Using satellite data to map the seafloor
In recent years, the introduction of high-resolution satellite constellations, such as Sentinel-2, have facilitated the development of new methodologies for examining the marine and terrestrial environments through their high revisit times and freely available data. One such application is the evaluation of the depth of the ocean or bathymetry.
Click the title to keep on reading...
A robust understanding of bathymetry is fundamental in monitoring the marine environment through uses in detecting underwater geo-hazards, environmental change, sediment transport, tsunami forecasting and ocean circulation. As of 2020, less than 20 per cent of the world’s bathymetry has been mapped which demonstrates a concern for fulfilling the UN Sustainable Development Goal #14, to conserve and sustainably use the oceans.
Satellite Derived Bathymetry (SDB) is the estimation of the depth of the seafloor using these space-borne sensors. This method provides an efficient and cost-effective technique to survey large extents in shallow water areas. The concept of SDB utilises the upwelling signal of light to measure the reflectance of the seafloor. A relationship exists between the intensity of reflected energy recorded by the sensor and the depth of water which can be exploited to determine the water depth through a physics-based look-up table approach using Radiometric Transfer Equations (RTEs).
The results from SDB are limited by the penetrating capacity of light and hence limited to coastal areas to a depth of ~30m. This cut-off can vary depending on the properties of the water column. In order to account for this discrepancy ARGANS has developed a bespoke technology, called Depth of Penetration (DOP), which can be used to evaluate the parameters in the water column to quantify the range of appropriate depths produced by SDB in specific waters.
Our workflow has been developed using six key stages:
- Atmospheric Corrections
- Masking of anomalies
- Physics based Bathymetry
- Cartographic evaluation
Coastal areas are dynamic and changing environments and therefore the SDB produced for a given image acquisition may not demonstrate a true picture of the environmental conditions. Furthermore, several possible depth estimates can be afforded by the RTEs and there is a need to apply validation for this. Full utilization of the large Sentinel-2 time-series has been considered through the development of an innovative merge technique. This technique allows us to compensate for the impact of irregular anomalies (such as: sediment plumes, clouds and boats) by calculating a weighted average of pixel values.
Work at ARGANS has focused on the development of International Hydrographic Office (IHO) compliant nautical charts through projects support by ESA and SHOM. Updating old and inaccurate charts has been of paramount importance to aid economies and safety of maritime navigation, especially in areas where access from survey vessels may be difficult or prohibitively expensive.
For more information on SDB, check out our website: https://sdb.argans.co.uk/