Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Sensitivity to the near-surface is a mixed blessing. On the one hand, it accentuates the influence of
topography, at the expense of geology, as illustrated at Red Dog, Alaska. Consequently, “terrain
correction” is normally applied prior to interpretation of gravity gradient data. Sensitivity to the nearsurface
can be equated to a loss of sensitivity at depth. This was illustrated by examining the
detectability of a hypothetical 50 Mt orebody as its depth was increased from 0 to 400 m. When
superimposed on real FALCON data from Broken Hill, Australia, the synthetic orebody response was
unrecognisable as a target of interest when buried to 200 m.
Sensitivity to the near-surface is advantageous for definition of outcropping or sub-cropping targets.
This has been illustrated in the context of kimberlite exploration in the vicinity of Ekati, Canada. The
lakes in the area can complicate the Gzz response. The apparent bathymetry of lakes was derived via
geometry inversion. Several of the known kimberlites coincide with depth anomalies. This provided a
means for ranking other Gzz anomalies as potential kimberlite targets.
One consequence of the extremely small magnitude of gravity gradients is that the data are lowpassed
filtered along flight lines. Therefore for quantitative modelling, it is important to apply an
equivalent low-pass filter to calculated gravity gradients, and thereby avoid introducing short
wavelength contributions which are absent from the measured data. This was illustrated using
synthetic data in the context of terrain correction at Red Dog.
Depending on the acquisition system, the interpreter may have a choice between a number of different
“gradient quantities” for interpretation. For quantitative interpretation, our experience suggests that
there appears to be little benefit in looking beyond Gzz or perhaps the full tensor amplitude, ||G ~ ||, if
this is available.
Acknowledgments
Geological data from Red Dog were compiled during the cited CAMIRO project, and are included by
kind permission of Teck Corporation. FALCON data from Ekati are published by kind permission of
BHP Billiton Canada. We are grateful to Jon Carlson and Greg Walker of BHP Billiton for their support
and technical input in relation to the Ekati example. The Broken Hill Airborne Gravity Gradiometry
Survey was a joint project between the NSW Department of Mineral Resources, pmd*CRC,
Geoscience Australia, Gravity Capital and BHP Billiton. The partners in this project have kindly made
the data available to the public.
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