Observation and Local Prediction of the Vertical Gravity Gradient: Review Paper

The knowledge of the vertical gradient of gravity (VGG) at observation points (benchmarks) in the field or at indoor pillars at observatories is needed across various earth science disciplines. The accurate value of VGG is required to reduce the precise measurements of gravity carried out by various modern gravimeters from the sensor height of the instrument to the level of the benchmark on the ground. No portable and easy to use instrumentation is available for measuring the VGG in the field, although portable quantum devices may change the situation in the future [1]. However, the average value of the VGG over a short vertical span near the surface, which approximates the point VGG value, can be observed by relative gravimeters in the so-called tower mode. Such observations call for the use of special-design (custom-made) tripods, or sturdy geodetic tripods, and repeated measurements of gravity by a relative gravimeter in the upper and lower positions, vertically separated by about one meter, or in several positions along the vertical, separated by decimeters. The in situ VGG measurements are demanding to carry out in the field, especially under harsh conditions. Particularly, strong wind can compromise or even halt the VGG observation, due to introducing high levels of background noise in the data. Therefore, the in situ VGG values are often missing, although highly needed for geophysical, geodetic or geodynamic applications.