Prediction of offshore gravity from bathymetry

Abstract

The definition of the shape of the geoid is a fundamental objective of geodesy, since it allows for the conversion between orthometric and ellipsoidal height systems. The geoid can be computed from gravity values measured over the surface of the earth, and considerable effort continues to achieve a global coverage of gravity values. One technique that has been very successful in recent years in providing gravity coverage in areas which previously have been too difficult to access is airborne gravimetry. This technique has proved very useful in covering near offshore regions, for example. The coastal regions of Australia are recognised as locations where airborne gravimetry has the potential to fill in missing gravity data. A pilot survey using an airborne gravity meter was undertaken off the north east coast of Australia. In areas that remain unsurveyed it is sometimes useful to fill in the missing gravity data values with predicted gravity values. Previous research has examined the possibility of predicting gravity values from other observed quantities. The best success has been achieved by using the gravity effect calculated from bathymetric information. Often the corresponding isostatic compensation is computed, and the combined bathymetric-isostatic gravity effect is used. However, the type and extent of compensation that exists in any particular region mostly remains unknown. Theoretical considerations indicate that the short wavelength part of the gravity field may be adequately modelled by the gravity effect of the bathymetry alone, without reference to an assumed compensation mechanism. With this in mind, a prediction scheme has been developed which utilises the short wavelength gravity field information implied by the bathymetry, combined with the long wavelength gravity field information from existing observed gravity. This scheme allows the prediction of fill-in gravity values in areas with limited observed gravity. The prediction technique was used on a test set of data off the east coast of Greenland. The prediction technique was seen to outperform a simple interpolation of gravity values by approximately ten percent. Geoid computations performed with the predicted gravity values indicate that the prediction technique can provide significant improvements in computed geoids.