This campaign aims to recover 16 Ocean Bottom Seismometers (OBS) at the Central Indian Ridge. The instruments record seismic activity at this slow spreading ridge where new ocean crust is being formed by phases of magmatic accretion and tectonic stretching.

These OBS have been deployed during a previous cruise of the German RV SONNE in December 2019. Originally, RV SONNE should recover them in September 2020 within research project SCIROCCO. However, due to the Corona pandemic, research vessels were ordered back home and projects were cancelled. Because the emergency time release of the OBS was scheduled to the meanwhile cancelled RV SONNE cruise, all OBS with their valuable data would automatically ascent to the sea surface, drift away, and be lost end of September 2020.

The French RV Marion Dufresne is the only European research vessel remaining in the Indian Ocean at La Reunion relatively close to the Central Indian Ridge. The Ocean Facilities Exchange Group (OFEG), an European oceanographic research organization which organizes barter exchange cruises between members, made it possible to conduct this OBS rescue mission in August 2020 on RV Marion Dufresne.

The OBS are set up in two arrays: a local array around the Kairei hydrothermal vent field and a regional array along the southernmost segment of the Central Indian Ridge, extending from the Kairei hydrothermal field in the north to the Rodriguez Triple Junction in the south, where three tectonic plates meet (Somalia, Indo-Australia, and Antarctica).

The local array around the active Kairei hydrothermal field shall resolve the origin of hydrothermal activity and image pathways of the fluid circulation. The regional array along the first segment of the Central Indian Ridge shall resolve active zones of magmatism including the Rodriguez Triple Junction.

The cancelled research project SCIROCCO (Seismic Imaging at the Central Indian Ridge: Structure and Formation of Oceanic Core Complexes) will be conducted with RV SONNE in future years. This experiment will expand the passive seismic experiment by reflection seismics with an 8 km long hydrophone streamer and refraction seismics including two OBS profiles and two OBS arrays.