Gravity data reduction, Bouguer anomaly, and gravity disturbance

Abstract

Each point on the earth has a gravity and gravity potential value. Surfaces formed by connecting points with equal gravity potential values are called equipotential surfaces or level surfaces. Determination of gravity field of the earth and the geoid which is one of the earth’s equipotential surfaces is very important for physical geodesy. Gravity values measured on the physical earth are not directly included in studies; firstly, they must be converted into gravity anomalies. For this, in this study precise leveling, gravity and GPS measurements were made in the field. Heights (H) with precise leveling measurements, gravity values (g) with gravity measurements and geographical latitudes (j) with GPS measurements were recorded. Then, gravity reductions (free-air, Bouguer) were calculated at the points. The actual gravity g measured on Earth is not immediately directly comparable to the normal gravity of the ellipsoid surface. Gravity values must be reduced to the geoid. Since there are masses outside the geoid, reduction methods differ according to the way these topographic masses are handled. Bouguer gravity anomalies and gravity disturbances are derived. The gravity anomaly (Dg) is defined as the scalar difference between the Earth’s gravity on the geoid and normal gravity on the surface of the reference ellipsoid.  Gravity disturbance (dg) is defined as the difference between the actual gravity magnitude measured on Earth and its equivalent normal gravity in the normal gravity field for the same point. The changes and magnitudes of the calculated quantities are compared. Changes such as the observed gravity and height data, observed gravity changes versus calculated normal gravity changes, normal gravity on ellipsoid versus geographic latitude, observed gravity changes versus latitude changes, Bouguer gravity anomaly and gravity disturbance versus latitude and elevation, free-air reduction and Bouguer gravity reduction versus latitude and elevation have been investigated.