When researchers measure the vibrations
of a structure, they usually measure its acceleration
using special instruments called accelerometers.
(20 mm diameter and 12mm long)
mounted on a steel block.
These extremely sensitive electronic instruments can detect
tiny movements of a very stiff structure, like a concrete
dam, when there is hardly any wind blowing on it at all (Suprisingly,
do move in the wind).
Here is one placed inside the gallery of a concrete dam.
It is protected in a plastic bag to stop moisture affecting
the electronics. Electrical signals from the accelerometers
run along the cables back to the monitoring and recording