Marine Gravity And Bathymetry Inversion Using Observations From Multi-satellite Altimetry Missions Over The Gulf Of Guinea

The ocean is the largest source of natural resources available to mankind.It also is the largest source of livelihood which shapes the economies of most countries and economic blocs.Because of these reasons,countries and economic blocs aggressively protect,and spend heavily in research funds to study their demarcated maritime boundaries.Up an until three decades ago,maritime research especially ocean floor topography surveys have been traditionally conducted by developed countries through ship cruises.In 1992,the advancements in space technologies brought forth a new paradigm in remote sensing,specifically satellite altimetry,which has revolutionized the geoscientific community.Satellite altimetry provides the geoscientist abundant,ubiquitous,and less expensive sea surface height datasets for ocean studies.It has henceforth become the best available option for developing economic blocs like West Africa to use for studying the Gulf of Guinea.The present study constructed an enhanced bathymetric model of the Gulf of Guinea(15°W-5°E,4°S-4°N)to fill in the gaps of sparse shipborne ocean depths.It initially constructed the gravity field model(comprising deflections of the vertical and gravity anomalies)of the study area from five altimetry missions(i.e.,SARAL/Alti Ka,Jason-1/GM,Cryosat-2,HY-2A/GM and Envisat).The remove-restore technique of regional gravity field modelling was adopted;however,it was only applied on the gravity anomalies,and not vertical deflections.Analysis of obtained results showed that the north component of vertical deflections was more accurate than the east-component by approximately 3 times.The contribution of each satellite in construction of the vertical deflections was analyzed,and results showed that each satellite can contribute equally in resolving the north component.However,Cryosat-2's high spatial resolution,Jason-1/GM's low inclination,and SARAL/Alti Ka's high Ka-band range accuracy rendered them the dominant satellites in resolving the east component.With a mean deviation of-0.36 m Gal,the computed gravity anomaly model compares well with reference models from DTU13,SIOv28 and EGM2008.Also,it compares well with shipborne gravity data after a second-degree polynomial adjustment.The geophysical capabilities of HY-2A/GM was particularly analyzed.Results showed that to obtain more accurate gravity anomalies from this satellite,higher weights must be assigned to the north component of its vertical deflections.These weights must be assigned based on the components'error variances relative a reference gravity field,and their covariance.The resultant HY-2A/GM-derived gravity anomaly deviated from reference models by approximately 0.05 m Gal.This proves that the satellite is geodetically and geophysically reliable.The bathymetric model is a fusion of results from gravity-geologic method(GGM),and Smith and Sandwell method(SSM).The SSM-derived model was slightly more accurate than GGM-derived model in the study area.GGM depths ranged from-5657.75?-1253.23 m with standard deviation of 582.65 m,whilst the depth range of the SSM model was-5754.81?-1682.78 m with standard deviation of 555.07 m.By merging results from both methods,the final bathymetry range was-5754.81?-1253.23 m with a standard deviation of 540.12 m.Mean error and standard deviation of errors at test points were-5.79m and 81.42 m respectively,with 98.91%correlation coefficient.A coherency plot between the final bathymetry and ship soundings showed negligible error bars;indicating the effectiveness of the bathymetric model.Investigation into the lower accuracy of the GGM-derived model showed that the use of a common density is not always optimal.Results showed that mountainous regions(15°W-8°W,4°S-2°N;1°E-5°E,4°S-0°N)require lower density contrasts of1900 and 1100 kg/m~3 respectively;whereas low-lying region(8°W-1°E,2°S-2°N)demands 3900 kg/m~3 as density contrast.This indicates that the study region's density contrast is inversely proportional to topography.The study also indicates that more control points are needed in the area where topography varies largely in order to obtain same accuracy as other regions.This study finally informs decision makers and other stakeholders which areas within the study region that warrant shipborne depth densification in order to improve the accuracy of future bathymetric models.