Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Lines accepted
Five out of 24 passes flown in the Turner Valley tests were not considered in calculating noise levels
because they were short segments. This left us with 19 full-length passes of which we rejected nine
due to high gravity residuals caused by excessive turbulence, although we also rejected two of these
passes due to poor GPS data quality. This left us with a final ten full-length passes which we accepted
for comparison with the AIRGrav system results: 5 out of 5 passes on Line 100; 3 out of 4 passes on
Line 200; and 2 out of 4 passes on Line 400.
Studinger et al. (2007, 2008) include results in various graphs and tables for GT-1A data which fall
well outside the dynamic range limitation of this instrument. For example, Figures 4 (b) and (d) in
Studinger et al. (2008) include data for all GT-1A passes, whereas we found it necessary to reject
some of those passes. Our analysis of the GT-1A data is summarised in Table 3 and Figure 11. These
results show that the GT-1A data which were collected within specified flying conditions of ± 0.5 g
meet the instrument’s specifications for straight and level flying.
Table 3. The RMS noise levels achieved by the GT-1A when considering only those lines flown
within its specified dynamic range of ± 0.5 g, compared with the results achieved with the
AIRGrav system taken directly from Studinger et al. (2007). In the absence of indications to the
contrary, we have assumed that all data used in computing the results shown for the AIRGrav
system were within the dynamic range of that instrument.
Line
Passes Flown
(Accepted)
GT-1A
RMS (mGal)
162
AIRGrav
RMS (mGal)
100 10 (5) 0.49 0.50
200 4 (3) 1.77 1.83
400 4 (2) 1.19 1.61
Figure 11. A comparison of the noise levels achieved by the GT-1A compared with the AIRGrav
when only lines within the specified dynamic range of the GT-1A are taken into account.
Finally, we note that in Figure 8 of Studinger et al. (2008), results from 6 passes along a short
segment of Line 200 are shown. However, we rejected flight 4 pass 1 due to an excessive number of
coarse channel saturations (51) and a high residual. This pass, coloured purple in Figure 8, has large
excursions from the mean of the other passes. We rejected this pass for the same reason it would be
rejected in a commercial survey, i.e., the data were collected in flight conditions outside the
specifications of the GT-1A. It should be noted that flight 2 pass 2, as well as flight 4 pass 2, were