The topography of the seabed, under thousands of feet of water, is known thanks to the low undulations of the ocean surface. Some satellite altimetry measurement as Cryosat or Jason allow to measure the height of the ocean surface.
Knowing precisely the average height of the ocean surface, we deduce the relief of the seabed. How?
To obtain a signal corresponding to the underwater relief, a complex digital processing should be done to remove all effects related to satellite track variations and disturbances such as ocean tides, currents, winds, atmospheric pressure or the expansion of the water due to the temperature. These variations in sea level are of the order of a few centimeters to several meters.
« It takes years of measurement, with several successive passes the same place to remove the effects of currents, tides and other disturbances, to keep in the end that the mean sea level », said David Sandwell, researcher at Scripps Institution of Oceanography UC San Diego, California.
Once the average height of the surface of the sea precisely known, the measures indicate that this height is not the same across the surface of the oceans, it varies depending on the seafloor and this corresponds surprisingly, to variations in topology of seabed.
In other words, on the surface there are small undulations in relation to the average height of the sea and this corresponds to a mini relief (a flattened relief) of the seabed. Satellite measurement, uses this property of the surface to reflect the topography of the seabed, the funds have an influence on the level of the water surface that covers them.
« We measure actually fine variations of the earth's gravity. When there is an underwater mountain, it has a greater density than the surrounding water and attracts more water than the surrounding environment. Since water is incompressible, there is more that accumulates above the seamount, which causes a slight increase in the local level of the ocean. », explains Stéphane Calming, Research Director at IRD (Institute of Research for Development) in Toulouse, France.
NB: the ocean observing satellites are exceptional sources of continuous global observations, very precise of the physical and biological ocean state (status and sea level, ocean currents, salinity, surface temperature of the sea, ocean color and sea ice) and atmospheric parameters (winds at the ocean surface, air-sea fluxes, precipitation) that determine oceanic circulation. In 2014, the sea level rises by an average of 3.27 mm per year, while it was only 1.7 mm in 1993-1994.
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Image: Geographic map of the ocean floor and plate boundaries. The relief of the seafloor creates ripples on the surface of the sea, which correspond to a mini topology of of background. An ocean ridge corresponds to a water bump on the surface of several meters in height, and an oceanic trench corresponds to a hollow of water to the surface. Thus the satellites of altimetry measurement of high-precision (of the order of cm) allow to map the submarine relief. The method of mapping of the seafloor via satellite making a significant improvement, because ≈80% of the ocean surface was not covered by onboard bathymetric sounders on ships. Bathymetry is the underwater equivalent of altimetry, referred to measurement ocean depth.