Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Table 2. The results achieved for test lines flown at Geraldton with the new shock mount readily
met the GT-1A specifications in all cases. The average RMS repeat line noise level of all results
on three gravimeters over five flights was 0.44 mGal.
IAS RMS Flight mean
Flight SN Pass RPM (knots) (mGal) (mGal)
1 SN005 1 1800 130 0.47
2 1800 130 0.58
3 1800 130 0.67
4 1800 130 0.56 0.57
2 SN005 1 1850 130 0.55
2 1800 130 0.41
3 1750 130 0.48
4 1700 130 0.51
5 1650 115 0.31
6 1600 115 0.36 0.44
3 SN009 1 1850 130 0.34
2 1800 130 0.63
3 1750 130 0.43
4 1700 130 0.49
5 1650 115 0.48
6 1600 115 0.55 0.49
4 SN009 Line A 1750 130 0.35
Line B 1750 130 0.48
Line C 1750 130 0.33
Line D 1750 130 0.23 0.38
5 SN001 1 1750 137 0.39
2 1650 149 0.30
3 1600 131 0.36
4 1550 146 0.29
5 1500 122 0.30
6 1700 155 0.40 0.34
Shock mount development summary
The new two-stage vibration isolation module works extremely well to isolate frequencies above
approximately 25 Hz, with isolation ranging from -20 to -39 dB over frequencies from 80 and 500 Hz. It
has the added advantage of also blocking non-orthogonal vibrations. The original mechanical spring
shock mount on the other hand has an average isolation of -11 dB across the same frequency band.
We proved that acoustic dampers, i.e., heavy aluminium panels to replace the original thinner access
panels on the sides of the gravimeter, are very effective for reducing the effects of acoustically coupled
vibrations. The success of these dampers eliminated the need to pursue an alternate option to use
heat-retaining acoustic foam. The latter option would have had detrimental side effects in that it would
have caused the gravimeter to overheat when operated in conditions of high ambient temperature.
Overall, a combination of the new shock mount and acoustic dampers has effectively removed any
signs of offsets and high noise levels originating from aircraft vibrations in all aircraft tested to date.
158