# Scanning Inside Volcanoes with Synthetic Aperture Radar Echography Tomographic Doppler Imaging
## 4 Validation of the Sonic Tomographic SAR Results
### Vibration Validation  
Vibrational validation is carried out by comparing data estimated by the satellite with those observed by the \"in situ\" seismological stations installed at Vesuvius. In situ data are provided in open source from the Italian National Institute of Geophysics and Volcanology (INGV), and are downloadable from the following website: [[https://eida.ingv.it/it/](https://eida.ingv.it/it/)]([https://eida.ingv.it/it/](https://eida.ingv.it/it/)), accessed on 23 April 2022. In this context, the vibrational data estimated by the satellite and those provided by the seismic in situ stations listed in Table 2 are synchronized in space and time.  
Figure 17 shows the map where the Vesuvius seismic stations are located. IV-VCRE and VBKN in situ station data are considered in this stage and are the only ones compatible with line-of-sight radar visibility. All other measurement stations were either not visible or were located outside the SAR imaging scene. Figures 18a,b and 19a,b represent the optical and radar observations, respectively, of the seismographic stations. The first facility is located at the east side of the crater and the second one is installed toward the northern side. The two stations are successfully represented on both the optical and radar images (indicated both by arrows number 1). In Figure 20a-c, the vibrational spectrum streaming recorded by the IV-VCRE seismographic station, superimposed to the respective spectrogram recorded by radar, is depicted. Figure 20a represents the unfiltered full-bandwidth spectrum, while Figure 20b is the filtered spectrum; finally Figure 20c represents a narrowed detail spectrum at approximately 1 kHz of bandwidth. The blue color function is that of the \"in situ\" station, while the brown color function is the radar-estimated vibrational spectrum. The native time trend (1 s of synchronized streaming) is shown in Figure 21a. The filtered time-domain plot (approximately composed of 1-s wide synchronized data streaming) is reported in Figure 21b, finally the IV-VCRE in situ station versus SAR vibrational streaming measurement errors is proposed in Figure 21c. The blue function represents the unfiltered error, while the one in brown is the filtered error. It can be seen that this error, while oscillating around zero, remains confined to very low values. In Figure 22a-c, the spectrum of the vibrational streaming recorded by the IV-VBKN seismographic station is represented. Figure 22a represents the spectrum in its entirety, Figure 22b is the filtered spectrum, while Figure 22c represents a narrowed detail dataset at approximately 1 kHz of bandwidth. Also, in this case the blu and the brown functions are the in situ and the radar estimated vibrational spectrum, respectively. The unfiltered time-domain sampled vibrations (approximately 1-s synchronized streaming) are shown in Figure 23. Although it is very difficult to perfectly synchronize two different measurement instruments--the first one is located on the ground, while the second one is located in space--such time synchronization turns out to be very acceptable. Both instruments measure the seismic background of the area. This result brings the SAR's effective measurement accuracy to attention.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**”In-Situ” Station**} & **Network** & **Channel** & **Location (Lat, Lon)** \\ \hline Vesuvius-East Crater & IV-VCRE & HH & 40.818999\({}^{\circ}\), \\ Vesuvius-North & & & 14.431419\({}^{\circ}\) \\ Bunker & IV-VBKN & HH & 40.829959\({}^{\circ}\), \\  Bunker & & & 14.429881\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Characteristics of the “in situ” seismographs.  
Figure 19: IV-VBKN seismograph station, located on the north side of the crater. (**a**) Optical image. (**b**) SAR image.  
Figure 17: “In situ” seismograph stations.  
Figure 18: IV-VCRE seismograph station, located on the east side of the crater. (**a**) Optical image. (**b**) SAR image.