Modelling Local Gravity Anomalies from Processed Observed Gravity Measurements for Geodetic Applications

As the application of gravity data in applied sciences such as geodesy, geodynamics, astronomy, physics and geophysics for earth shape determination, geoid model determination, computation of terrestrial mass displacement, orbit computation of natural and artificial celestial bodies, realization of force standards and derived quantities and density distribution in the different layers in the upper crust and having considered the cost of direct gravity survey, the study presents modelling local gravity anomalies from processed observed gravity measurements for geodetic application in Benin City. A total of 22 points were used. The points were respectively observed with CHC900 dual frequency GNSS receivers and SCINTREX CG-5 Autograv to obtain their coordinates and absolute gravity values. The theoretical gravity values of the points were computed on the Clarke 1880 ellipsoid to obtain their local gravity anomalies. The free air and the Bouguer corrections were applied to the computed gravity anomalies to obtain the free air and the Bouguer gravity anomalies of the points. Least squares adjustment technique was applied to obtain the model variables coefficient/parameters, as well as to fit the fifth-degree polynomial interpolation surface to the computed free air and the Bouguer gravity anomalies. Kriging method was applied using Surfer 12 software to plot the computed and the models' free air and Bouguer gravity anomalies. Microsoft Excel programs were developed for the application of the models in the study area. The Root Mean Square Errors (RMSEs) and the standard errors of the two models were computed to obtain the dependability, as well as reliability of the models. It is recommended that whenever either free air or Bouguer gravity anomalies of points within Benin City are to be obtained for application in applied sciences, the determined models should be applied.

Automatic gravity optimization of 2.5D strike listric fault sources with analytically defined fault planes and depth-dependent density

An automatic gravity inversion technique in the space domain simultaneously estimates the parameters of strike-limited listric fault sources and regional gravity background from a set of observed Bouguer gravity anomalies. The fault profile and regional effect are described by unknown polynomial functions of arbitrary but prescribed degree. Furthermore, the density contrast within the fault structure is presumed to be known, according to a prescribed parameterized nonlinear function of depth, in geologic settings where the detached downthrown block consists of a series of sedimentary beds whose density increases with depth. The inversion is automatic in that it initializes and determines polynomial coefficients for the fault boundary and regional gravity background from a set of observed Bouguer gravity anomalies and improves them iteratively until the modeled gravity anomalies mimic the observed anomalies. An analysis of a set of gravity anomalies attributable to a synthetic model of a listric fault structure in the presence of pseudorandom noise with and without regional background has disclosed that the algorithm yields reliable interpretations with modest error in model geometry, even in the presence of pseudorandom noise. In the presence of regional gravity background and pseudorandom noise, the estimated parameters of the structure deviate marginally from the true ones. The derived density-depth model of the Jharia coal basin in India, a pull-apart basin, has been used to analyze the observed Bouguer gravity anomalies of a boundary fault. The interpretation has yielded information consistent with drilling results and geologic setting of the basin.