Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
GT then undertook a detailed study of the design of the main gravimetric sensing element, or vertical
accelerometer, and found that under certain conditions of induced non-orthogonal vibrations, i.e.,
vibrations non-orthogonal relative to the base of the gravimeter, the accelerometer output could be
offset by several tens of mGal. They concluded that the diagonal spring/damper assemblies should be
removed as they could potentially induce non-orthogonal vibrations.
New shock mount design
In order to eliminate the problems associated with vibrations, in particular non-orthogonal vibrations,
and to thus improve the performance of the gravimeter under a wider range of installations and flying
conditions, we designed a totally new shock mount during the fall of 2006.
The new shock mount (Figure 3) has two stages of isolation that make use of special closed-cell
polyurethane foam of a specific density which has exceptional damping properties and durability. We
designed the shock mount, or two-stage vibration isolation module, to specifically remove nonorthogonal
vibrations. We obtained design assistance from two independent consultants in Canada
and Australia who are considered experts in the field of vibration isolation.
The natural frequencies of the two isolation stages differ by several Hz which helps to isolate
vibrations, and the new shock mount has a different weight distribution. These two features are both
important attributes of a two-stage vibration isolation system. Both stages are further isolated from
aircraft vibrations by eliminating contact between the main structural bolts and the base-plate. The
prototype shock mount used a heavy steel plate in the centre stage, but we replaced this with two
thinner aluminium plates to remove potential interference with magnetometers which may also be on
board the survey aircraft. The thinner aluminium plates are much lighter and are thus easier to handle,
ship and install.
Figure 3. The prototype of the new two-stage vibration isolation module or ‘shock mount’. The
steel plate seen here above the first stage isolator was later replaced by two lighter aluminium
plates for ease of handling.
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