Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
� improving velocity models in terms of density models;
� mapping in fold/thrust belt terranes;
� detecting salt domes, carbonate platforms and pinnacle reefs, and
� assisting in mapping of the base of salt and salt geometry.
AirG and AGG gravity technologies are both useful in these applications, with the prime differentiator
being the wavelength of interest. AirG is generally used for applications requiring longer wavelengths.
Routine conforming of FALCON gravity to regional fields has made this distinction from AGG less
critical (Dransfield, 2010).
The most common application for gravity in oil and gas explorations remains basement mapping.
Fernandez et al. (2009) describe an application using the FALCON AGG in an area near Chirete in
Argentina, and the example in the next section describes the use of the GT-1A system in Ghana.
3D inversions and interpretation – the Volta Basin study
Interpretation of potential field data has made increasing use of 3D inversion technology over the last
decade and it is increasingly common for 3D geological models to be developed from AGG data (see
for example, Nicholls et al., 2007) and AirG data. The Volta Basin study is based on airborne gravity
and magnetic data collected in early 2008 by Fugro Airborne Surveys over the Volta and Keta Basins
in Ghana (Figure 5). The survey was flown for the Mining Sector Support Program of the Ghanaian
government and was funded by the European Union. The GT-1A airborne gravity survey in this case
study was specifically flown to aid mapping of basement geology and structure at several kilometres
depth under the sedimentary basin cover. The gravity data were acquired with a GT-1A vertical
component gravimeter using 5 km line spacing and 50 km tie line spacing at approximately 500 m
above ground level. The survey area, digital terrain model, magnetic total field data, and complete
Bouguer anomaly gravity data are shown in Figure 5.
Figure 5. Satellite image of Ghana showing (from left to right) the survey area outline, digital
terrain model, magnetic total field data, and complete Bouguer anomaly gravity data.
Although the main aim of the project was to develop the mining sector in Ghana, one of the minor
objectives was to analyse the oil prospectivity of this deep sedimentary basin. Using the Fugro LCT
software package a 3-D gravity modelling exercise was carried out to evaluate the basement
structures, depth to basement surface, and apparent density.
The model was developed by applying both a structural inversion of the basement topography and by
a property inversion of the basement apparent densities. The structural inversion was guided by the
integrated geological basement interpretation. Additional constraining information included eleven
wells (three of which penetrated the basement), one seismic line, and magnetic depth estimates along
selected lines of the airborne survey.
The depth to basement modelling results (Figure 6) show that the Volta Basin is comprised two
smaller sub-basins, with the north eastern sub-basin being somewhat smaller than the southern sub-
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