The Role and Capability of Absolute Gravity Measurements in Determining the Temporal Variations in the Earth’s Gravity Field

The temporal variations of the gravity field are the result of a superposition of the effects of a large number of processes in the atmosphere, the hydrosphere, the cryosphere and the solid Earth. A number of geodynamic processes, capable of being detected and studied as a result of the changes in gravity they produce, can be specified in terms of their principal frequencies and spatial wavelengths (Figure 1). The processes influencing gravity are a mixture of primary and secondary effects. For example, polar motion is thought to be a response to mass movements in the atmosphere, the hydrosphere and the Earth’s core (e.g., Eubanks, 1993). Those processes having periods greater than 100 years generally appear as secular variations with rates of 1 µGal/yr or less (lµGal = 10 nm/s2). Typical peak-to-peak gravity variations of several µGal are associated with seasonal groundwater movement, atmospheric processes and polar motion. The body tides have an associated peak-to-peak gravity variation of around 100 µGal. Although monitoring of gravity with relatively drift-free instrumentation is still in its infancy, spectral analysis of existing data indicates that the non-tidal, gravity spectrum is “red”, i.e., gravity variations are generally larger at the low-frequency end of the spectrum. This is consistent with the fact that very-long-period “geological” processes are associated with large displacements and movements of mass resulting in gravity anomalies of the order of mGals.