Wavelet-based fusion of altimetry-derived and shipborne gravity data around Egypt

This study presents a wavelet-based fusion approach for enhancing the marine gravity field over the Red and Mediterranean Seas around Egypt by integrating altimetry-derived free-air gravity (AFAG) and shipborne free-air gravity (SFAG) data. The shipborne dataset was first denoised and gridded to match the resolution of AFAG from DTU21. Then, seven wavelet families across six decomposition levels were evaluated against independent shipborne gravity measurements. All wavelets exhibited progressive improvement up to three-level decomposition, while higher levels led to over-decomposition and noise amplification. The dmey wavelet at three-level decomposition provided the optimal fusion result, reducing the RMS of residuals from 6.27 mGal to 5.27 mGal, achieving an improvement of 15.95%. Among tested fusion rules, selecting the larger absolute value of high-pass coefficients between AFAG and SFAG best preserved dominant local features, while a weighted-average rule also improved accuracy but produced smoother outcomes. Power-spectrum analysis revealed that the fused grid retains regional trends of AFAG while significantly enhancing short wavelengths toward SFAG levels. Comparisons with the GEBCO2025 bathymetric model along ship tracks and randomly selected ship profiles demonstrated that the fused anomalies more closely reflect seabed topography, particularly where ship coverage is dense. In regions with sparse ship data, the fused field smoothly transitions toward AFAG, maintaining overall consistency. The results confirm that wavelet-based fusion effectively combines the broad-scale accuracy of altimetry with the fine-scale resolution of shipborne data, providing a more accurate and detailed representation of the marine gravity field.