Twenty-Five Years of Progress in Sea Floor Mapping by Satellite Altimetry

Abstract

Although ships equipped with deep-water multi-beam echo-sounding (MBES) swath mapping systems and satellite (GPS) navigation have been around for the last 25 years, they rarely collect data in unexplored ocean areas. The most accurate and detailed sea floor sounding data are mostly confined to shallow coastal areas around developed countries, and a few deepwater areas that have been the focus of particular efforts (such as the search for the missing Malasia Airlines flight MH370 aircraft). Almost all the global ocean floor area lies more than a few hundred kilometers from the nearest GPS-navigated MBES data, and global ocean floor depth models must rely on older, low-tech single-wide-beam echo soundings recorded on analog scrolls and often positioned with only celestial navigation (most of the available data in the remote oceans was collected prior to 1965). If the ocean floor area is divided into squares one nautical mile (1.85 km) on a side, and all data, both GPS-MBES and old, low-tech data, are combined, only 8 percent of squares have any data at all. For this reason, global ocean floor mapping relies on satellite altimetry to guide the interpolation of gaps in unmapped areas. The largest variations in sea surface topography are time-invariant and exhibit geoid height anomalies produced by the Earth's gravity field. At high wavenumber (full wavelengths approximately 10 to 160 km) these anomalies are usually correlated with sea floor topography, but can also arise from sub-seafloor tectonic structure buried beneath seafloor sediment. Resolving anomalies at this scale requires satellite altimeter profiles of sea surface height along a dense network of ground tracks, so that the inter-track spacing adequately samples scales as short as 5 km or less. The first altimeter mission to yield a dense network of tracks was the European Space Agency's ERS-1 mission, completed in March of 1995. Marine gravity maps made from these data were exhibited at the Spring 1995 Americal Geophysical Union meeting, and this may have prompted the U.S. Navy to release the Geosat dense track data, collected in 1985-86 but classified Secret until July 1995. Some southern ocean Geosat data had been previously released in 1990 and 1992, allowing algorithm development for bathymetric estimation from dense track altimetry. After the 1990s there was a long period with no new dense-track altimetry, and so seafloor mapping made incremental improvements as geodesists learned to improve the along-track resolution at high wavenumber using specialized retrackers and high-data-rate filters designed to extract the seafloor topography signal. With CryoSat-2 in a long-repeat orbit since 2010, and the Jason-1, Jason-2, and SARAL/AltiKa missions also going into dense-track orbits at the end of their primary missions, there is now a rennaissance in seafloor mapping. Efforts are underway to see how many previously uncharted seamounts may be found, and how much resolution may be squeezed out of the newer mission data.