Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Integrated software processing and interpretation
methods for airborne gravity
Summary
Desmond FitzGerald 1 and Rod Paterson 2
1 Intrepid Geophysics (des@intrepid-geophysics.com)
2 Intrepid Geophysics (rod@intrepid-geophysics.com)
The increased usage of airborne gravity methods (incorporating both airborne gravity and airborne
gravity gradiometer instruments in this context) over the last four to six years is pushing the evolution
of downstream processing and modelling software.
The approach to signal processing of airborne gravity data is expanding from scalar to full
vector/tensor methods. Gravity data are being acquired largely for basin studies in all sorts of terrain
including transition zones, over sand dunes and rugged volcanic country. Adaptive vector and tensor
processing methods are being used to take advantage of the physics of the measurements rather than
shoe-horning everything into a scalar world.
Software tools have been developed to help assess the quality of airborne full tensor gravity gradient
(FTG) signals in either line or grid format. Rotational noise is minimized by using a spherical linear
interpolation (SLERP) gridding method followed by the application of a filtering method for minimizing
tensor residual errors (MITRE). The new process has some similarities with the minimum curvature
gridding method but is designed to check for the consistency of the tensor components. Fourier
domain FTG methods that are optimized and purpose built for vector/tensor data are available and
include low pass, band pass and integration functions. Particular attention has been paid to vector and
tensor gradient terrain corrections for airborne gravity. It is critical to apply corrections for aircraft
movements and terrain effects before attempting interpretation and integration with other data.
GeoModeller has been used to harmonize all geoscientific data and thinking for projects. With well
prepared gravity survey data at hand, a 3D geological model can be used to constrain inversion
processing to provide insight into the sub-surface.
Several case studies are presented, one from Libya and one from Papua New Guinea. They
demonstrate how the new generation software tools help to achieve a consistent interpretation of
geological strata and faults beyond drill holes and seismic lines. The ability to rapidly predict the
gravitational response of a model and compare this response to the observed data, be that vector or
tensor in nature, is a vital requirement for the uptake of this workflow element by users.
Comprehensive and integrated software is required so that any measured component of gravity or its
curvature gradients can be preserved and utilized during the processing and modelling stages, and
thereby avoid the distortions and approximations of the past. Downstream users require 3D high
definition geological models with quantified uncertainties as the deliverables so that they can
subsequently target specific features for further testing, or perform simulation and resource
optimization calculations.
Introduction
The currently available airborne gravity acquisition systems are proving cost effective and adequate
for the task of delineating prospective sedimentary basins for oil and gas, uranium and iron ore
exploration efforts. Future improvements will not only arise from hardware and acquisition system
development, but through purpose built software to extract the maximum signal content. A key to this
is honouring the vectorial nature of the gravity field, in particular the directly measured curvature
gradients, using tensor components. Gravity processing is sometimes assumed to be easy, but for
those that grapple with it day-to-day, the ability to be discerning down to better than 1 part in 10 9
always causes issues. Adopting a formal vector/tensor representation for all data processing
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