Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Figure 4. Effect of depth of burial on Gzz response. Excerpt from the Broken Hill regional gravity
gradient survey, showing observed Gzz (top left). Observed Gzz plus calculated Gzz for a
hypothetical 50 Mt orebody at the surface (top right), at 200 m depth (bottom left), and 400 m
depth (bottom right). The colour scale in units of Eo is the same for all images.
Sensitivity to near-surface density variations
Inverse cube spatial dependence renders gravity gradients more sensitive to shallow features.
Sensitivity to near surface density variations is desirable when exploring for shallow targets, e.g.,
kimberlites, UXO, etc. However, all near-surface density variations are emphasised, which is not
necessarily beneficial, especially since the near-surface is often characterised by rapid and marked
variations in density, e.g., due to weathering or geomorphology. .
The mixed blessings arising from sensitivity to shallow density variations are illustrated using FALCON
Gzz data from a portion of the Ekati mine leases, NWT, Canada (Figure 5). Kimberlites in this area are
less dense (~2.4 g/cc) than the Archaean basement (~2.7 g/cc on average). However, Gzz lows
associated with lakes can complicate and sometimes obscure lows due to kimberlites. If the lake
bathymetry were known, the contribution to Gzz from the lakes could be modelled as a component of
the “terrain”.
In the absence of lake bathymetry data, VPmg geometry inversion (Fullagar et al., 2008) was
employed to determine the water depths that would be required to account for the Gzz variations
observed over the lakes. Gridded Gzz data, terrain corrected for uniform 2.0 g/cc ground, were
inverted, assuming an 80 m drape. The geology was represented as a water layer (1.0 g/cc) overlying
basement, which was assumed to be homogeneous with respect to density. The water layer was
initially zero thickness everywhere, and was only permitted to grow in thickness over the lakes.
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