Fig 1.
Measurement positions: (a) lateral; (b) frontal.
Fig 2.
Measurement layout during lateral investigation.
Zones {1, 2, 3} allowed to distinguish the interferometer echo.
Fig 3.
PS vs frequency of the displacement measured with the radar at three positions of the statue (Fig 2): (a) zone 1; (b) zone 2; (c) zone 3. Three resonant frequencies are visible at 3.55 Hz, 6.00 Hz and 8.4 Hz.
Fig 4.
Frequency vs time of the displacement measured with the radar at three positions of the statue (Fig 2): (a) zone 1; (b) zone 2; (c) zone 3. Persistence over time of three frequencies (3.55 Hz, 6.00 Hz and 8.4 Hz) is visible.
Fig 5.
PS of the displacement measured by the radar (red) and the accelerometer (blue), and the result of the correction (green).
The peak at 6 Hz disappears in the corrected signal, and the signals between 10.5 Hz and 12.5 Hz are attenuated.
Fig 6.
PS of the acceleration measured by the accelerometer placed directly on the floor of the Accademia Gallery, near the radar station.
Fig 7.
PS vs frequency of the displacement measured with the radar at position #3 (Fig 2): (a) 3 s time-windows (the amplitude of the corresponding 3.55 Hz peak is shown); (b) 1 s time-windows (the amplitude of the corresponding 8.40 Hz peak is shown).
Fig 8.
Displacement measured with the radar at position #3 (Fig 2).
Table 1.
Summary of the experimental results.
Fig 9.
Measurement layout during frontal investigation.
Zones {0, 1–2, 3} allowed to distinguish the interferometer echo.
Fig 10.
PS vs frequency of the displacement measured with the radar at three positions of the statue (Fig 9): (a) zone 0; (b) zone 1/2; (c) zone 3. Four resonant frequencies are visible at 3.55 Hz, 5.94 Hz, 8.40 Hz and 11.88 Hz.
Fig 11.
Frequency vs time of the displacement measured with the radar at three positions of the statue (Fig 9): (a) zone 0; (b) zone 1/2; (c) zone 3. Persistence over time of three frequencies (3.55 Hz, 5.94 Hz, 8.4 Hz and 11.88 Hz) is visible.
Fig 12.
PS of the displacement measured by the radar (red) and the accelerometer (blue), and the result of the correction (green).
The peaks at 5.94 Hz and 11.88 Hz are attenuated, while the ones at 5.12 Hz, 11.05 Hz and 14.24 Hz disappear after the correction of the signal.
Fig 13.
PS vs frequency of the displacement measured with the radar at position #3 (Fig 9): (a) 3 s time-windows (the amplitude of the corresponding 3.55 Hz peak is shown); (b) 1 s time-windows (the amplitude of the corresponding 8.40 Hz peak is shown).
Fig 14.
Displacement measured with the radar at position #3 (Fig 9).
Fig 15.
PS vs frequency of the displacement measured with the radar at three positions of the statue (Fig 2): (a) zone 1; (b) zone 2; (c) zone 3. Three resonant frequencies are visible at 3.54 Hz, 5.92 Hz and 8.38 Hz.
Fig 16.
PS of the displacement measured by the radar (red) and the accelerometer (blue), and the result of the correction (green).
The peak at 6.0 Hz disappears, and the signals between 10.5 Hz and 12.5 Hz are attenuated after the correction of the signal.
Fig 17.
Displacement measured with the radar at position #3 (Fig 2).
Fig 18.
First mode shape of the David (different colors refer to different displacement amplitudes): f1,Num = 3.55 Hz.
Fig 19.
Second mode shape of the David (different colors refer to different displacement amplitudes): f2,Num = 8.39 Hz.
Fig 20.
Third mode shape of the David (different colors refer to different displacement amplitudes): f3,Num = 23.11 Hz.