Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Case studies
There are many case studies in which airborne gravity has played an important part in helping create a
coherent 3D geological model to help delineate economic resources. GeoModeller is revolutionizing
the ability to create a 3D structural geology interpretation quickly and then have this tested using
independently collected gravity data.
The first of two cases mentioned here is from Uganda and the recent near-surface oil that is
associated with Lake Victoria. The horst and graben controls are readily seen in the initial FTG data.
Utilising an initial 3D geological interpretation to produce a forward prediction of the gravitational
response, we see a misfit with the observed FTG data. An iterative refinement of the 3D geology
model is carried out until a primary reconciliation of both the geological constraints underpinning the
3D geological interpretation and the observed FTG data is achieved.
The second case study is from on-going work in PNG, where it is exceedingly expensive and
frustrating work to acquire 2D seismic data over densely forested limestone country. Some of this work
has previously been reported at McInerney et al. (2007).
Figure 6a shows the forward modelled Tzz response for a proposed 3D geology model and how it
corresponds to the observed airborne gravity signal (Figure 6b). Several iterations of refinement have
been carried out to achieve this degree of correspondence between the observed and predicted
geophysical data sets.
(a)
Figure 6. Bwata Gas Field observed versus predicted vertical gravity gradient signatures, with the
gravity high associated with the Bwata Anticline highlighted. Image (a) is the first vertical
derivative of the fully terrain-corrected Bouguer gravity anomaly data from the airborne gravity
survey. The modelled gravity response (b) is the Gdd component of the gravity gradient tensor.
The two images are displayed using the same colour stretch. Map dimensions are 41 by 40 km.
Conclusions
The methods outlined in this paper have proven to be very useful for signal processing, gridding and
interpretation of FTG data. They require the correct treatment of (non-commutating) tensors as
compound objects without resorting to processing the components separately. This is achieved as a
direct result of recognizing the rotational components within the signal.
When processed correctly, airborne gravity or its gradients can help guide the interpolation geological
units and structures (fault correlations) between seismic lines and/or boreholes, help identify new
features, and assist in the production of depth estimates for basement structures.
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(b)