To date, approximately 20% of the ocean floor has been surveyed by ships at a spatial resolution of 400 m or better. The remaining 80% has depth predicted from satellite altimeter-derived gravity measurements at a relatively low resolution. There are many remote ocean areas in the southern hemisphere that will not be completely mapped at 400 m resolution during this decade. This study is focused on the development of synthetic bathymetry to fill the gaps. There are two types of seafloor features that are not typically well resolved by satellite gravity: abyssal hills and small seamounts (< 2.5 km tall). We generate synthetic realizations of abyssal hills by combining the measured statistical properties of mapped abyssal hills with regional geology including fossil spreading rate/orientation, rms height from satellite gravity, and sediment thickness. With recent improvements in accuracy and resolution, It is now possible to detect all seamounts taller than about 800 m in satellite-derived gravity and their location can be determined to an accuracy of better than 1 km. However, the width of the gravity anomaly is much greater than the actual width of the seamount so the seamount predicted from gravity will underestimate the true seamount height and overestimate its base dimension. In this study we use the amplitude of the vertical gravity gradient (VGG) to estimate the mass of the seamount and then use their characteristic shape, based on well surveyed seamounts, to replace the smooth predicted seamount with a seamount having a more realistic shape.

The Geodetic Mission (GM) of Jason-2 was planned to provide ground-tracks with a systematic spacing of 4 km after 2 years and 2 km after 4 years to increase the spatial resolution of global altimetric gravity fields. Jason-2 ceased operation after 2 years of GM but provided a fantastic dataset. We highlight and evaluate the improvement to the gravity field which has been derived from the GM. The ageing Jason-2 suffered from several safe-holds and instrument outages. Here, we try to quantify the effect of safe-holds on marine gravity and discuss suitable approaches advising future GM like Jason-3. We evaluate the importance of attempting to “rewind” the mission to recover missing tracks as well as the possibility to continue an existing GM by using the same orbital plane. The latter idea would allow bisecting the already 2-years Jason-2 GM creating a 2 km grid after 2 years of Jason-3 GM.

Seamounts can be habitats and hazards to submarine navigation, and their distribution reveals the volcanic history of the oceans. As only a few percent of ocean floor has been sounded, seamount distribution must be mapped by satellite altimetry. Wessel (doi:10.1029/2000JB000083) looked at data from an earlier generation of altimeter technology and suggested that all seamounts 2 km and taller had been found, but there might be as many as 50,000 seamounts between 1–2 km tall that were not yet found. The AltiKa altimeter delivers more precise sea level measurements at a higher along-track sampling rate than previous altimeters. These data resolve small seamounts not previously resolvable (Smith, doi:10.1080/01490419.2015.1014950), particularly if repeat-track profiles are “stacked” and band-pass filtered (Marks and Smith, doi:10.1007/s11001-016-9293-0). These two studies looked at only a few isolated locations where multibeam acoustic depth sounding surveys were available for “ground truth” for tuning a band-pass filter to detect the small seamount geoid signal. In the new work we present here we have stacked 32 repeat cycles of SARAL AltiKa data world-wide, and band-pass filtered the stacks, to yield 75,208 potential seamount locations distributed between +/- 81.5 latitude throughout the global ocean. These locations are detected as local maxima in the filtered geoid at least 2 cm above background and with a full-width at half-maximum (FWHM) at least 4 km wide. Of these, 4824 detections were over multibeam surveys. We assign a proxy seamount height to each by subtracting the regional SRTM30 depths from the multibeam depths. These proxy heights follow a Poisson statistical distribution similar to that which fits acoustic bathymetry profiles over seamounts (Jordan et al., doi:10.1029/JB088iB12p10508). We are currently investigating how to derive proxy heights from anomaly amplitude and FWHM, optimizing the trade-off between false negative and false positive detections, and whether it is possible to identify potential seamounts that may pose hazards to submarine navigation.

Sentinel-3 A&B radar altimeters yield sea surface height measurements in both a high-precision Synthetic Aperture Radar Mode (SARM), and a Pseudo-Low Resolution Mode (PLRM). We stacked repeat cycles from both missions and in both modes to compare their resolution of small seamounts. Stacking entailed removing non-geoidal heights and height errors, testing for consecutive measurements over ocean, aligning to common locations at 1 km intervals along a synthetic track, and forming a median height profile. These profiles are available from the National Centers for Environmental Information (NCEI) data repository. Global maps show that, over the oceans, the median height is usually derived from more than 49 cycles, and the typical error in an individual PLRM measurement is approximately 1.9 times greater than an individual SARM measurement. We applied a seamount detection bandpass filter to the median profiles and compared their spectral resolution to that of the Satellite for ARgos and AltiKa (SARAL) AltiKa mission. Small seamounts are similarly resolved by Sentinel-3 A&B SARM data and by the SARAL/AltiKa data.