Regional seafloor topography around New Zealand deduced by mixing ERS-1 radar altimeter data and ship track measurements (source: Ramillien et al., 2000).
While seafloor topography gives valuable information for understanding the submarine geological processes, large oceanic areas, in particular in the Southern ocean, remain completely unexplored. It would take hundreds of years to survey depth in these regions by echo sounding during expensive campaigns. Current maps of seafloor based on ship measurements suffer from three problems: irregular data distribution revealing many gaps between surveys, poor quality of echo sounding records in remote areas and archaic methods for map projection. In addition, the resolution and accuracy of the data are variable. Since the 90’s, the scientific community has turned to other techniques based on satellite observations that ensure a systematic and global coverage instead of sparse acoustic swath records. Radar altimeter missions have been measured the static variations of the sea surface with high accuracy and moderate spatial resolution of 15 - 25 km. Once they are corrected from the permanent oceanic currents, these undulations correspond to the marine geoid which is an irregular surface of constant potential of the Earth gravity field. At short wavelengths (i.e., a few hundreds of kilometers), the measured geoid heights are correlated to the sea floor variations, so that radar altimeter measurement can be used to estimate the sea floor topography by inversion.
So far, two types of strategy for inversion have been proposed; the Fourier filtering (i.e. Smith and Sandwell, 1994) and the non-linear least square adjustment (i.e. Baudry and Calmant, 1991; Calmant, 1994; Ramillien, 1998). However, these methods have important limitations.
We propose to combine all accessible radar altimeter databases for sea floor topography determination from the launch of the first repetitive Topex/Poseidon and ERS-1 missions to the recently launched missions such as SARAL (February 2015), Jason-3 (January 2015) and Sentinel-3 (February 2015). Even if we will focus on the missions with a geodetic phase (ERS-1, Jason-1, Cryosat-2), we will examine the interest of Ka-band (SARAL), SAR and InSAR techniques (Cryosat-2, Sentinel-3) to retrieve the parameters at a higher resolution. In particular, we will benefit of the orbits of improved quality from the recent ESA/MSSL-UCL REAPER reprocessing. Cumulating a large amount of sea level observations, geographically dense, would amplify the signal-to-noise ratio, and consequently decrease the error on the marine geoid heights. Accurate mean sea surface would enable the recovery of the set of topographic heights more precisely, by resolving the fine scale details of ~10 km dimensions.
The main challenge we want to address are:
- the redundancy of the observations for bringing valuable information in the inversion;
- the classical inverse problem of gravimetry that is known to be non unique, it is solved by a trade-off between geophysical parameters which effects compensate each other;
- the development of new algorithms that are robust and efficient enough to manage the inversion of a very large number of data.