Evaluating the gravity anomalies over Nigeria from global gravity field models and new terrestrial gravity anomalies

The gravity anomalies over Nigeria range from -47.54 mgals to 70 mgals in the flat region and from 70 mgals to 157.26 mgals in regions of significant topography. In geodesy, combined global gravity field models (GGMs) are used for gravimetric geoid modelling. Due to the multitude of GGMs, evaluation against independent data is very important for the quality description of the different models and to establish their precision for application in those localities. It is pivotal that these gravity datasets be evaluated within the study area before they are used for gravimetric geoid modelling. This study aims to evaluate, for the first time, five combined GGMs (EGM2008, SGG-UGM-2, EIGEN-6C4, XGM2019e_2159 and GECO) over Nigeria using a new set of terrestrial gravity anomalies. There exist few evaluations of gravity anomalies based on state boundaries. However, this gravity dataset has not been used to evaluate the above five combined GGMs over the entire country of Nigeria. Our strategy is to compare GGM-derived gravity anomalies at a spectral resolution of d/o 300 in steps to d/o 2190 with terrestrial (free-air and Bouguer) anomalies in terms of standard deviation (SD) and root mean square (RMS) for determining the best fit global model in the study area. The gravity datasets cover the study area, Nigeria within longitudes 3° to 14° E and latitudes 3° to 14° N. In the computation of the free-air gravity anomaly, the effects of atmospheric corrections, free-air reduction and latitude correction were considered. Since it is important to consider the topographic correction in an area where the topography is large, we considered the topographic corrections based on Shuttle Radar Topography Mission (SRTM 30) data set when computing the Bouguer anomaly. From our comparison, it was observed that XGM2019e_2159 derived gravity anomalies have a best-fit relationship with the terrestrial data than the other four GGMs. While further analysis is needed, the preliminary results show that our method has the potential to effectively evaluate and indicate gravity field models that can be useful for modelling the gravity field of the Earth over the study area.

Keywords: Free-air gravity anomaly, Bouguer gravity anomaly, Global gravity field models.