# Multi-Parametric Imaging of Etruscan Chamber Toms:
## 4 Discussion
### Geophysical and Geochemical Data Overlay  
Figure 8 shows the comparison between the models obtained via ERT, CCR, and GPR techniques. The anomaly due to the Eb1 excavated chamber is clearly recognizable in all geophysical datasets. However, it differs in shape and depth depending on the adopted geophysical method. Meanwhile, the ERT provided a good result in terms of anomaly position and its roof and floor depth detection; the Eb1 CCR anomaly was not correctly reconstructed by the inversion algorithm due to the lack of survey data due to the presence of the metallic fence obstruction along the path. Two main differences were identified between the CCR and ERT output models. Owing to its higher shallow resolution, the ERT resistivity model highlighted the presence of an uppermost lower-resistive layer (\(\rho<200\) Ohm.m), approximately 1 m thick throughout the section, that was not imaged by CCR data. The western resistivity anomaly (e7) is suggested to have geometrical aspects similar to that of the adjacent anomalies e5 and e6 as compared with the CCR model, which does not precisely locate this anomaly. Note that the CCR dipoles have a minimum dimension of 5 m and thus are unable to depict the resistivity signature of the uppermost layer due to the inherent lower sensitivity at very shallow depth. As for the e5 resistivity anomaly, the GPR profile showed continuous strong reflections, locating a small void between the roof chamber and the filling material. It also provided an accurate view of the chamber width, but not the floor position, even though the diffuse diffractions due to the chaotic filling material suggest a buried volume. Meanwhile, the ERT model showed a very similar result as the roof anomaly generated a resistive thin layer at a depth of approximately 1.5 m. Similar to GPR, the chamber shape was not recognizable, meaning that the host and fill materials had similar electric properties. Further, the uppermost resistivity variations highlighted by the ERT model in correspondence of the e5 anomaly probably affected the accurate imaging of this anomaly. Conversely, the CCR model is practically insensitive to this effect as no apparent resistivity data were measured for that portion of the ground. As a matter of fact, we could invert a simpler resistivity context (averaged top layer with a void inside) that was largely affected by the imposed starting model (\(\rho\simeq 600\) Ohm.m) in the uppermost portion of the model. The e6 anomaly is practically undetected in the GPR radargrams because of the slight difference (less than 0.7 m) between the GPR2/GPR3 and CCR2 paths in that area. This confirms the dependence of our results on the precision in profile overlays when using high-resolution methods. Figure 9 shows the comparison between the results obtained along the profiles CCR1 and GPR1 acquired on the small northwest-southeast oriented trail. The GPR1 radargram provides a clear image of the separated anomalies (G1, G2, and G3), which is probably due to the presence of partially filled voids. Conversely, in the CCR model, the e2 anomaly is similar to G2, but it appears more confined in space and magnitude. The G3 anomaly shows some relation to the e3 CCR anomaly, which appears to be related to the G3 lateral extension. Such differences could be reasonably attributed to the change in survey direction and the presence of obstacles above the surface, which probably caused inaccurate positioning of the CCR anomalies along the profile with respect to the GPR line. Figure 10 reports the Th/Rn ratio from stations that overlay the CCR2 profile. We found a good correlation between the high values measured in samples GC9 (A1), GC11 (A2), and GC13 (A3) and the three resistivity anomalies recovered by the CCR2 and ERT profiles (e7, e6, and e5, Figure 8) and confirmed by ERT and GPR datasets. Figure 8: Geophysical data comparison. Comparison between all sets of output geophysical models recovered over a coincident line (CCR1, GPR2, GPR3, and ERT profiles in Figure 3).