Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Using Solid Earth to enhance interpretation
Summary
Gregory B. Walker 1
1 BHP Billiton (Greg.B.Walker@BHPBilliton.com)
BHP Billiton deploys a set of applications and associated software tools for interpreting geophysical
data, which are collectively known as Solid Earth. Solid Earth provides a framework for the
inversion of geophysical data into physical property models that can be presented as 3-Dimensional
volumes. In particular, Solid Earth contains a module dedicated to the inversion of FALCON
Airborne Gravity Gradiometer and magnetics data. Recovered density and magnetic susceptibility may
be treated using a Self Organizing Maps algorithm and related to petrophysical lithology classifications
to produce 3-Dimensional volumes representing pseudo-geology.
Introduction
BHP Minerals (now BHP Billiton) developed the FALCON Airborne Gravity Gradiometer (“AGG”)
system in the 1990’s in conjunction with Lockheed Martin (van Leeuwen, 2000). FALCON was the first
airborne gravity gradiometer system designed with noise and resolution characteristics suited to
minerals exploration. Test flights took place in 1999 and the first system (Einstein) commenced a
production survey later that year. Einstein and the subsequent systems (Newton, Galileo and
Feynman) have acquired over 1.5 million line-km of data over a range of exploration deposit styles in 5
continents. The systems were sold to Fugro Airborne Surveys in 2008.
The FALCON AGG consists of two complements of accelerometers mounted on a near-horizontal
slowly rotating platform. The instrument measures the differential curvature gravity gradients, referred
to as GUV and GNE (Lee, 2001). The two complements, referred to as A and B, deliver four
measurements of the gravity gradient potential (ANE, AUV, BNE, BUV). The difference in output between
the A and B complements is monitored as an indicator of the system noise level.
The curvature gravity gradients are not very intuitive for qualitative interpretation, so the data are
transformed using two methods (Fourier transformation, and an equivalent source method) to produce
2D grids of the vertical gravity gradient (GDD) and vertical gravity (gD). Interpretation of potential field
data can be fundamentally improved by 3-Dimensional (3D) visualisation of recovered density and
magnetic susceptibility source volumes. Furthermore, Self Organizing Maps (SOM) methods (e.g.,
Kohonen, 1995; Chang et al., 2002; Fraser and Dickson, 2007; Fraser and Hodgkinson, 2009) can be
applied to define the relationships between density and susceptibility in 3D space, leading to
geological characterisation of survey results.
Application development and method
After the launch of the FALCON systems, BHP Billiton Principal Geophysicists Rob Ellis and Peter
Diorio commenced development of 3D inversion algorithms and user interfaces which adapted the
University of British Columbia Geophysical Inversion Facility (“UBC-GIF”) magnetic and gravity
inversion algorithm (Li and Oldenburg, 1996, 1998) to the total magnetic intensity (TMI) and differential
curvature gravity gradients measured by FALCON. The developments were incorporated into a set of
applications and associated software tools for interpreting geophysical data, which are collectively
known as Solid Earth. Solid Earth provides a framework for inversion of geophysical data into
physical property distributions that can be presented as 3D models.
188