# Scanning Inside Volcanoes with Synthetic Aperture Radar Echography Tomographic Doppler Imaging
## 4 Validation of the Sonic Tomographic SAR Results  
### Topographic Validation  
In this section, the topographic validation of the estimated tomograms is performed. The topographic elevation lines are provided by the digital elevation model (DEM) generated by the Shuttle Radar Topography Mission (SRTM). The topographic lines are superimposed on the estimated tomographic sections. In this specific case, the topographic line 1 visible in Figure 14a has been extrapolated from the global SRTM topographic surface,  
Figure 13: Tomographic results and analysis. (**a**) Tomographic analysis of the external eastern slope of Vesuvius, where potentially 11 magmatic fluid fractures are visible. At the bottom of the tomogram, the presence of a magma chamber is evident. (**b**) Tomographic results and analysis. (**a**) Tomographic analysis of the internal eastern slope of Vesuvius, where potentially five magmatic fluid fractures are visible. At the bottom of the tomogram, the presence of a magma chamber is evident. (**c**) Detail of the eastern slope showing an internal feeder conduit. (**d**) Detail of the deepest part of the conduit showing a splitting of the magmatic fluid conduit. (**e**) Detail of the external magmatic fluid conduit (upper part). (**f**) Detail of the eastern slope on the external side (lower part). which is coincident with the tomographic line 1 shown in Figure 11a. The topographic line has been merged on the tomogram and the total information is visible in Figure 15b. The visual analysis shows the surface tomogram overlapping almost perfectly with the topographic line, which is represented by yellow line 1. The next validation case is to compare the position of the topographic line 1 visible in Figure 16a (always extrapolated from the SRTM-DEM) with the top of the tomography coincident to line 1 shown in Figure 8a and the consequent merged information is visible in Figure 15b. The visual analysis shows the almost perfect overlap of the superficial component of the tomogram with the topographic line, which is represented by yellow line 1. The last performance analysis case study is shown in Figure 14a,b. For this case, the tomographic line is perfectly oriented along the range direction. The objective was to study the tomographic reconstruction robustness under heavy foreshortening and layover effects. The tomographic picture is reported in Figure 14b. In this case, the eastern side of the crater (the one characterized by layover) is better seen than the western side (the one characterized by foreshortening). However, possible magmatic fluid conduits communicating with both the inner and outer parts of the main volcano cone remain visible on the eastern side of the crater. The tomogram has been divided into three portions: result numbers 2, 3, and 4, which are compartmentalized by three red squares visible in Figure 14b. Each tomographic surface is pointed at its tomographic line reference, which is visible in Figure 14a. The portion of the tomogram characterized by layover is number 2, while result number 3 is  
Figure 14: Tomographic results and analysis. (**a**) SLC SAR data of Vesuvius. (**b**) Tomographic results and analysis. the one representing the interior of the main crater. Finally, result number 4 refers to the descending side of the volcanic cone. Results confirm the presence of vibratory material, similar to rock, probably composed of cooled compact solidified magma that produces a substantial blockage mass located deeply inside the main volcanic conduit. This blockage is visible within box 5 of Figure 14b. Figure 16: Range-line echographic tomography result (magnitude), and validation through DEM. **(a)** DEM results in the slant-range coordinates. **(b)** Tomographic map (in magnitude), and topographic height (yellow line). The result is calculated over the white line down on the DEM reported in Figure **(a)**. The yellow tomographic line is indicated by the number 1. Figure 15: Range-line echographic tomography result (magnitude), and validation through DEM.