Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Gravity survey specifications and resolution
The anticipated half wavelength resolution of the airborne gravity data was about 2 km. This led to the
specification of a survey flight line spacing of 2 km to provide similar resolution of anomalies both
along and across flight lines. Survey lines were flown east-west along UTM northings divisible by
2 km. A target noise level of 0.7 mGal after grid processing was specified. Final data were supplied as
grids of free air, Bouguer, and Bouguer isostatically corrected gravity.
The survey was sole-sourced in 2007 and 2008 and competitively bid in 2009. Sander Geophysics
Limited (SGL) was the successful bidder in all cases. The AIRGrav surveys were flown with a fixed
wing platform in 2007 and with a combination of fixed wing and helicopter platforms in 2008 and 2009.
About 25,000 line km were flown in each survey campaign, bringing the line km total for the 4 surveys
(i.e., including the Nechako survey block funded by the Geological Survey of Canada) to almost
100,000 line km. The instrument used was the SGL AIRGrav system.
Magnetics, Helicopter TEM and radiometrics
The acquisition of additional regional aeromagnetic data was considered by GBC but the incremental
value of flying more aeromagnetic data was not considered to be very cost effective. Almost all of the
area has been flown at 800m line spacing. Some areas have been flown at 400m line spacing. All of
the data are available from the Canadian national aeromagnetic database. There were insufficient
funds to fly the entire area of interest with line spacing significantly smaller than that of the existing
good quality data, and these data were deemed to be suitable for regional interpretation work.
Similar reasoning precluded flying detailed radiometrics. To be useful, radiometrics would have to be
flown at tight line spacing, taking cognizance of the many variables that need to be considered when
acquiring radiometric data. On a cost/benefit basis for the large area of interest, radiometrics did not
compete with airborne gravity when ranking possible new regional geophysical data sets.
A Helicopter TEM survey was commissioned for the QUEST (2007) survey area on four km line
spacing, using the Geotech VTEM system (Geotech Ltd, 2007). This high-powered system was
chosen to map the depth of overburden and to provide information about the bedrock geology. It was
recognized that if the system crossed a porphyry deposit, then the alteration associated with it might
be detected. Direct detection of massive sulphide deposits was not a primary survey objective. To
assist the interpretation of overburden depths and the mapping of the bedrock geological response a
LEI (Layered Earth Inversion) was commissioned by GBC. A typical inversion section is shown in
Figure 4. One encouraging result of the TEM survey interpretation was that the overburden cover,
while ubiquitous in much of the area, was not as thick as previously thought and considerable areas
have been identified as practical for follow up exploration.
The helicopter TEM system did acquire magnetometer data. This was not a primary data set, but is
high quality low level aeromagnetic data that provides useful information along profiles and it is
available in the data releases. Even though it is only collected on 4 km spaced traverses, it is a useful
data set and has been released with the TEM data.
As a result of the successful QUEST TEM survey, a helicopter TEM survey was flown in the QUEST-
West area (2008) using similar parameters (Aeroquest Limited, 2009). The survey was flown using the
AeroTEM III system.
AEM inversions for the QUEST area were performed using the 1D electromagnetic inversion program
EM1DTM, developed at the University of British Columbia – Geophysical Inversion Facility (UBC-GIF)
(Farquharson and Oldenburg, 1993; Farquharson, 2006). This program is capable of inverting data
from 3 component magnetic-dipole sources. The algorithm is designed to invert for a model with many
more layers than input data so that the character of the recovered model is determined by a model
objective function and not solely by the goal of fitting the data.
In this work, a laterally parameterized method was used to invert the data (Phillips et al., 2009). Late
time decays are calculated to establish an estimate of background conductivity for each sounding.
These are smoothed laterally to establish a smooth late time model for use as a starting model for the
inversion of each TEM sounding. To establish a depth of investigation, a total conductance rule was
established, with a depth cut-off chosen as a function of the total conductance of the layers to that
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