Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Operational improvements - turbulence and noise
In both AirG and AGG surveys, the dynamic behaviour of the aircraft, driven by air turbulence, is the
major source of noise in the data. In low-level surveys, survey line turbulence levels (measured as the
RMS variation of the vertical acceleration in the frequency band [0.3 – 10.0] Hz over an entire survey
line) are most often in the range from 0.4 ms -2 to 1.0 ms -2 (Figure 1). The gravity field of interest is four
or five orders of magnitude smaller.
Figure 1. A histogram of average turbulence along 14,437 separate AGG survey lines acquired
from Cessna Grand Caravan aircraft during the period September, 2004 – October, 2008. Survey
ground clearances were typically around 100 m. The acquisition occurred in all seasons and in
many countries. FALCON AGG data are not usually acquired if turbulence exceeds 1 ms -2 so that
the histogram is skewed to lower turbulence. Nevertheless, it is clear that turbulence levels in
low-level geophysical surveys are usually above 0.4 ms -2 .
In AirG surveys, a range of tactics are employed to minimise the impact of turbulence on the gravity
data. Primary tactics are the use of differential GPS to track aircraft motion and correct for these
effects in the gravity data, low-pass filtering to remove the higher frequency effects of turbulence, and
survey design to minimise the exposure of the system to turbulence. Improvements in GT-1A
operational procedures and optimised filter design have led to improved consistency in delivering low
noise AirG data. Good data processing holds the key to producing the desired results and typically a
107 second filter is run through GT-1A data to yield 0.5 mGal accuracy at about 3.5 km spatial
resolution. Depending on the ultimate target depths and sizes, some flexibility exists in designing
surveys to provide appropriate fit-for-purpose solutions for the client (Figure 2).
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