Bathymetry estimation from satellite geodetic missions using gravity geologic method

Abstract

Bathymetry information is essential in understanding the physics of the Earth and the ocean process. However, the bathymetry data are difficult to obtain at the restricted, complex and vast area. The conventional bathymetry surveys which used single beam echo sounder and multibeam echo sounder required high expenditure, consumed much time and the bathymetry data obtained were sparse. This study aims to map the bathymetry over the Malaysian seas by using the space-based approach. Six satellite missions namely Jason-1, Envisat1, ERS-2, Jason-2, Cryosat2 and Saral covering 11-year data period (2005-2015) have been used. Gravsoft software was utilised in the derivation of free air gravity anomaly (FAGA), using Fast Fourier Transform technique. Next, the derived FAGA was validated against the marine FAGA model developed by the Department of Survey and Mapping Malaysia. The Gravity-Geologic Method (G-G method) was then performed for the estimation of bathymetry and a density contrast of 1.67 g/cm3 was used. Area of the estimated bathymetry was along the latitude and longitude of 5ºN – 10ºN and 107ºE – 114.6ºE, respectively. National Geophysical Data Center shipborne data was used utilizing 12362 bathymetry data points. 6584 points were used in the G-G method process while 5778 points as the validation points (check points). Minimum curvature interpolation was utilized in establishing the regional FAGA surfaces. The assessment on the accuracy of the results obtained was made using Root Mean Square Error (RMSE) and correlation coefficient analysis. The mean sea surface height (MSSH) obtained shows a strong correlation with Technical University of Denmark 2015 MSSH model with values of 0.9980. The RMSE for the computed FAGA achieved ±11.52606 mGal, with the use of EGM2008 (full degree and order) Global Geopotential Model and with this value, it gives a reliable derived FAGA information. The final estimated bathymetry produced the RMSE value of ±96.949 m, which is estimated to be large, perhaps due to the dynamic of the ocean and the depth variations. However, this estimated bathymetry can improve the depth accuracy by approximately 69% and 38% based on the comparison made with Earth Topography 1-minute and Technical University of Denmark 2010 global bathymetry model respectively. The final estimated bathymetry is known as Universiti Teknologi Malaysia 2018 bathymetry model. The study confirms that the estimation of bathymetry using the space-based approach is reliable and the mapping of the bathymetry is more effective and time-saving as it can cover non-accessible and restricted area in a mesoscale. The information collected from satellite altimeter can be delivered to the Malaysian Bathymetry Database System as the product from this study.