Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Figure 2. This accuracy versus resolution graph for an aircraft travelling at 69 m/s for filters of
varying lengths (in units of seconds) applied to GT-1A data shows the practical limit determined
by in-flight measurement characteristics and noise-levels of 1.1 mGal accuracy at about 1.7 km
spatial resolution. Sufficient spatial over-sampling allows 2- and 4-fold stacking to further improve
accuracy to 0.8 and 0.55 mGal respectively, as shown by the dashed curves.
AGG instruments are specifically designed to remove aircraft dynamics and consequently cope with
turbulence better than AirG systems. However, residual dynamic sensitivity remains the major source
of noise. The ability to provide accurate gravity and gravity gradient measurements at turbulence
levels up to 1.0 ms -2 has been demonstrated with the FALCON technology. Dransfield and Lee (2004)
reported productivity comparable with aeromagnetic surveys and accuracy of 4 - 7 E RMS in a 0.18 Hz
bandwidth (equivalent to a Nyquist sample spacing of about 150 m). The variation in accuracy is well
correlated with variations in turbulence. Boggs and Dransfield (2004) reported the error in FALCON
gravity at 0.1 mGal/√km. Since 2004, improvements have been made in processing and instrument
control so that difference error levels today range from 2 – 5 E in a 0.18 Hz bandwidth in turbulence up
to 1 ms -2 . The average difference noise in all surveys flown in 2006 to 2009 is 3 E RMS in this
bandwidth providing a Nyquist sampling rate of 150 m. Gravity gradient error has halved since 2004.
Minerals and water exploration
Applications in airborne exploration for water and minerals generally require AGG data because of its
higher spatial resolution. FALCON AGG data have been successfully used in exploration for a very
wide range of deposit styles and commodities (Dransfield, 2007) including iron, coal, copper, leadzinc-silver,
gold, nickel and diamonds.
Over the last 6 years, there have been two major advances in the deployment of FALCON AGG
systems of direct relevance to mineral exploration: HeliFALCON and joint FALCON – EM deployment.
The digital AGG and HeliFALCON
One limitation of all airborne gravity gradiometers before 2005 was their large volume and mass.
Boggs et al. (2005) announced the development of a digital AGG (called Feynman) in which the large
rack of AGG electronics was replaced by a smaller set of electronics contained within the AGG
binnacle. Feynman has less weight and occupies less space than all other AGG systems. In addition,
Feynman has better sensitivity at low turbulence.
The smaller mass and footprint of Feynman makes it possible to carry the system in a light, readily
available and cost-effective helicopter such as the Aerospatiale 350-B3 (Figure 3). Initial test results
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