Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
This paper describes the developments made to Air-FTG ® technology since the introductory
publication (Murphy, 2004). A re-assessment and renewed understanding of full tensor gradiometry
led to the development of strategies to best exploit what it offers. The improved acquisition procedures
prompted development of new and improved QC, data processing and interpretational tools to
facilitate an efficient workflow that could produce high quality data in a timely fashion.
Full tensor gravity gradiometry
Murphy (2004) describes full tensor gravity gradiometry (FTG) as a means of measuring changes in
the gravity field in all directions of the field, i.e., to simultaneously measure changes and influence of
changes in each of the vertical and horizontal components of the gravity field (Figure 1). This is a
fundamental difference to conventional gravity in that it highlights shorter spatial wavelength aspects
of the gravity field in comparison to conventional gravity meters measuring only the vertical component
of the gravity field vector.
Individual tensor components are often represented as elements of a nine component tensor matrix
(Figure 1) and since the gravitational potential honours the Laplace Equation, the sum of the diagonal,
or trace, is zero. Further, there is symmetry about the diagonal elements with the Txy, Txz and Tyz
being equivalent to Tyx, Tzx and Tzy respectively. Thus, there are truly only 5 independent
components in the Tensor field, i.e., Txx, Tyy, Txy, Txz and Tyz. It is these components that are
measured by an FTG instrument.
Figure 1. Schematic diagram showing the gravity field tensor components (red) in relation to the
gravity vector elements (gold). The nine tensor components are summarised in matrix form,
where only 5 components are truly independent.
The benefits of measuring the individual tensor components are two-fold, i.e., (1) additional
information for geological interpretation and (2) improved constraints for extraction of clear signal from
measured data. The geological application is described by Murphy (2004) who prescribes use of Tzz
for mapping of geological bodies and the horizontal component data to image attributes of a target’s
geological setting, i.e., orientation, thickness, depth, dip (if any) and shape. Murphy (2004) also
describes how the individual response patterns are unique to each tensor component and how these
may be used for interpretation.
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