As soon as the mission begins, part of the Arc-en-Sub scientific team is busy on the corridor… The deployment of the ocean bottom seismometers (OBS for short) must be prepared.
These seismometers are self-contained instruments which, when placed on the seabed, will record its movements. These movements, or seismic vibrations, can be caused by natural processes, such as earthquakes or changes in water pressure due to waves, or man-made processes, such as compressed air cannons.
During the Arc-en-Sub mission, we will launch 5 OBS at 9 deployment points. Ballasted, they will free-fall to the seabed. Once on the ground, the seismometer will be released from the structure using an electrified release system. This will improve the accuracy of the measurement, making it as insensitive as possible to movements of the OBS structure. Thanks to an acoustic sensor, we will also be able to locate the OBS precisely. Our instruments will measure, over a period of 5 to 30 days depending on the station, the variations in water pressure on the ocean floor, modulated by gravity waves from the ocean surface, as well as the ground movements in response to this forcing.
Once a seismometer has completed its mission, we will send it an acoustic command to return to the surface. Then an electrolysis-activated system will open the clamp that connects it to the ballast grid, and it will rise to the surface via a flotation system of glass spheres. A flag and light and radio signals will be activated to help us locate and recover the OBS.
The data recorded by the seismometers and pressure gauges are valuable. By studying the response of the solid Earth to ocean tides, it is possible to constrain the internal structure of the first few kilometres of the Earth. Indeed, the amplitude of the movements of the sea floor is related to the mechanical properties of the oceanic crust. Thus, determining the relationship between a load, in this case oceanic gravity waves, which are 100 m to a few tens of kilometres long and a few millimetres high, and the deformation of the ground that it causes, of the order of a few micrometres, makes it possible to better characterise the materials that make up the first few kilometres of the ocean floor. The less rigid the soil is, and in particular the lower its shear modulus, the more the seafloor will move. This method, known as “seafloor compliance”, is therefore particularly relevant for studying hydrothermal systems or molten bodies with very low shear moduli.
One of the objectives of Arc-en-Sub is to deploy several OBS in the area of the Rainbow Massif, a hydrothermal site in the Mid-Atlantic Ridge, and to use the seafloor compliance method to image the presence of hydrothermal or melt fluids in the first 8-10 km of the oceanic crust. By using several seismometers and pressure gauges, and by correlating their observations, we aim to obtain a 3-dimensional image of these fluids.