Geodetic gravity

Geoscience Australia and the Australian National University jointly undertake the geodetic gravity component of the AuScope program, aiming at improving the understanding of the temporal gravity changes across the Australian continent. The geodetic gravity program has two separate outcomes, they are to:

• use absolute gravimetry as an independent measure for vertical movements of the crust and/or other long-term gravity signals of interest. This objective is based on the principle that gravity changes as deformation occurs between the surface and the centre of the Earth

• improve our understanding of temporal deformation of the Earth's surface induced by tidal changes in gravity caused by the movements of the Sun and Moon. These movements also cause ocean tides which deform the surface when large tidal masses of water are moved around the Earth. This deformation can be in the order of centimetres and can affect the other fundamental precision geodetic measurement systems of Global Navigation Satellite System (GNSS), Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR).

The instrumentation and the observation program are separated into two streams: absolute and relative. The absolute gravity program involves the recording of 24 to 48 hours of observations, using an FG5 absolute gravimeter at around 10 geodetic sites across Australia. These sites are co-located with GNSS Continually Operating Reference Stations (CORS).

The FG5 gravimeter can measure gravitational acceleration (g) to one part in 10⁹ (8th decimal place in m.s-2), or the equivalent of ~3 millimetres of height change. Therefore, monitoring small gravity changes over extended time periods will allow changes in surface height to be measured accurately and independently of other survey techniques.

The relative gravity program consists of a series of relative gravimeters suited to tidal gravimetry that will be used to improve the understanding of temporal gravity variations caused by tidal forces. Researchers rely on predictive models that have yet to be tested over most of the Australian continent. Currently, there is some doubt about the accuracy of the models, particularly in the northern part of the continent.

The greatest improvement to the understanding of the tidal models will come from observation of the dominant tidal frequencies, which are in the diurnal and semi-diurnal tidal bands, around one and two periods/cycles per day respectively.

Up to six months of data are required from each site to obtain sufficient precision in the local tidal parameters. Data is collected over many years of observations, from sites around the continent. These are necessary to gain a greater understanding of the Ocean Loading models, which are applied across Australia. Although uncertainties in these models provide the most pressing need for Earth tide measurements across the country, building a tidal catalogue across the country will also provide other benefits, potentially including additional knowledge of the Earth's sub-surface structure, used for geodynamics research.