Great Barrier Reef Airborne Gravity Survey (BRAGS’99). A gravity survey piggybacked on an airborne bathymetry mission

Abstract

Generous support from Australian Hydrographie Office made it possible to install and operate a gravimeter in the aircraft, a Fokker F-27, carrying their Laser Airborne Depth Sounding (LADS) system. The LADS system operated in the Great Barrier Reef region, northeast Australia, at that time. This shallow water area is a major gravimetric data void, and thus degrades the gravimetric geoid models in and around the area. The aim of the airborne gravity measurements was both to gather new data in the area to improve the geoid and to examine the feasibility of combined airborne bathymetry and gravity surveys. An important consequence of this is that the existing bathymetry may be used to predict the gravity field in areas lacking gravity — thus doing away with the need for direct gravity measurement in offshore areas well mapped with bathymetry.

This paper outlines the gravity system setup and the applied sensor modelling. The system performance is validated by internal crossover analysis and comparison to existing data. Special emphasis is put on modelling of non-linear terms in the La Coste & Romberg S gravimeters response to aircraft phugoid motion. The phugoid motion of this specific aircraft generates a noise term in the gravity system output that overlaps more with the gravity signal than experienced in smaller aircraft. A simple lowpass filtering of the noise will result in an unsatisfactory resolution of the final data. It is shown that the phugoid motion induced noise can be sufficiently reduced by including non-linear terms in the modelling of the sensor response.