Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
program to redefine the North American vertical datum. Photograph supplied by Theresa Diehl, NOAA
- National Geodetic Survey.
The HeliFALCON airborne gravity gradiometer (Dransfield et al., 2010) and RESOLVE airborne
electromagnetic systems photographed at Norm’s Camp, Ekati, Canada. The digital AGG is mounted
behind the pilot’s seat, with twin LiDAR systems in a pod mounted below and behind the cabin. The
RESOLVE AEM bird is resting on the ground in front of the Aerospatiale 350-B3 helicopter.
Photograph by G. Gooch, supplied by Mark Dransfield, Fugro Airborne Geophysics Pty Ltd.
Bottom row, left to right
Bell Geospace Air-FTG ® airborne gravity gradiometer system mounted in a Zeppelin airship (Hatch
and Pitts, 2010). The stable, slow moving airship provided a perfect platform for acquiring high quality
data for the De Beers Group in Botswana during 2006. Photograph supplied by David Hatch, Gedex
Inc. (formerly with De Beers Group Services).
Images of residual gravity, total magnetic intensity and electromagnetic B-field response in channel 9
from the joint FALCON digital AGG, TMI and TEMPEST survey at the Kauring Test Site (Dransfield
et al., 2010; Howard et al., 2010). Each image covers the prospect-scale core region of the test site,
an approximate area of 2 km by 10 km. The survey was flown by Fugro Airborne Surveys as a proofof-concept
experiment in a CASA 212 twin-engine aircraft in late 2009. The response of features with
anomalous density, magnetisation and conductivity properties is evident in the central portion of each
image. Images supplied by Mark Dransfield, Fugro Airborne Geophysics Pty Ltd.
VK1 airborne gravity gradiometer system mounted in a Cessna 208 aircraft (Anstie et al., 2010). The
system, being developed by Rio Tinto in collaboration with the Department of Physics at the University
of Western Australia (UWA), has the ambitious goal of achieving a noise level better than 1 Eo in a
bandwidth of 1 Hz and is undergoing final preparations prior to flight trials. Photograph supplied by
Theo Aravanis, Rio Tinto Exploration.
Image of regional Bouguer data acquired with an AIRGrav airborne gravity system operated by Sander
Geophysics on behalf of Geoscience BC and the Geological Survey of Canada. The survey, covering
a substantial part of British Columbia, Canada, was flown to stimulate mineral and petroleum
investment in British Columbia (Kowalczyk et al., 2010). Approximately 100,000 line km of data were
acquired on lines spaced at 2 km intervals in four campaigns during the period 2007-2009. The survey
covers an area of approximately 150 km by 1000 km. Image supplied by Luise Sander, Sander
Geophysics.
Units
Physical quantities should be expressed according to the Système International d'Unités (International
System of Units, international abbreviation SI). The Bureau International des Poids et Mesures (BIPM)
is the custodian of this system (http://www.bipm.org/en/si/). As part of a collaborative effort, the U.S.
adaptation of the SI is managed by the National Institute of Standards and Technology (NIST)
(http://www.nist.gov/physlab/div842/fcdc/si-units.cfm).
The SI unit for acceleration is “metre per second squared” (m/s 2 ). The signals encountered in gravity
surveys for exploration are small, and the prefix “micro” is commonly used (micrometre per second
squared, μm/s2). The gal (or Gal), equal to 1 cm/s 2 , is a derived unit for acceleration in the CGS
system of units. A prefix of “milli” is commonly used (milligal, mGal). In rare cases in the literature, a
“gravity unit” (gu) may be encountered. In this publication, the μm/s 2 has been the preferred unit for
gravity measurements, but mGal has been accepted.
1 μm/s 2 = 10 -6 m/s 2
1 mGal = 10 μm/s 2
1 gu = 1 μm/s 2
The gravity gradient is a gradient of acceleration and so the appropriate units are acceleration units
divided by distance units. Thus, “per second squared” (s -2 ) is appropriate in the SI system. Typical
gravity gradients measured in exploration are extremely small, and the prefix “nano” is appropriate in
most circumstances (per nanosecond squared, ns -2 ). The eotvos unit (E or Eo), although not
recognised in either the SI or CGS systems, is used almost universally in geophysics as the unit for
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