Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Figure 3. Comparison of Levelled and Full Tensor Noise Reduction processed Txy component
data. The FTNR processed data has reduced noise levels and is better able to image subsurface
geological features.
Fast track interpretation of full tensor data
Murphy and Brewster (2007) described a procedure for interpreting Tensor component data that, like
the FTNR process, exploits the properties of full tensor data to compute new and innovative tensor
representations from the final processed and tensor component data. Pedersen and Rasmussen
(1990) recognised this possibility in the days before tensor data acquisition was truly possible and
described the computation of two invariant tensor quantities that makes use of all tensor components.
The output quantities are invariant, i.e., independent of the observer’s frame of reference.
If we assume that the FTNR processed data not only produces a high S/N ratio data set, but one that
is also internally stable and truly representative of the gravity gradient tensor, then it becomes possible
to compute new tensor representations as additional images of the sub-surface geological contribution
to the gravity signal.
Dickinson et al. (2009) summarise the methodology and discuss their application. The rotational
invariant tensor, I2 is a combination of the vertical and horizontal tensor components and is used for
imaging signature patterns arising from 3D shaped geological targets such as fault blocks, igneous
intrusives, salt bodies, and ore bodies. Invariant tensor representations combining just the horizontal
component data facilitate lineament mapping by imaging geological contact information generated
from lateral density contrasts.
Figure 4 shows an example of where the rotation invariant tensor computation was used to isolate
signature patterns associated with an igneous intrusive centre in eastern Canada for Celtic Minerals.
Mataragio and Kieley (2009) describe the compound feature as a series of steeply dipping high
density bodies associated with a prominent fault system. Their close proximity to each other yields the
long wavelength, high amplitude positive Tzz anomaly shown in Figure 4(a). The rotation invariant I2
tensor response is shown in Figure 4(c). The corresponding 1 st vertical derivative response in Figure
4(d) images these individual steeply dipping igneous intrusives particularly clearly.
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