Airborne Gravity 2010 - Geoscience Australia

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Airborne Gravity 2010 Figure 4. Colour images of (left to right) the residual gravity, total magnetic intensity and electromagnetic B-field response in channel 9 from the joint FALCON AGG, TMI and TEMPEST survey at Kauring. The Kauring feature (just right of centre in each image) includes material of high density, high susceptibility and high electrical conductivity. All images are oriented with north up the page and all have a linear colour scale: gravity -0.1 - 0.2 mGal; TMI 58,400 - 67,000 nT; B-field -0.1 - 1.0 fT. Oil and gas exploration There are numerous applications for airborne gravity in oil and gas exploration. Examples include: � reconnaissance over areas that present difficulties for seismic acquisition (e.g., transition zones, rough terrain, areas subject to cultural/community ground access restrictions) in order to target the seismic more cost-effectively; � mapping intrusive and buried volcanics within sediments; � mapping of major delta and fluvial channels; � mapping of deep basement structures; 53
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- 54 250km
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Record Record 2010/23 2010/23 GeoCa
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