Assessment of HY-2A GM data by deriving the gravity field and bathymetry over the Gulf of Guinea

Abstract

Nine cycles of Haiyang-2A (HY-2A) IGDR (Interim Geophysical Data Record) data are used to derive gravity products over the Gulf of Guinea (15° W–5° E, 4° S–4° N). Firstly, the sea surface heights (SSH) and vertical deflections are derived and their precisions are evaluated. The comparison results show that the east component of vertical deflections has a poorer precision than the north component by 4.15 times. A theoretical proof was given to explain this point according to the error propagation rule. Gravity anomalies are then derived from vertical deflections using the remove–restore method. The precision of the derived HY-2A gravity anomalies is evaluated with SIO, DTU13, EGM2008, EIGEN-6C4 products. The results showed that the differences between HY-2A-derived gravity anomalies and these models have mean values larger than 0.5 mGal and std values around 7.0–7.3 mGal. In order to improve the precision, an improved new version of gravity anomalies was derived by assigning a small weight to the east component of vertical deflections, since the precision of which is poorer than the north component. Comparison with the initial model showed that the precision of the new gravity anomalies is an improvement of the initial model by approximately two times. When compared with EGM2008, EIGEN-6C4, SIOv28 and DTU13, the mean values of the differences are close to zero and standard deviation of the differences are around 2.7–3.0 mGal. The improved gravity anomalies were used to invert the bathymetry of the region using the gravity-geologic method. The modeled bathymetry compared well with a previous bathymetric study by the authors that used DTU13 gravity anomalies. It also performed well against ETOPO1 and SRTM15+V2; with difference means, standard deviations and correlation coefficients of 26.67 m, 183.09 m, 0.9562; and 12.26 m, 174.55 m, 0.9590, respectively. This implies that SSH data from HY-2A are geophysically reliable; and hence, can be incorporated with SSH data from other satellite altimeters.

Recovering Marine Gravity Over the Gulf of Guinea From Multi-Satellite Sea Surface Heights

“…This is attributable to the inclinations of the satellites which usually are towards the north. The effect of this accuracy imbalance propagates to the resultant gravity anomalies derived from the deflections of the vertical (Small and Sandwell, 1992;Zhang, 2017;Wan et al, 2020b). In order to nullify this effect on the derived gravity anomaly model, weights are assigned to the vertical deflection components based on the covariance between them, and their error variances relative to the reference gravity field model.…”

“…The advancements in satellite altimetry since the past 4 decades has led to the proliferation of sea surface height (SSH) data which has contributed significantly to geodesy, geophysics, oceanography, and hydrology. Some of its applications include: sea level variation research (Cazenave et al, 2014;Ablain et al, 2015;Watson et al, 2015;Passaro et al, 2018), determination of mean dynamic topography (MDT) and mean sea surface (Andersen et al, 2016;Ophaug et al, 2021), construction of marine geoid (Dadzie and Li, 2007;Chander and Majumdar, 2016), monitoring of lakes and rivers (Villadsen et al, 2015;Zakharova et al, 2019) and inversion of marine gravity field (Sandwell et al, 2013;Sandwell et al, 2014;Zhu et al, 2019;Nguyen et al, 2020;Wan et al, 2020b).…”

Validation of Just-Released SWOT L2 KaRIn Beta Prevalidated Data Based on Restore the Marine Gravity Field and Its Application

“…Annan & Wan [17] utilized an improved gravity geology method, combining airborne air-free GA and shipborne depth data, to construct a depth model for the Gulf of Guinea. Wan et al [18] used the GA data restored from the HY-2A/GM data to compute the bathymetry model over the Gulf of Guinea. Their results indirectly demonstrate that the altimetry data provided by HY-2A can be used for ocean gravity field inversion.…”

Bathymetry Inversion Using Vertical Deflections: a Case Study in South China Sea

“…Bathymetry information is important for economic, military and Earth science. Many methods are developed for deriving bathymetry information, such as multibeam echo sounding, the derivation based on gravity data (Smith and Sandwell 1997;Hwang 1999;Hisao et al 2016;Wan et al 2019Wan et al , 2020a, detection using laser technology (Kervern et al 1992), inversion based on imagery (Monteys et al 2015;Jawak et al 2015). Among these methods, altimetry derived gravity products have played a great role in global bathymetry inversion.…”