the observatory's subsea components, the two SEAMON nodes, in 2002. As the SEAMON stations are challenged to autonomously operate at the sea-bed for the whole duration of the MoMAR project, IFREMER decided to replace the outdated underwater acoustic modems, transferring SEAMON data to the surface. The energy efficiency, bits of correctly transmitted information per joules of consumed energy, was a crucial factor for the upgrade.

IFREMER contacted thirteen acoustic modem manufacturers to take part in the benchmark programme. Among them, three participated in a two-week test cruise in 2008, two of the modems being later installed on the sea-bottom for a longer (seven months) testing cycle in 2009-2010.

After long-term trials, EvoLogics S2C underwater acoustic modems were approved for application as the most efficient and reliable solution. During their mission, the MoMARSAT team successfully installed EvoLogics S2C modems at the depth of 1700 m. The established acoustic sensor network comprises two modems that transmit data from the SEAMON units to a modem at the surface buoy, connected to the shore via satellite.

EvoLogics S2C acoustic modem units started operating at the Mid Atlantic Ridge on October 12, 2010, transferring around 100kB per day from the SEAMON East station and around 3.5 kB from the SEAMON West. The communication sessions between the sea-bed and the surface buoy take place every 6 hours. The modems' performance, being the key component of the project's success, has already received positive feedback from the MoMAR team.