Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
rejected by Studinger et al. (2008) for noise analyses due to large navigation errors, whereas we
included flight 4 pass 2 in our calculations of noise.
Data collected within specified flying conditions
Table 3 shows the results from the ten accepted passes with the GT-1A compared to the data from the
AIRGrav system as reported by Studinger et al. (2007). Note that these ten accepted GT-1A passes
are the lines which were flown full length and which pass the standard QC procedures for GPS data
quality, for the number of coarse channel saturations, and for the value of the GT-Grav residual. The
data on these ten lines fall within the specifications of the GT-1A dynamic range of ± 0.5 g and would
normally be accepted in a commercial survey.
Flight turbulence and bandwidth
In Table 1 of Studinger et al. (2008), the authors show that the absolute value of the 2-Hz GPSderived
vertical acceleration did not exceed 0.44 g for all flights, and that the average over all lines
was 0.04 g, both values being well within the GT-1A limit of ± 0.50 g. Studinger et al. (2007) state that
the four passes on Line 100 were flown under “near ideal conditions”, and that “vertical accelerations
did not exceed 0.1 m/s 2 during these flights”.
The GT-1A gravimeter samples vertical acceleration at a rate of 300 Hz (fN = 150 Hz) and detected a
total of 384 occasions when the vertical acceleration exceeded ± 0.5 g during the 24 passes flown.
These accelerations occurred on average every 20 to 30 seconds on the more turbulent lines and led
to a rejection of 9 lines, 2 of which were apparently flown under “ideal conditions” as reported by
Studinger et al. (2007).
During normal commercial operation, the GT-1A is flown during those hours of the day when local
flight conditions are within ± 0.5 g as measured by the GT-1A at 300 Hz. This is readily determined by
experience within a few days after the start of each survey.
We carried out a model study of random vertical aircraft accelerations as would be measured by GPS
at 2 Hz compared with those measured by the GT-1A at 300 Hz. We used accelerations with various
durations ranging from 0.1 to 2.0 seconds and with various magnitudes of up to ± 1.2 g. We concluded
from this study that the amplitude of vertical acceleration determined with a 300 Hz sampling rate can
be several times greater than 2-Hz GPS-derived acceleration data. We therefore conclude that vertical
accelerations reported by Studinger et al. (2008) were most likely derived from low frequency GPS
data sampling (whether 2 Hz or 10 Hz).
Analysis of the May 2007 LDEO test flight data
The results of the Turner Valley repeat line tests carried out in 2007 showed the limitation of the
GT-1A during turbulence and when changing altitude in drape mode. However, this gravimeter has
typically been used to carry out commercial surveys at a constant barometric altitude and there have
been very few re-fights due to turbulence because of careful attention to pre-flight planning and flight
timing. When considering only those data acquired under turbulence conditions which did not exceed
GT-1A tolerances, the gravimeter consistently produced high quality data within specifications.
An important point to note when referring to measurements made by digital sampling is that the
sampling frequency, and hence the measurement bandwidth, determines the accuracy and resolution
of the parameter being measured. The magnitude of accelerations sampled at 2 Hz can be
significantly smaller than those sampled at 300 Hz due to under-sampling of the high frequencies of
inertial motion experienced by a small survey aircraft during turbulence.
Studinger et al. (2008) concluded that “Both systems, the AIRGrav and GT-1A, can produce higher
resolution data with improved flight efficiency than can the BGM-3 and LaCoste & Romberg
gravimeters.” However, we would add a further conclusion, subject to possible adjustment of the
AIRGrav performance figures presented in Studinger et al. (2007, 2008) by Sander Geophysics: that
the GT-1A system performs as well as or better than the AIRGrav system in conditions where the
GT-1A does not exceed its maximum specified vertical acceleration of ± 0.5g (Figure 11).
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