# Scanning Inside Volcanoes with Synthetic Aperture Radar Echography Tomographic Doppler Imaging
## 3 Experimental Results  
In this section, all the experimental results are described by processing a SAR image acquired by the COSMO-SkyMed Second Generation (CSG) satellite constellation. The data concerns a spotlight-2A acquisition mode, in the horizontal-horizontal (HH) polarization, with a Doppler band of about 22.5 kHz, and a chirp band of about 450 MHz. For this first case study, both the optical and the SLC image in magnitude are depicted in Figure 5a,b, respectively. The characteristics of the employed SAR image are listed in Table 1. Figure 6a is the representation of the SAR (magnitude) image where Vesuvius with its main crater is visible. Yellow line 1 is where the seismic tomogram is calculated. The test dataset is constituted by an entire range line belonging between the near range and the far range of the SAR image. Figure 6b is the result representing the estimated seismic tomography. The depth of investigation is about 3 km. In the figure, the surface topography of the volcano and its entire interior are visible. From the tomogram, it is possible to observe the crater of the main conduit inside red box 1. From this result, we observe the presence of vibration energy accumulations located in depth. This discontinuity suggests the presence of material that vibrates more concerning the background. This anomaly could be associated with the presence of denser material, and therefore with a sort of cap located below the main crater of Vesuvius. On the left side of the cap, there are two ducts, probably created by the pressure of the vapors or fluid gasses coming from underground. The maximum depth measured by the tomogram is approximately 3 km from the maximum topographic surface height of the volcano. Figure 6a,b is a detailed analysis of the of the volcano feeder conduit. In this figure, two apertures, probably excavated by high-pressure magmatic fluid gases,  
Figure 4: Computational scheme of tomographic imaging. and the presence of a large portion of a denser material (having higher vibrational energy), residing below the surface of the main crater at a depth of about \(1\) km, are visible. \begin{table}
\begin{tabular}{c c} \hline \hline
**SAR Parameter** & **Value** \\ \hline Chirp bandwidth & 450 MHz \\ Doppler Bandwidth & 22 kHz \\ PRF & 2-(Doppler bandwidth) \\ Antenna length & 6 m \\ Type of acquisition & Spotlight \\ Polarization & HH \\ Acquisition duration & 14 s \\ Platform velocity & 7 km/s \\ Observation height & 650,000 m \\ Acquisition date & 17 January 2022 m \\ Acquisition geometry & Right-ascending \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the SAR acquisitions. Figure 5: Environment images. (**a**) optical image. (**b**) SLC SAR image in magniture. Figure 7a shows the SAR image (in magnitude) of Vesuvius according to a new tomographic line inserted, slightly more inclined, to cover as much as possible, in radar visibility, the volcano crater diameter, on the condition of having the greater scattering energy. On those pixels, we calculated the seismic tomogram visible in Figure 7b. In this case, the possible presence of a cap constituted by dense rocky material and the presence of an energetic gap where the seismic waves oscillate with less intensity is visible. Figure 8a represents the SAR SLC image where the tomographic line 1 is shown where we investigate the acoustic response in depth. The result obtained, thus the tomography of the entire line is proposed in Figure 8b. The topographic height is correctly detected through line 2. In Figure 9a, the detailed seismic tomography of the Vesuvius crater is depicted (particolar of box numer 1 of Figure 8b), and details of the SAR image are proposed in Figure 9b. Figure 9c is the sonic tomogram of a range line spanning the entire crater. Through yellow arrows 3 and 4, pointing to the SAR image, two conduits are shown; finally, through arrow 4, the presence of a massive debris accumulation is detected. This facility appears to be composed of several layers having different physical compositions.