OHA-SIS-BIO(Observatory of HydroAcousticity from SISmicity and Biodiversity in the Indian Ocean) is a long-term hydroacoustic program for monitoring the seismic activity and the vocal activity of large marine mammals in the southern Indian Ocean. This cruise was dedicated to the final recovery of the OHASISBIO network of autonomous hydrophones deployed since 2010 between La Réunion, Crozet, Kerguelen and Amsterdam islands, taking advantage of the yearly journey of R/V Marion Dufresne to these southern islands (9 moorings). Instruments will now be updated and the mooring lines checked and refit (on shore).

The hydrophone array was designed to continuously monitor the earthquake activity associated with 3 contrasted spreading ridges (16 to 70 mm/yr) and with the intraplate deformation in the southern Central Indian Basin.  This approach proved very effective for detecting and locating low magnitude (>2.5) earthquakes, which are not recorded by the land-based seismological networks, and for deciphering magmatic from tectonic events.  The objectives were to characterize the seismic climate of mid-oceanic ridges with ultra-slow, slow and intermediate spreading rates, and the temporal and spatial distribution of the intraplate deformation, and to test the occurrence of low-magnitude precursors prior to large earthquakes along active sub-marine transform faults.

This observatory jointly monitored the vocal activity of large baleen whales. The analysis of our hydroacoustic records showed that their typical acoustic calls can be used as a proxy for monitoring the abundance and seasonality of 5 populations of blue whales. The overall objective was to improve our general knowledge on the presence, abundance and migration pattern of large-mammal endangered species over several year cycles in the southern Indian Ocean.

Our data also displayed unexpected information on the sea-state and icebergs (calving, collision, dislocation). These two applications also require long time-series to validate sea-state prediction models or to monitor the evolution of sound sources (natural or man-made).

These objectives had in common to use hydroacoustic records in the same low frequency-band (< 120 Hz) and to require continuous time-series as long as possible to be representative of the seismic regime, mammal activity or ambient noise changes in these remote areas.

Geometry of the OHASISBIO network of hydrophones (circles) since 2017 (SSWIR was not redeployed after 2019).

Example of a spectrogram and waveform of a 30 minute-long acoustic record acquired at the WKER site, April 29, 2018. One can see : the arrival of P and S seismic waves at 22h26m30s followed by energetic T-waves at 22h39 of a large earthquake; two other T-wave arrivals at 22h45 and 22h48 from two other earthquakes (barely visible in the waveform); a continuous and energetic frequency band between 18 and 26Hz corresponding to a continuous chorus of Antarctic blue whale calls; a continuous chorus at about 100Hz of fin whale calls; and a series of vertical lines corresponding to distant seismic airgun shots every 20s.