Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
wells have been drilled in the basin; the Pandora well, which was a discovery, and the Uranus well,
which terminated inside salt. Recent discoveries in other nearby basins suggest potential for further
hydrocarbon discoveries within the Nordkapp Basin (Hokstad et al., 2009). Improved seismic imaging
changed the structural interpretations of the salt diapirs. What were initially thought of as wide salt
stocks with vertical flanks turned out to be more complex geometries with broad diapirs overhanging
narrow stems. Much of the exploration risk associated with these structures results from distortions in
the seismic imaging, and subsequent ambiguity of the salt isopach.
A full tensor gradient (FTG) survey was acquired over the Nordkapp Basin with the intent of
delineating salt geometry. The Tertiary rocks in the area have a density between 2.30 and 2.38 g/cm 3 .
The salt diapirs are characterized by negative density contrasts relative to the surrounding sediments
and can be identified from the gravity gradiometry data. In this paper, we focus on results for the
Obelix prospect in the southwest of the basin, particularly the G2, F1 and F2 salt diapirs shown in
Figure 3.
We performed 2D migration of gzz and gzx data along profile A-A’ (Figure 4) and the nine west to east
profiles (Figure 5). The profile locations are shown in Figure 3. We also show the cross-sections
corresponding to the nine west to east profiles that were obtained from 3D regularized inversion in
Figure 6, where the entire 28 km x 17 km survey area was discretized into approximately 2.8 million
100 m x 100 m x 100 m cells (Wan and Zhdanov, 2008). For comparison, the same typical negative
density contrasts are seen in both of these figures.
Figure 4 is the 2D gravity migration along profile A-A’. This is co-rendered with the corresponding
seismic depth migration image. Salt diapir F2 is clearly identified in both the gravity and seismic
migration images. Each 2D gravity migration ran in the order of minutes, whereas the 3D joint
inversion of multiple components from all 48,051 observation points took less than a day. The quoted
runtimes are for a 64-bit desktop PC running Windows Vista with a 2.4 GHz serial processor and 8 GB
memory.
Conclusions
We have introduced a new method for interpreting gravity gradiometry data based on potential field
migration. This method is based on an integral transformation of gravity gradiometry data into an
image of subsurface density distributions. Potential field migration is fast and stable, requiring just
minutes for 2D migration of entire profiles of data. It can be used for real-time imaging or for preparing
an a priori model for subsequent 3D regularized inversion. This method can be naturally extended to
3D, as well as to magnetic fields and their gradients, and this constitutes one of our future research
activities.
We also have also demonstrated the use of focusing stabilizers for large-scale 3D joint inversion of
entire datasets to models with millions of cells within a day on a desktop PC. Our implementation
naturally lends itself to large-scale parallelization, and we are currently in the process of distributing
our software on massively parallel architectures. This will allow for further decrease in the runtime and
will allow us to jointly invert even larger gravity and gravity gradiometry datasets.
We have demonstrated both migration and inversion with a case study for salt mapping from the
Nordkapp Basin in the Barents Sea.
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