The OBS experiment

23:27 on Wednesday, July 17 2019 : it’s time for the first OBS deployment !
During the SMARTIES cruise, 19 OBS (Ocean Bottom Seismometers) will be deployed. It is one of the first things to do when we arrive at the study zone. Indeed, once it reaches the seafloor, the OBS will record every felt seismic tremor all throughout the mission.
An OBS deployment usually follows this scheme :
First the ship stops above a previously defined spot.
The OBS is connected to a winch and is then raised outside the ship. It is placed on the surface of the water and when the cables are released, its the beginning of a long trip for him ! The instrument descent is followed by the team on watch through continuous acoustic interrogation. This allows knowing when it reaches the bottom.
After about an hour of descent towards the abyss, depending on the depth of the area, the OBS reaches the seafloor. It’s now time for the triangulation which will inform on the precise location of the OBS. Since it dives very deep (several thousands of meters) it can drift from its original location. To re-locate it, the ship drives a circle at low speed around this original location and sends acoustic signals to the instrument that answers back.
Once the OBS is precisely located, it is time to head towards a new deployment spot. We also use this short transit to acquire a new bathymetric profile in order to complete existing maps.

GEBCO bathymetry of the SMARTIES area showing the positions of the deployed OBS.

By F. Bonnet